CE317_Domestic Low & Zero Carbon Technologies 2010

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Domestic Low and Zero CarbonTechnologies

Technical and practicalintegration in housing

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1. Introduction 41.1 What are low and zero carbon technologies? 41.2 Why should we use them? 41.3 Drivers for using low and zero carbon technologies in housing 5

2. Low energy design 72.1 Insulation 72.2 Ventilation 72.3 Thermal mass 82.4 Heating systems 92.5 Flue gas heat recovery systems 9

3. Typical household energy consumption 10

4. Solar photovoltaic (electricity) 114.1 The solar resource 114.2 Basic principles 114.3 Types of photovoltaic cell 124.4 A typical solar PV system 13

5. Solar thermal (hot water) 145.1 Basic principles 145.2 Types of solar thermal collector 145.3 Heat transfer medium 155.4 Circulation and control system 155.5 A typical solar thermal hot water system 16

6. Wind 176.1 The resource 176.2 Basic principles 186.3 Large and small-scale turbines 186.4 A typical wind turbine system 18

7. Micro-hydro 197.1 The resource 197.2 Basic principles 197.3 A typical micro-hydro scheme 19

8. Biomass 208.1 The resource 208.2 Types of fuel 208.3 Basic principles 218.4 Types of system 218.5 Liquid biofuels 228.6 A typical biomass system 22

Table of contents

Domestic Low and Zero Carbon Technologies 2010 edition

Cover: VELUX solar hot water collectors

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9. Heat pumps 239.1 Ground source 23

9.2 Heat pump and distribution system 249.3 Water source 259.4 Air source 259.5 A typical ground source heat pump system 25

10. Community heating and combined heat and power 2610.1 Community heating 2610.2 Biomass-fired boilers 2610.3 Combined heat and power (CHP) 2610.4 A typical community CHP system 27

11. Micro-CHP 28

11.1 A typical micro-CHP system 2812. Standalone and grid-connected systems 2912.1 Standalone 2912.2 Grid-connected 29

13. Suitability: urban and rural environments 3013.1 Solar thermal and PV 3013.2 Wind 3013.3 Micro-hydro 3113.4 Biomass 3113.5 Ground source heat pumps 31

13.6 Air source heat pumps 3113.7 Community heating and CHP 3113.8 Micro-CHP 31

14. Capital and maintenance costs 3214.1 Individual dwellings 3214.2 Community scale 33

15. Sources of grant funding 3415.1 Feed-in tariff and renewable heat incentive 3415.2 Bio-energy Capital Grants Scheme 3415.3 Scottish Biomass Heat Scheme 34

15.4 Community Sustainable Energy Programme 3415.5 Scottish Community and Householder Renewables Initiative 3415.6 Renewable Obligation Certificates 3415.7 Reduced rate of VAT 3415.8 Energy Saving Trust database 34

16. Developing a renewable energy strategy 35

Appendix A Further information 36

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1. Introduction

Home energy use is responsible for over a quarter of UK carbondioxide (CO2) emissions, which contribute to climate change. Tohelp mitigate the effects of climate change, the Energy SavingTrust has a range of technical solutions to help UK housingprofessionals build to higher levels of energy efficiency.

This guide gives an overview of low and zero carbon (LZC)

generation technologies available for use in domestic applications,their costs, technical and practical limitations and integration. It isaimed primarily at building professionals, housing associationsand developers, who will benefit from considering how toincorporate renewable energy at an early stage in the designprocess.

The guide covers all types of electricity and heat generatingtechnologies that are commonly available, including both thosesuited to the individual building and community level. It alsoidentifies which of these technologies are best suited to urban,rural, newbuild or refurbishment applications.

1.1 What are low zero carbon technologies?The scope of technologies covered within this guide includes boththose that are zero carbon in operation (powered by 100%renewable energy) and those that are considered to be lowcarbon in operation 1 (powered at least in part by fossil fuels).

Renewable energy is energy derived from naturally occurring andnaturally replenished energy flows that cannot be exhausted.These include solar, wind, hydro and biomass all of which areultimately driven by solar energy.

Low carbon sources: Use fossil sources to generate heat or electricity far more

efficiently than conventional alternatives and thus producefewer carbon emissions (e.g. heat pumps, district heatingand CHP).

Use renewable energy as a principle source but also require aproportion of fossil energy input (e.g. transport of biomass,solar thermal hot water with a mains-powered pump).

In some scenarios, low carbon sources may be more cost -and/or carbon-effective than zero carbon sources. The suitability ofLZC technologies is site-specific, and may be dependent on factorssuch as location, orientation, topography, development size, energydemand profile and mains gas availability.

1.2 Why should we use them?We are consuming energy in ever-increasing quantities, mostlyfrom fossil fuels. Home energy use represents more than aquarter of UK carbon emissions.

1.2.1 Climate changeBurning fossil fuels leads to a significant increase in the levels ofcarbon dioxide (CO2) within the atmosphere, which is the principalcause of climate change. As climate change progresses,established weather patterns will become more unpredictable,with serious impacts on water supply, the built environment,biodiversity, health, agriculture and business. (See the SternReview on the Economics of Climate Change 2 and the EnvironmentAgency publication The climate is changing: time to get ready 3).

Table 1: Low and zero carbon technologies

1. All equipment, whether low or zero carbon in operation, has carbon emissions associated with its manufacture, distribution, installation and decommissioning. Anydifferences in embodied or life cycle carbon emissions are not considered here.2. hm-treasury.gov.uk/stern_review_report.htm3. publications.environment-agency.gov.uk/pdf/GEHO0205BIRS-e-e.pdf

Heat output Electrical output

Zero carbon Solar thermal hot water Solar photovoltaicgeneration (photovoltaic pump or Wind

thermosyphon) Micro-hydro

Low carbon Solar thermal hot watergeneration BiomassHeat pumpsDistrict heating

Micro or community combined heat and power

Energy Mechanical ventilationefficiency and heat recovery

Passive flue gas heatrecovery device

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Domestic Low and Zero Carbon Technologies 2010 editionIntroduction

1.2.2 Energy securityThere are significant amounts of fossil fuels locked away in theearths crust that can be extracted at an almost unchecked rate.Ultimately, however, they represent a finite resource. The UK hasfor many years had access to large deposits of oil and gas underthe North Sea. However reserves are declining fast and we nowhave to rely on increasing amounts of fossil fuel sourced from

outside our borders. By 2020 we could be dependent on importedenergy for three quarters of our total primary energy needs 4.

Many of the regions where the largest deposits remain are distantand have a history of political instability. In order to maintain thesecurity of our energy supply into the future it is vital that wereduce our usage and diversify our supply.

1.3 Drivers for using low and zero carbontechnologies in housingDrivers for increasing the uptake of LZC technologies in housing

exist at several levels. Legally binding international commitmentsto reduce emissions such as the Kyoto Protocol and EU EnergyPerformance of Buildings Directive have given rise to a number ofUK, national and regional level drivers and incentives, which areexplained in further detail below.

1.3.1 UK Building Regulations Part LApproved Document Part L of the UK Building Regulations requiresreasonable provision to be made for the conservation of fuel andpower in buildings.

Part L1A sets out minimum energy performance requirements fornew dwellings, in the form of limiting fabric U-values and airpermeability, efficient building services, effective controls andoverall target CO2 emission rates. Part L1B, which has slightlyless onerous requirements in many cases, deals with therefurbishment of dwellings 5.

The overall target CO2 emissions and those predicted to beachieved by the dwelling are measured using the StandardAssessment Procedure (SAP) 6. These standards are set to rise,leading to increased use of LZC technologies in domestic buildings.

1.3.2 Renewables ObligationThe Renewables Obligation is a UK government requirement forlicensed electricity suppliers to source a specific and annuallyincreasing percentage of the electricity they supply fromrenewable sources. The current level is 9.1% for 2008/09, risingto 15.4% by 2015/16 7.

Renewables Obligation Certificates (ROCs) are issued to electricitygenerators in order to demonstrate that they have reached therequired level. Alternatively, these can be purchased on the openmarket from those who generate renewable energy, or a buyoutcan be paid 8.

1.3.3 Code for Sustainable HomesThe Code for Sustainable Homes is the Governments key strategyvehicle for driving sustainability in the domestic construction sector.

It is a credits-based rating system that encompasses manyaspects of sustainability, from energy and water through to

pollution and the use of materials. There are nine categories intotal. Each of the discrete credits is either achieved or withheldand the final score translates into a star rating from one to six.(See energysavingtrust.org.uk/housing/thecode )

1.3.4 Planning requirements and the Merton ruleNational planning policy in England is outlined in a series ofdocuments known as Planning Policy Statements (PPS). Thefollowing three PPSs are relevant to energy planning for newresidential developments:

PPS 1: Delivering Sustainable Development (2005).

PPS 22: Renewable Energy (2004). PPS: Planning and Climate Change a Supplement to PPS 1(2007).

These documents set out the principles that local planningauthorities should apply when considering new development intheir regions.

4. Department of Trade and Industry (2003) Our Energy Future Creating a Low Carbon Economy [online], HMSO, Available at:dti.gov.uk/energy/whitepaper/ourenergyfuture.pdf5. See planningportal.gov.uk/england/professionals/en/1115314110382.html6. See bre.co.uk/sap20097. BERR.berr.gov.uk/whatwedo/energy/sources/renewables/policy/renewables-obligation/what-is-renewables-obligation/page15633.html8. The buyout value is set by OFGEM each year. For 2008/09 it was 35.76/MWh.

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Introduction

The Merton Rule takes its name from a requirement introduced bythe London Borough of Merton in October 2003. It required allnew non-residential developments above a threshold of 1,000m 2to incorporate renewable energy production equipment to provideat least 10% of predicted energy requirements.

In practice and over time, the Merton rule can apply to both

commercial and residential developments, and the term is nowgenerally used to express a requirement for a 10% renewableenergy component (or more in some cases and usually in additionto any Code for Sustainable Homes requirements) in any newbuildcontext. For calculation purposes, a conversion of energyrequirement to CO2 emissions is used to discourage the use ofelectricity for heating.

The most recent PPS, Planning and Climate Change, has takenimportant steps forward in recognising that policies are mosteffective if they:

Allow both low and zero carbon sources of generation. Include elements of both on and near site generation. Are not applied as a blanket rule, but instead can be tailored

to specific sites depending on the available potential.

The term Merton plus may be used to describe this approach innational policy. It outlines the opportunity for local planningauthorities to require higher building standards ahead of thenational timetables set by central government 9 (e.g. that allhomes should be zero carbon from 2016) but only where this isjustified and by using national standards such as the Code forSustainable Homes, rather than developing local standards.

National planning policies for Scotland, Wales and NorthernIreland relating to LZC energy are contained in the followingdocuments:

Scotland: Scottish Planning Policy (SPP) 6: Renewable Energy(2007).

Wales: Technical Advice Note (TAN) 8: Renewable Energy(2005).

Northern Ireland: PPS 18: Renewable Energy.1.3.5 Feed-in tariffsIn October 2008, the UK parliament published the Energy Act 10,which enabled the Government to introduce a feed-in tariff. Thiswas introduced in April 2010. A feed-in tariff is an incentivestructure to encourage the adoption of renewable electricitygenerating technologies below 5MW capacity, e.g. solarphotovoltaic (PV) panels and wind turbines. Regional or nationalelectricity utilities will be obligated to pay generators for all theelectricity they generate in addition to buying any electricityexported both at rates set by the government. This reduces thepayback times of eligible technologies, making them morefinancially attractive.

9. These are outlined in the Department for Communities and Local Government publication, Building a Greener Future.10. Full text (amendment 5) available at publications.parliament.uk/pa/ld200708/ldbills/072/amend/ml072-ir.htm

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Low energy design

This:

Provides stable daily temperatures in both summer and winter. Delays the daily peak temperatures relative to the external air

until later in the day.

Reduces demand on heating systems and improves thermalcomfort during summer.Peak temperaturedelayed by up to 6 hrs

Up to 6-8C differencebetween peak externaland internal temperature

Internal temperature with

high thermal mass

Internal temperature withlow thermal mass

External temperature

Day

15 C

30 C

Night Day

Figure 2: The impact of increasing building fabric thermal mass 12

12. Butler, D. (2007) Ventilation and building fabric using thermal mass and hybrid ventilation, presented at: Low Carbon Technology Briefings: Low energy cooling -keeping our buildings cool in a warmer climate, BRE, Watford, 17 April 2007.

Key considerations

New dwellings should, wherever possible, be positioned totake maximum advantage of solar gains, daylight and anyexisting protection from external elements.

Houses should be oriented so that their long axis lieseast/west where possible, with the main glazed elevationfacing as close to south as possible.

Appropriate solar shading should be installed on southfacing glazing to reduce the risk of summer overheating.East and west facing glazing can be difficult to shadeeffectively.

Over-shading within 30 of south should be avoided.

Exposed internal thermal mass should be used to provide amore stable internal temperature and reduce the risk ofoverheating.

To maximise passive solar gain, the dwellings should beplanned internally so that main living areas are on the southside of the building, with unheated spaces or infrequentlyused spaces on the north side. Dwellings should also bespaced at least twice their height apart (north to south).

Heating systems and controls which respond to internal andexternal temperature (e.g. thermostatic radiator valves, weathercompensators) should be specified. Rooms subject to highsolar gain should have their own zone temperature control.

2.3 Thermal massA central feature of any building designed to make the most ofpassive solar energy is high thermal mass. Exposed thermalmass elements have the ability to absorb and store heat from,and release heat to, the internal spaces, increasing theirthermal inertia.

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2.4 Heating systemsHeating systems must be sized correctly for the actual heat lossfrom the dwelling (with allowance for warm-up). Over-sizing islikely to waste energy, whilst under-sizing will not achieve thedesired temperature. Ideally, a professional heating engineer shouldcalculate heat demand and emitter (e.g. radiator) size, although theEnergy Saving Trust Domestic Heating Design Wizard can be usedto help determine the heating system capacity needed 13.

Heating controls that respond to incidental and solar gains andprovide adequately zoned heating to all parts of the dwellingshould be provided.

As fabric performance is improved, reducing space-heating losses,the energy used in heating domestic hot water becomes relativelymore significant. Condensing boilers are typically less efficient inhot water-only mode, as the return feed from the heating coil inthe hot water storage tank is at a relatively high temperature,which reduces condensing performance. SAP 2009 recognises this

by having separate summer and winter seasonal efficiencies.In a similar way, the efficiency of a condensing boiler in heatingmode can be improved by employing low-temperature deliverymethods like underfloor heating.

2.5 Flue gas heat recovery systemsFlue gas heat recovery systems (FGHRS), which can be fitted to newor existing condensing boilers, typically consist of a heat exchangerdevice located between the boiler and the flue. They can increaseboiler efficiency by capturing some of the heat in boiler flue gasesthat would otherwise be wasted, and make use of it to reduce theamount of fuel that has to be burned when providing hot water.

Passive flue gas heat recovery devices (PFGHRDs), a sub-section ofFGHRS, have no active elements and therefore require nomaintenance or adjustment by the occupier.

The heat recovered is mostly from the condensation of watervapour in flue products and the application of FGHRS is restrictedto condensing boilers, because non-condensing types are notgenerally adequately protected against the corrosive effects ofcondensate. Where the device has a heat store within it, energyrecovered during space-heating production can also be used to

later offset the heat required for providing domestic hot water.

13. See energysavingtrust.org.uk/housing/boilersizing14. SAP Appendix Q provides a route for recognition of new or improved technologies since the previous version was released (2005). It is likely that FGHRS and MVHRtechnologies will be included with SAP 2009.

Figure 3: Passive flue gas heat recovery device

Domestic Low and Zero Carbon Technologies 2010 editionLow energy design

There are three ways the system can save energy:

Instant savingsDuring production of hot water, heat is recovered from the flueproducts and instantly used to warm the cold water feed.

Deferred savingsWhen the boiler is firing for space heating purposes, the systemrecovers heat from the flue products and stores it. The heat is latertransferred to pre-heat the cold water feed of the domestic water.

Reduced wasted hot waterInstantaneous combi boilers, despite their name, take a little timeto provide hot water, mainly because the water and heatexchangers within the boiler require warming before the domesticwater can be heated to an acceptable temperature. This meanssome water may be wasted because it is not warm enough. FGHRSmay reduce the amount of rejected lukewarm water. This option isonly applicable to combi boilers without a keep hot facility.

The extent to which savings are made depends on a number ofinteracting factors, such as the boiler fuel and efficiency, relativedemand for space and water heating, and the hot water draw-offpattern. These are taken into account in a calculation procedurewhich has been included in Appendix Q of SAP.

Further information on SAP Appendix Q, the test methodologies

and test results of eligible systems can be found atwww.sap-appendixq.org.uk14

Domestic hot water

Mains cold water Mains cold water

Condensingboiler

Hot fluegasses

Centralheating

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3. Typical household energy consumption


The average household electricity consumption for lights,appliances cooking, heating and cooling varies across the UK butaverages to around 4,700kWh per annum 15.

For dwellings built to 2006 building regulations, overall energyconsumption by end use has been estimated as follows 16:

Hot water consumption and draw-off patterns vary depending onthe number, age and lifestyle of occupants in a given household.

15. The total UK domestic electricity consumption of 117.589 terawatt-hours (TWh) (The Digest of UK Energy Statistics 2005) and 25.2 million UK households (Mid-yearHousehold Estimates published in 2004 by the Office for National Statistics).16. Taken from CE190 Meeting the 10% target for renewable energy in housing a guide for developers and planners.17. BERR.berr.gov.uk/files/file16568.pdf18. DTI.berr.gov.uk/files/file16522.pdf

Table 2: Reference data

Dwelling type Top floor flat Mid-terraced End-terraced Semi-detached DetachedTotal floor area (m2) 60.9 78.8 78.8 88.8 104.0

Space 2,270 2,232 2,893 3,423 4,451Energy Water 2,813 3,228 3,228 3,412 3,762requirements Lighting and appliances 2,201 2,719 2,719 3,057 3,635(kWh/yr) Cooking 1,173 1,264 1,264 1,314 1,386

Total 8,456 9,443 10,103 11,205 13,233

Space 440 433 561 664 863CO2 emissions Water 546 626 626 662 730(kgCO2 /yr) Lighting and appliances 929 1,147 1,147 1,290 1,534

Cooking 227 245 245 255 269Total 2,142 2,452 2,580 2,871 3,396

Notes: Standard total floor areas (TFA) have been assumed per dwelling type. The figures provided are calculated for dwellings built to the Approved Document L1A 2006 regulatory standards. The CO2 emissions assume that the heating, hot water and cooking fuel is mains gas for all dwelling types. It is assumed that no secondary heating is provided and that all of the space heating and hot water requirement is met bythe main heating system (86% efficient). The energy requirement figure provided do not include production, delivery and appliance conversion losses. The CO2 emissions incorporate production, delivery and appliance conversion losses.

Typically, hot water demand is between 45 and 55 litres perperson per day 17, with little variation in hot water consumptionprofiles from winter to summer 18.

Only once energy efficiency and passive design have been appliedto the greatest extent practicable should LZC technologies beconsidered to meet at least part of the reduced demand. The

following sections discuss the range of LZC technologies suitablefor application to the domestic sector.

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4. Solar photovoltaic (electricity)

4.1 The solar resourceEach year, on average, every horizontal square metre of the UKreceives solar energy amounting to between 750 and 1,100 kWh,depending on the latitude and prevailing weather conditions.

Some of this energy is received as direct sunlight and some isdiffuse sunlight scattered by clouds, the atmosphere and reflectedby surrounding objects 19. Therefore, the amount of energy receivedon any given day varies seasonally and with cloud cover, but is

Figure 4: Typical seasonal variation in the solar resource in London(at optimum inclination from the horizontal)

19. The diffuse component varies between around 40% in summer to 80% in winter.

0%

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b e r

Figure 5: Variation in average monthly solar irradiation (London)

predictable in terms of average daily and annual patterns. It isgreatest at midday and during summer when the sun is highestand the days are longest.

The global solar irradiation (i.e. the total solar energy receivedper m2) and potential for photovoltaic (PV) generation (consideringthe efficiency of all components in a PV system) at the optimumangle in each location is shown in figure 4.

4.2 Basic principlesPV cells convert solar radiation directly into electricity through theinteraction of light with electrons in a semiconductor material.

Solar cells are built using layers of semiconductors which createa small potential difference between the layers. When exposed tosolar radiation, an array of cells connected together (called amodule) creates a usable amount of direct current (DC).

This is usually converted to alternating current (AC) for householduse or for export to the National Grid. A typical domestic systemconsists of several modules, an AC/DC inverter, fuse box, isolatorand an import/export meter.

The performance ratio of a solar PV system describes the actualenergy yield as a proportion of the theoretically expected yield.

System losses can be caused by deviation from module nominalefficiency, module soiling and inverter and wiring losses. Theperformance ratio is typically 0.7 to 0.75.

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Solar photovoltaic (electricity)

Using integrated solar modules, e.g. solar roof tiles, can offsetcost and embodied energy by replacing materials that wouldotherwise have to be used in construction. PV modules arecommercially available in a range of different types: traditionalaluminium framed modules, roof integrated systems, solar tilesand semi-transparent atrium roof systems.

Figure 6: Solar roof tiles

4.3 Types of photovoltaic ce llMonocrystallineThe most efficient and e xpensive form of silicon for produci ngsolar cells, monocrystalline silicon is grown from a single crys taland cut into wafers. The man ufacturing process is both time andenergy intensive.

Figure 7: Monocrystalline solar cell

PolycrystallinePolycrystalline silicon cells are manufactured from many differentcrystals rather than a single one. The cells are less expensive tomanufacture and have lower efficiencies compared tomonocrystalline cells, meaning that a slightly greater roof area isrequired per kilowatt peak (kWp) power. However, where availableroof area is not a restriction, polycrystalline cells are generally

more cost-effective.

Figure 8: Polycrystalline solar cell

Back-contact modulesMost crystalline PV modules consist of cells electrically connectedon both front and back surfaces of the module. These contacts onthe front of the module shade part of the PV cells (up to 8%)reducing the power output.

The back-contact cell improves module efficiency by eliminatingthis shading, and has the potential to increase automation of themodule assembly process, since all connections are in the sameplane. Some manufacturers report production efficiencies of over19% using this technique.

Amorphous siliconAmorphous silicon is deposited as a thin strip or film. Cells areproduced more quickly and cheaply but have significantly lowerefficiency. They are ideally suited to applications where there is alarge surface area available, so tend to be more suitable forcommercial buildings.

Amorphous cells perform slightly better in diffuse sunlight than

mono or polycrystalline modules. They do, however, have a shorterlifetime and suffer an initial drop in efficiency (15-35%) in thefirst few months, which then stabilises. Manufacturers take thisinto account, quoting an initial, stable product power output.

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Hybrid siliconHeterojunction with intrinsic thin layer (HIT) modules, whichinclude both amorphous and crystalline layers, tend to have thegreatest seasonal efficiency in temperate climates such as the UK.They are correspondingly more expensive, but lose lessperformance at high temperatures than crystalline modules.

Second generation thin filmSecond generation thin film technology uses lower-temperaturemanufacturing techniques such as vapour deposition and ultrasonicnozzles to deposit several very thin layers of semiconductormaterials on glass or ceramic substrates. This has reduced thecost of manufacture compared with first generation PVs.Amorphous silicon, copper indium diselenide (CIS) and cadmiumtelluride (CdTe) cells are already being manufactured with otherpromising materials at the research and development stage.

0

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Global solar cell demand (GW)

Crystalline solar

2007 2008 2009 2010 2011 2012 2013 2014 2015

Thin-film solar

Figure 10: Crystalline silicon has led the way for solar PV, butfuture solar growth will mostly come from thin-film. Source:Photovoltaics World

4.3.1 Relative Performance and DurabilityCrystalline modules are commonly warranted for 20 or 25 yearsbut may be expected to have a useful lifetime of 40 years ormore. Towards the end of this cycle, the output does tend to dropoff somewhat, an effect observed more keenly with amorphousand hybrid modules.

Domestic Low and Zero Carbon Technologies 2010 editionSolar photovoltaic (electricity)

0%

Si monocrystalline

Si HIT monocrystalline

Si polycrystalline

CIS/CIGS

CdTE

Amorphous Si

Production module efficiency Best lab result

5% 10% 15% 20% 25%

Figure 11: Relative efficiencies of different solar cells

4.4 A typical solar PV systemA typical system installed on a newbuild house might have thefollowing characteristics:

Capacity 2kWp

Capital cost 7,000 9,000

Output 1600kWh/annum

CO2 saving 840kg CO2 /yr*

*assuming grid displaced electricityemissions factor of 0.529kg of CO 2equivalent per kWh (kgCO2 /kWh).

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Second-generation thin-film technologies are newer and less wellunderstood. They may be expected to have a similar lifetime toamorphous modules, of some 25 to 30 years. The inverter andother components within the PV system have a lower durability, ofaround 15 years. They may need to be replaced one or moretimes during the lifetime of the PV modules.


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5. Solar thermal (hot water)

5.1 Basic principlesSolar energy can also be used to heat buildings, swimming pools,or most commonly, domestic hot water. A collector absorbs thesolar radiation and converts it into heat energy, which is removedby a heat transfer fluid such as water, antifreeze or air.

In most systems, a small pump is required to circulate the heattransfer fluid to where it is immediately needed, or to a storefrom which it can be removed and used later. In the case ofsolar water heating systems, this store is usually a hot watercylinder. A back-up heat source is required to ensure that thewater is heated to a sufficient temperature both to eliminate therisk of leigionella 20 and maintain the water temperature duringperiods of low solar radiation.

Figure 12: Solar thermal system

Note that the diagram shows one of the most commonarrangements, although there are many other possibleconfigurations21:

Forced (pumped) or thermosyphon circulation. Direct or indirect solar loop (indirect shown above,

incorporating a heat exchanger).

Direct or indirect primary domestic hot water loop (directshown; a thermal store would be indirect).

Single (either twin coil or with direct acting heat source suchas combi boiler) or primary and secondary stores.

Freeze protection employed: freeze tolerant pipes, use ofair/antifreeze or drain back.

Stagnation protection employed: drain back (open vented) orfully filled (sealed and pressurised with an expansion vessel).

5.2 Types of solar thermal collectorThere are three main types of collector for use in domestic solarhot water (SHW) systems - unglazed, flat plate and evacuated tube.

5.2.1 Unglazed collectorsSome manufacturers supply unglazed solar collectors for heatingswimming pools. These simple collectors require larger surfaceareas but can be a very cost effective method for providing solarheated water at relatively low temperatures.

5.2.2 Flat plateFlat plate collectors consist of flat, dark-coloured absorber plates

(made from metals such as copper or aluminum, polymers or acombination) attached to tubes through which the heat transferfluid passes. The plates are enclosed within a glazed, insulatedbox that behaves like a mini-greenhouse to retain heat withinthe collector.

The addition of glazing reduces the amount of light that reachesthe collector, but also reduces the heat loss significantly, allowinggreater temperatures to be achieved. Most commercially availablepanels also have a selective coating to reduce the amount of heatre-radiated into the atmosphere.

20. Although intended for commercial premises where storage volumes are typically greater than for domestic applications, Approved Code of Practice L8: Legionnaires' disease: The control of legionella bacteria in water systems provides further guidance (see Appendix A.).21. For further details see Appendix A.

Solarcollector

Pipes to andfrom heating

boiler

Supply to hotwater taps

Hot waterheader tank

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1 Automatic air vent2 Flow meter3 Expansion vessel4 Pressure release valve5 Pump6 Pump gate valves7 Non-return valve8 Solar drain9 Cold feed cut off valve10 Hot water drain valve11 Fill valve and gauge12 Differential controller

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5.2.3 Evacuated tubeEvacuated tube collectors consist of rows of parallel glass tubes,each containing a metal absorber with a selective coating. Duringmanufacture, air is evacuated from the tubes. The resultingvacuum reduces conduction away from the collector, improving itsperformance, especially at high temperatures.

Evacuated tubes can have either heat pipes (where each tube isconnected into a manifold through which the heat transfer fluidpasses externally) or direct flow and return internal pipeworkconfiguration. Heat pipes mean that tubes can be easily replacedwithout draining the system, however direct flow and returnsystems are more versatile in terms of mounting position.

Domestic Low and Zero Carbon Technologies 2010 editionSolar thermal (hot water)

Figure 13: Evacuated tube collector

5.3 Heat transfer mediumCommercially available systems are either direct or indirect. In adirect system, it is the water in the store that passes through thecollector. In the more common, indirect system, the heat transferloop through the collector is separated from the store by a heatexchanger.

Water is less viscous than glycol, so required pump power can bereduced, minimising the energy clawback from pumping. Directconnection with the solar loop also improves stratification withinthe store, ensuring that the collector inlet temperature is as lowas possible, which increases performance and pump on time.

Direct heating is the more efficient arrangement. However, there isa risk of scalding water reaching the taps unless an indirectdomestic hot water loop, such as a thermal store, is used.Freeze-tolerant (e.g. polymer) pipes must also be specified withappropriate controls to prevent these reaching very hightemperatures.

5.4 Circulation and control systemThe circulation system can be either active or passive. Activesystems use an electric pump to circulate the heat transfer fluid,(some installations use a small PV module to generate electricityfor driving the pump and solar controller). In a typical system acontroller will compare the temperature of the solar collectorswith the temperature of the water in the storage cylinder(s). If thecollector temperature is a certain number of degrees hotter thanin the storage cylinder, the controller will switch on the pump.

Passive systems do not require a pump at all. In a thermosyphonsystem, the collector is placed lower than the store and theheat transfer fluid circulates through the collector loop bynatural convection.

Off-the-shelf passive systems, common in some warmer parts ofEurope, are not sold in the UK, although if the roof arrangementallows the storage tank to be located at a greater height than thecollectors, a bespoke system may be designed using commonlyavailable components. Pipe runs between the collectors and thestore must be kept short, of wide diameter and continuallyrising or falling as appropriate to enable the thermosyphon to

operate spontaneously.Lacking any control system, thermosyphon systems are not wellprotected against freezing and overheating.

Both flat plate and evacuated tube collectors perform well in bothdirect and diffuse sunlight.

In general, evacuated tube collectors are both slightly moreexpensive and around 10 to 20% more efficient than flat platecollectors. They are best suited to situations where availableunshaded roof area is limited. However, some modern flat platecollectors are double glazed and the performance of theseapproaches that of evacuated tubes.

Glazed flat plate collectors usually integrate into the buildingfabric better than evacuated tubes. Both wall and roof-integratedflat plate options are available.

Sunlight absorbed as heat bythe dark inner surface of theevacuated-tube

C o p p e r

h e a t

p i p e

N o n - t

o x i c l i q u

i d

C o l d v

a p o u r

l i q u i f i e

s a n d

r e t u r n

s t o t h e

b o t t o m

o f t h e

h e a t p i p

e t o r

e p e a t

c y c l e

H o t v

a p o u r

r i s e s

t o t h e

t o p o

f t h e

h e a t p

i p e

G l a s s

e v a c u a

t e d - t u

b e l o c

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Solar therma l (hot water)

5.5 A typical solar thermal hot water systemA typical system installed on a newbuild house might have thefollowing characteristics:

Area 4m2

Capital cost 3,000 5,000Output 1,400kWh/annum


*assuming 80% efficient gas boiler isdisplaced; emissions factor of0.198kgCO2 /kWh for mains gas.

During the summer months, solar-thermal hot water systemswill typically provide over 80% of domestic hot waterdemand, but less during winter. Over the course of the year,a solar fraction the proportion of energy required fordomestic water heating that is provided by solar means ofaround 35% could be expected with the system illustrated.

Solar fractions of up to 70% have been reported in somestudies, depending on the collector area relative to waterconsumption and the means of pumping.

The Energy Saving Trust is undertaking a field trial tomonitor the actual performance of up to 100 domestic solar-thermal hot water systems across the UK. The trial willidentify the actual energy and carbon sayings achieved bydifferent manufacturers systems when installed in a varietyof property types, with a varying profile of households. Thefield trial will include both flat-plate and evacuated tubesystems. Results are anticipated in summer 2010.

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6. Wind

UK is representative of short grass terrain. The NOABL databaseshould not be used for suburban or urban areas or forinstallations with nearby obstacles.

6.1.2 SitingIdeal locations are exposed (for example coastal sites or at thetop of a smooth hill) with no obstructions in the immediate vicinity,

especially in the direction of the prevailing wind. The turbine shouldbe mounted at the greatest height possible, since wind speedincreases with height. The least expensive way to increase thepower output from a wind turbine is to increase its tower height.

6.1 The resourceThe UK is by far the windiest of any country in Europe, withapproximately 40% of the total on and off shore resource. Thelarge majority of this is available only to large-scale wind farms,although in some locations generation on a domestic scale isfeasible. Resource availability is well matched to demand patterns,with average wind speeds greater during the winter months.

0%

2%

4%

6%

8%

10%

12%

P r o p o r t

i o n o

f a n n u a

l w

i n d p o w e r

Month

J a n u a r y

F e b r u a r y

M a r c

h

A p r i l

M a y

J u n e

J u l y

A u g u s t

S e p t e m

b e r

O c t o b e r

N o v e m

b e r

D e c e m

b e r

Figure 14: Variat ion in average monthly wind power output

6.1.1 Assessing the resourceBefore considering any installation, the proposed site should beassessed for wind resource, in relation to the size and budget ofthe development. Regular measurement using on-site anemometry,taken over a period of at the least several months (preferably ayear), is the most accurate way to assess the feasibility ofinstalling a wind turbine at the proposed site.

To give an approximate measure of feasibility, mapping tools areavailable to help predict the likely wind resource at a givenlocation. The Carbon Trust has released an online Wind YieldEstimation Tool22, which incorporates details about the location,surrounding landscape, hub height and the type of turbine tocalculate the annual mean wind speed as well as the likelyenergy generation and carbon savings. This is the most rigoroustool currently available when applied to urban or suburban areas.

The NOABL wind speed database was created to identify suitableplaces to locate large turbines and wind farms. This databasetakes into account undulations of the terrain, i.e. hills and valleys,but does not account for local topography elements or obstaclessuch as trees or buildings. As such, it assumes the whole of the

Turbulence will decrease performance and may reduce expectedequipment life. The turbine should also be sited away from theturbulent air flow, preferably upwind or a long way downwind(considering prevailing wind direction).

Height above surface

P o w e r

i n c r e a s e

f a c

t o r

4

3.5

3

2.5

1.5

11 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

2

Figure 15: How power increases with height

Region of highlydisturbed flowPrevailing

wind

Zone of disturbed flow over a small building

H2H

20H2H

Figure 16: How buildings disrupt the wi nd

22. carbontrust.co.uk/wind-estimator

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6.2 Basic principlesA turbine consists of a rotor, which converts wind power intorotating shaft power, a gearbox, to increase the speed of rotation,and an electrical generator. Most turbines have a horizontalaxis with two or three blades similar in profile to the wing ofan aeroplane.

Turbines range in size from small, battery-charging applicationsused for caravans and boats the multi-megawatt turbines used inwind farms. Small-scale, vertical-axis turbines are alsocommercially available.

6.3 Large and small-scale turbinesLarge-scale turbines are one of the most mature and cost-effective renewable technologies, generating electricity at a costcomparable to fossil fuel power stations.

As the rotor size decreases, its area and available power outputis reduced in relation to the square of its diameter. So if thediameter of rotor is halved, power output is reduced by a factorof four at given wind speed. Differences in wind speed betweenlocations have an even greater effect on power output, whichvaries as the cube of wind speed.

Several recent studies have been undertaken in the UK toestimate wind resource in urban areas. One such study, theWarwick Wind Trials, monitored the performance of several typesof microgeneration turbine across 26 sites during 2007-08 andconsidered issues such as social acceptability. The projectconcluded that using unmodified wind speed data by postcode

from the NOABL database and manufacturer power curves forturbines can lead to overestimating likely energy output by factorsof between 15 and 17 23.

The Energy Saving Trust has collected data from an additional 58fully monitored sites and further sites where customers providemonthly inverter readings. Conclusions based on a total of 114sites can be summed up simply - wind turbines work, but onlywhen installed properly and in an appropriate location. 24


Wind

6.4 A typical wind turbine systemA typical small scale wind turbine installed on a 15m pole in arural area (>5m/s windspeed at hub height) might have thefollowing characteristics:

Capacity 6kWe

Capital cost 20,000 25,000

Output 10,000kWh/annum

CO2 saving 5,290kg CO2 /yr

A micro-wind turbine, pole-mounted in the same location mighthave the following characteristics:

Capacity 1kWe

Capital cost 2,500 5,000Output 750kWh/annum

CO2 saving 400kg CO2 /yr

23. Warwick Wind Trials:warwickwindtrials.org.uk/24. Energy Saving Trust micro-wind field trial: Location, Location, Location energysavingtrust.org.uk/Generate-your-own-energy/Energy-Saving-Trust-field-trial-of-domestic-wind-turbines

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7. Micro-hydro

7.1 The resourceOf all the technologies featured in this guide, hydropower isperhaps the oldest method of harnessing renewable energy formechanical and electrical power. Turbines can be used to extractpower from moving water in a similar way to wind turbines. Akey advantage is that if the waterway is dammed, water can bestored at height and its gravitational potential energy releasedperiodically to match demand patterns. Opportunities are fairlylimited for additional inland hydro schemes in the UK because thegood quality, most commercially attractive resource has beenalmost completely developed.

On a scale more suited to single or groups of dwellings, run-of-the-river schemes are inherently more environmentally friendlythan big dams. These methods do not flood land or obstructrivers in ways that restrict the movement of river and marine life.

Hydro power is more cost effective as the head (height of storedwater) increases, although if there is a waterway local to thedevelopment with a reasonable drop in height across the site (ofthe order of a several metres) it is almost certainly worthfurther investigation.

Determining the energy available from a micro-hydro scheme willdepend upon both the head and varying flow rate available at thesite. Estimating the resource can be complex and time-intensive,and therefore the advice of specialist micro-hydro consultants shouldbe taken whenever assessing the potential of a proposed scheme.

7.2 Basic principles

Hydro schemes range in size from a few hundred watts to severalhundred MW. Micro-hydro generally refers to schemes under 100kWcapacity, and most domestic systems are considerably smaller.

The maximum capacity of hydro system that can be installed at agiven site can be roughly estimated as:

Head (m) x Flow (m 3 /s)x 5 = kW

Figure 17 shows the main components of a typical micro-hydroscheme. The intake diverts a proportion of the water flow into alength of pipe, known as the penstock, from the normal river flow.Water intakes are often incorporated into a weir and have screensto prevent fish and rubbish from entering the penstock pipe.

Figure 17: Micro-hydro system

The penstock transports water under pressure from the intake tothe turbine. A small diameter penstock costs less than one oflarger diameter, but will incur significantly more head-loss due toincreased friction when water flows through a smaller diameter pipe.

The powerhouse contains the turbine and generator set. Manydifferent types of turbine are available and have specificcharacteristics that are suited to specific sizes of scheme. Afterpassing through the turbine the water is released through anoutflow back into the river or stream.

7.3 A typical micro-hydro schemeA typical low head system might have the following characteristics:

Capacity 10kWCapital cost 10,000 30,000*

Output 50,000kWh/yr**

CO2 saving 26.5 tonnes CO2 /yr

*Indicative cost; capital costs are highlysite specific.**Indicative output; highly dependent onflow and head. (Sowton Mill, Devon)25.

25. Sowton Mill:devon.gov.uk/renewable_energy_guide_case_study_5.pdf

Intake Weir

Penstock

TailracePowerhouse

Transformer

Transmissionlines

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8. Biomass

8.1 The resourceIn common with fossil fuels, biomass emits CO 2 and otherpollutants when it is burnt. However, the amount of CO 2 releasedis equal to that absorbed when the tree was growing, so theprocess is carbon neutral except for energy used in planting,harvesting, processing and transporting the biomass. Evenallowing for these emissions, replacing fossil fuel energy withwood will typically reduce net CO2 emissions by over 90%.

In order for biomass to be a truly renewable energy source, thefuel must come from a sustainable source (i.e. it is replenished)and should be used in close proximity to where it has beengrown. Unlike other renewable energy options, wood can bestockpiled and used as and when its needed.

Figure 18: The biomass cycle

Potential for energy cropsThe Biomass Task Force report to government 26 suggests thatof a total agricultural holding of 17 million hectares in the UK,around one million hectares of land may be available fornon-food uses equivalent to roughly half the area of Wales.At the present time less than 2,000 hectares of land is underenergy crop cultivation.

26. Biomass Task Force report (2005) defra.gov.uk/farm/crops/industrial/energy/biomass-taskforce/pdf/btf-finalreport.pdf27. cse.org.uk/pdf/sof1116.pdf28. berr.gov.uk/energy/statistics/publications/ecuk/page17658.html

Harvested on a three year cycle, farmed timber crops such asshort-rotation coppice willow currently yield around 20-25odt(oven dried tonnes) per hectare, although there is potential forthis to be improved. Each dry tonne provides the equivalent ofapproximately 5,000 kWh27, meaning that one million hectarescould meet the annual space and domestic hot water heatingrequirements (6,800kWh) for approximately 5.3 million semidetached houses built to 2006 building regulations (or a tenthof the total UK domestic space and water heating demand 28).

8.2 Types of fuelLogsLogs are the simplest and cheapest form of woody biomass, withthe minimum processing required. Freshly felled timber contains ahigh proportion of moisture, and should be seasoned or ovendried before use. This reduces the moisture content from around50% to 20%, improving combustion and reducing the amount ofsmoke and tar produced.

WoodchipWoodchip is widely available and relatively cheap. It is derivedfrom forest/park/garden waste, recycled wood waste, or farmedtimber crops such as short rotation coppice willow. It has thelowest energy density of the types considered here, meaning thata greater storage volume is required. There is also a need to payparticular attention to quality assurance; for problem-free use,woodchips need to achieve high uniformity in size and have a lowmoisture content.

PelletsWood pellets are typically made of compressed sawdust wastefrom manufacturing of timber products, which may otherwise havegone to landfill. The result is a drier, denser fuel (8-10% moisturecontent) with twice the energy density of logs and four times thatof woodchip. Burning cleaner, they also produce less ash, reducingthe cleaning and disposal effort. The use of pellets is wellestablished in Sweden, Austria, Denmark and North America andis gaining popularity in the UK, although there are currently alimited number of pellet suppliers in the UK. Supplier informationcan be obtained from logpile.co.uk

Atmospheric carbon dioxide,water and sunlight

Which is harvested and burnt

Carbon released backinto the hemisphere

Converted into new plant materialthrough photosynthesis

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Domestic Low and Zero Carbon Technologies 2010 editionBiomass

Figure 19: Comparative volumes of fuel required to supply annualspace heating and hot water energy requirement

The greater energy density of pellets means delivery results inlower emissions per km when compared to logs or woodchip.Unfortunately, the vast majority of pellets are imported fromoutside the UK, so the distances travelled will be greater. The netenergy balance (and its effect on carbon emissions) will therefore

need to be evaluated depending on the source. Life cycleassessment (LCA) studies have shown that where pellets aresourced from within 50km, pellet transport is responsible for 7%of total emissions (i.e. those related to planting, harvesting,processing and transportation). If this distance were increased to500km, the emissions from delivery would increase toapproximately 43% of the total 29.

8.3 Basic principlesThe combustion process can be extremely inefficient if notproperly controlled. Burning logs in open fires is very inefficient;

with 80% of the heat lost up the chimney. Controlling the process,and in particular the air supply, is a key factor in ensuring thatthis heat is not lost.

Domestic scale biomass systems are technically and aestheticallywell advanced, offering a highly efficient alternative to fossil fuelbased systems. Modern wood burning appliances can achieveefficiencies of 60 to 90%; they use microprocessors to set andmaintain temperature by regulating the rate fuel and air are fedto the combustion chamber. Output is variable and can bereduced to around 30% of the maximum to match demand,although efficiency is greatest, and the amount of tar and ashproduction least, when operated at maximum capacity.

29. Bennett, S. J. (2007) Review of Existing Life Cycle Assessment Studies of Microgeneration Technologies, (Energy Saving Trust)

8.4 Types of systemStovesA log or pellet burning stove is ideal for providing secondaryroom heating in conjunction with a main central heating system.Typical capacity ranges from 5-15kW although heat output can beregulated down to 2kW on some models. Higher output versionsmay have an integral back boiler to divert a proportion of theiroutput to heating water for domestic hot water or central heating,rather than 100% space heating.

Highly insulated dwellings may receive enough heat for the entirehouse from a centrally located stove (which can be integratedwith a back boiler to supply domestic hot water), freeing upspace required by a boiler, since the stove itself provides a roomfeature. A back-up source would be needed to provide domestichot water during the summer season when space heating is notrequired. Stoves are typically manually controlled.

Figure 20: Cutaway diagram of a typical stove

Automatic fuel feedat optimum rate

Correct air andfuel rates resultin clean burnand little ashfor disposal

Flames arevisible throughwindow

Convector fanensures moreeven distributionof heat

Fuel hopper

Air is drawn intothe burn chamber

Wood chip Logs Pellets Heating oilLow

High

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Biomass

BoilersFully automatic log, pellet and woodchip boilers are available forlarger scale applications. Designed for installation in kitchens orutility rooms, log and pellet boilers are suitable for largehouseholds where space and water heating demand is greaterthan 8kW (pellets) or 12kW (logs). Small-scale woodchip boilersfrom around 25kW and above are better suited to communal

schemes and apartment blocks.

Figure 21: Pellet-fed boiler

Biomass boilers are generally suitable for connection to anexisting central heating system. They work best in conjunction witha thermal store, allowing the boiler to operate at higher and moreconstant loads, which maximises its efficiency.

8.5 Liquid biofuelsBiofuels can be derived from non-woody biomass plant stock,such as rape seed, soya and palm oil (or on a very limited scalefrom used cooking oils). Biofuels known as fatty acid methylesters (FAME) are suitable for blending with fuel oil to be used inoil-fired boilers for home heating.

The blends are typically 5% FAME with 95% fuel oil. Although itis possible to convert boilers to run on 100% FAME, there arepractical difficulties to be overcome in that FAME tends todegrade rubber seals and cloud or gel at low temperatures.It is important that the combustion properties of biofuels areconsistent; FAME for heating purposes should meet therequirements of EN 14213. Further information is available fromOFTEC: OFTEC Information Sheet 56 Introduction to Bio-Liquidsfor Home Heating and Cooking (2009).

8.6 A typical biomass systemA typical pellet boiler for a newbuild house might have thefollowing characteristics:

*Equivalent to the heat demand for spaceand water heating for the dwelling.**Assuming 80% efficient gas boiler isdisplaced; emissions factor of0.028kgCO2 /kWh for biomass pellets.

22

Capacity 20kW

Capital cost 10,000

Efficiency 85%

Output 8,000kWh/yr*

CO2 saving 1,720kg CO2 /yr**


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9. Heat pumps

Heat pumps use electricity to move heat from one place toanother via a heat transfer medium, from lower to highertemperature. Heat energy is removed from a low temperaturesource and upgraded within the heat pump by a compression andevaporation cycle to heat air or water inside the building. Heatpumps can use ground, water or air as the heat source,depending on availability in a given scenario.

9.1 Ground sourceEnergy gained from solar irradiation is stored as heat in the earth.Its high thermal mass leads it to react more slowly to seasonalchanges in temperature than the air above it. Figure 22 illustratesthe seasonal variation in ground temperature.

T e m p e r a

t u r e

C

Time of year

24hr mean air

Ground @ 1.7m

Far field @ 75m

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

20

18

16

14

12

10

8

6

4

2

0

Figure 22: Ground temperatures throughout the year

Ground source heat pump (GSHP) systems consist of a groundloop, a heat pump unit and a heat distribution system. A water-antifreeze mixture circulates in a ground loop, extracting low-grade heat from the earth and passing through the heat pump(a schematic is shown in figure 23).

The ground loop can be either laid horizontally in trenches orvertically in boreholes. The choice of horizontal or vertical systemdepends on the land area available, local ground conditions andexcavation costs.

Horizontal ground loops require a relatively large area of land forthe trenches. Even if the most compact slinky type ground loop isspecified, the length of trench required is approximately 10m perkW capacity, around 40-50m for a medium newbuild detached

Figure 23: GSHP system

house. In order to avoid interference between adjacent trenches,they should be dug at least 3m apart. Ground water plays a partin conducting heat to the ground loops, so they should be locatedin an area that is either built on nor sealed from the rain.

Figure 24: Ground loop

Ground heatexchanger

Expansionvalve

Heatpump

Evaporator Condenser

Compressor

Heat distributionsystem

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10. Community heating and combined heat and power

10.1 Community heatingLarger-scale urban housing schemes present the opportunity forsignificant carbon savings by incorporating community heating orcombined heat and power (CHP). This relies on a central heatsource, a heat distribution network and heat exchangers withineach dwelling or building.

Dwellings connected to the heat network have a hydraulicinterface unit instead of a boiler. This regulates the heat takenfrom the heat main as required by the occupant for space anddomestic hot water heating. It typically takes up less space thana wall-mounted boiler. Internal distribution and control systemsare similar to those of a conventional boiler system.

The central heat source can operate on a variety of fuels. One ofthe advantages is that the central boilers can easily be replacedas technologies advance, leaving the distribution network in place.

10.2 Biomass-fired boilersOperating biomass boilers on a community scale (from 20kW toover 1,000kW) brings a number of advantages besides economiesof scale. A central storage and boiler house means co-ordinatingdeliveries and ash disposal and maintenance is morestraightforward and less expensive.

High capacity biomass boilers are generally designed for use withwoodchips. They will perform at their maximum efficiency whenoperated at, or close to, total capacity, and are therefore bestsuited to providing base-load heat demand, with alternativelyfuelled boilers operating as an auxiliary back-up to fulfil peak

load demand.

As technology advances, models are becoming available withsophisticated controls allowing the boiler to modulate and followthe heat load as it varies, without significant reduction in efficiency.

10.3 Combined heat and powerA vast amount of low-grade heat is wasted by fossil fuel-firedpower stations, as they are usually designed with high capacities(to capitalise on efficiencies of scale) and sited away frompopulation centres.

A coal fired power station typically converts only 30-35% of theprimary energy in coal into electrical power 30. Even the mostmodern and efficient fossil fuel power stations (Combined CycleGas Turbine plants) lose half of their energy in conversion.Transmission losses through the National Grid amount to a further2-3% of the total. This accounts for the high cost and carbonemissions associated with grid-supplied electricity.

30. World Energy Outlook 2004, IEA, ISBN 92-64-10817-3 (2004).

Conventionalpower generation

CHP

Used

Wasted

Figure 28: Proportion of primary fuel turned into useful energy

CHPworks on the prin ciple that generati ng electr icity locallyenables th at low-gra de heat to be put to use h eating homes andnon-domestic build ings.

CHP systems range in size f rom a few kilowatts to manymegawatts. Units a re usua lly based o n interna l combust ionengines which ar e able to supply heat netw orks servingeverything from clusters of dwellings throu gh to large community

sized systems. The majority of CHP systems run on natural g as,although other fuels including waste ma terials, bio mass fu els oreven hydrogen can be used. They have higher cap ital andmaintenance costs than conventional heating us ing central boilerplant (ho wever the additional capita l costs ma y be offset byfinancial suppor t).

The key technical chall enge in designing a CHP system is tobalance the thermal a nd elect rical loads . Systems are usua llydesigned to match t he base load heat requirement for thecommunal heat ne twork, with top-up boilers s upplying theremainder as req uired to meet the p eak ther mal load. Any

surplus of electr ic power during th e heatingseason c an be

exported to the grid.

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Domestic Low and Zero Carbon Technologies 2010 editionCommunity heating and combined heat and power

If there is little or no space heating demand (for example, outsidethe main heating season), then running the system for waterheating and power only may result in a heat surplus. Most CHPsystems use thermal storage for the purpose of smoothing outthe peaks and troughs in demand to allow for more continuousoperation.

Figure 29: Schematic of a domestic CHP system. In practice, heatand electricity from CHP systems are supplemented by top-upboilers and by connection to the electricity distribution network.This ensures reliability of supply and provides opportunities forelectricity sales to other customers.

10.4 A typical community CHP systemA typical system installed at a community level serving around500 flats in high-rise apartment blocks might have the followingcharacteristics:

Capacity 250kWe

Heat to power 1.5:1ratio

Overall efficiency 80%

Capital cost 2,500 3,500per dwelling*

Output 1,600MWh/yrthermal**1,100MWhe/yr

CO2 saving 300 tonnes CO2 /yr600kg CO

2 /yr

per dwelling

*Including he at network. The district heatnetwork t ypically accounts for the majority ofthe ca pita l cost associated with the project.Costs a re highly site specific.**Based on 4,500 hours operation per an num.

Boiler house

CHPTop-up

stand-byboilers

Heatand hotwater

Electricityfrom CHP

Electricity sales to customersImport/export

metering and control

Heatsupply

Electricity

Housing orother buildings

Fuel

Top-up andstand-byelectricity

Externalelectricity sales

27

10.3.1 Energy services companiesAn energy services company (ESCO) is an organisation or

contractual body set up to deliver a long-term energy service tohousing. An ESCO is able to design, finance, build, own andoperate local decentralised energy systems; it is contracted toprovide community heating and/or power to connected customers.

An ESCO may be set up specifically as part of a newbuilddevelopment or significant refurbishment involving construction ofa community heating/CHP plant, solar panels or wind turbines. Insome cases, ESCOs are set up by energy supply companies aswholly-owned subsidiaries; in others they are set up by landlordsor by the residents themselves.

ESCOs are often not-for-profit organisations and any financialsurplus goes to improve the energy efficiency of the housing theyserve or to expand the customer base.


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Micro-CHP units generate heat and power on a scale to suit asingle dwelling. These units replace the existing domestic boiler,perform the same space and domestic hot water heating functionand also generate electricity as a byproduct. Their operation iscontrolled by heat demand so that no heat is ever wasted. Extraelectrical power will often be required from the grid, butelectricity may be exported when an excess is generated.

There are a number of micro-CHP technologies including Stirlingengine, internal combustion engine and fuel cell, each of whichhave different operating characteristics, notably in terms of heatto power ratio. The majority of domestic scale systems availablein the UK are based on the Stirling engine, which generatesbetween 1 and 3kW of electric power (kWe) and 6 to 12kW ofheat (kWth). Matching the plant to building needs is critical togood performance, even more so than with other technologies. Ifa unit is oversized relative to heat demand, then running hourswill be low, and insufficient electricity will be generated tocompensate for the relatively low heating efficiency. If its under-

sized, then substantial amounts of top-up space and waterheating will be required from other appliances, which may berelatively inefficient.

11. Micro-CHP

Figure 30: Micro-CHP schematic energy flows

Micro-CHP is an emerging technology, with a very limited range ofproducts available at present, although a number are underdevelopment. Reliability and performance of early systems hasbeen an issue 31, however this will no doubt improve as thetechnology matures.

Cost effectiveness is improved by running the CHP for as manyhours per day as possible. This makes currently available micro-CHP units less feasible in smaller dwellings, where heat demandis lowest. Hours in operation can be maximised by slightly under-sizing the plant and combining it with a thermal store.

11.1 A typical micro-CHP systemA typical micro-CHP system installed in a newbuild house mighthave the following characteristics:

31. Refer to the Carbon Trust Micro-CHP Accelerator - Interim Report www.carbontrust.co.uk/publications/publicationdetail.htm?productid=CTC726

Capacity 1.1kWe

Heat to p ower 6:1ratio

Capital cost 4,000 - 6,000

Thermal output 8,000kWhth/yr

Electr ical output 1,330kWhe/yrThermal efficiency 75%


*Avoided emissions due to electricalgeneration of 700kg CO2 /yr.Additional heating emissions due to lowerheating efficiency of 130kg CO2 /yr.

Exhaust 5%

Electricity15%

Heat80%

MicroCHP unitGas

100%

Electricityimport/export

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12. Standalone and grid-connected systems

It is unlikely that the amount of electricity generated by an LZCsystem exactly matches demand at any given time of the day, letalone for the duration of it. So a major consideration is how tostore that energy.

LZC technologies generating electricity can be either standalone orgrid-connected.

12.1 StandaloneStandalone systems operate completely independently of thenational grid. The LZC technologies are connected directly to anarray of batteries which stores the electricity generated and actsas the main power supply for the dwelling. The size of the batteryarray will determine the amount of energy that can be stored.

An inverter may or may not be used to convert the direct current(DC) electricity produced into alternating current (AC) for domesticuse. The size of the inverter will determine the number and power

of appliances that can run concurrently. Many systems currently inuse operate in conjunction with an auxiliary generator to providebackup during periods of low output.

Standalone systems are rare. The most common application foroff-grid systems is at the scale of individual dwellings in remotelocations, where the cost of grid connection is prohibitive. Furtherinformation is available at off-grid.net

32. All grid-connected generators must comply with connection agreements G83 (less than 3.7kW) or G59 (greater than 3.7kW).33. For the latest market price see e-roc.co.uk/trackrecord.htm - currently 51.81 per MWh (Jan 09)34. Further information on current tariffs for exporting electricity to the grid is available at energysavingtrust.org.uk/Generate-your-own-energy/Sell-your-own-energy

12.2 Grid-connectedGrid-connected systems do not need a battery array to storeenergy. Any unused or excess electricity can be sold to the localelectricity supply company and exported to the grid. Althoughenergy is not stored as such within the national grid, at anygiven time it can be used elsewhere. When local demand exceedsgeneration, electricity is imported from the grid.

The National Grid is controlled under very tight tolerances interms of voltage and frequency, so an inverter is needed toconvert DC power to AC at a quality and standard acceptable tothe grid 32.

The Distribution Network Operator (DNO) typically pays for eachunit exported at a lower rate than the cost of imported electricity.ROCs may be claimed for any electricity generated, which can besold to electricity supply companies at the market rate 33.

An alternative available from several electricity supply companiesis a fixed rate for every kWh of electricity generated regardless ofwhether it is exported or used on site. This simplifies meteringarrangements 34.

Net metering arrangements may be available in future, where thenet of the electricity generated and consumed is calculated over aspecified period of time, and the customer billed for the balance.Feed-in tariffs have the potential to increase the price paid byDNOs for exported electricity beyond the cost of that imported.

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13. Suitability: urban and rural environments

13.1 Solar thermal and PVSolar thermal hot water and PV systems are suitable in mostsituations. Key considerations are the orientation of available roofspace and any shading from trees, buildings or chimneys. Evenpartial shading of a PV array is to be avoided, since yield will fallby more than the area shaded due to the electrical arrangementof the modules.

The greatest amount of solar energy is captured if panels are asclose to perpendicular to the suns rays as possible. Ideally thepanels should be oriented as close to south as possible; outputfrom panels on east or west facing roofs is typically 10% lessthan for south-facing (at optimum tilt). The optimum tilt in the UKis around 33 to 35 from to the horizontal, although this is notcritically important and panels are often mounted in line with theroof pitch. A tilt of between 15 and 50 will provide over 95% ofthe maximum annual output.

A steeper tilt will make panels comparatively more effective inwinter when the sun is lower in the sky, although total output forthe year will be slightly reduced. This can be an effective strategyto increase output of solar hot water systems, since solar energycaptured (in summer) that exceeds 100% of domestic hot waterdemand can be less easily stored or exported compared toelectricity from PV.

Installation is more straightforward for newbuild developments, orrefurbishments where roofing work must be carried out, since roofaccess is required. Newbuild also represents an opportunity tointegrate solar panels into the building fabric, reducing materialcosts. The structural strength of the roof should be confirmed forolder buildings to ensure that they can withstand the weight andpotential wind loading. Evacuated tube collectors of a similar sizetend to be heavier than flat plate.

Following recent changes to legislation, installation of building-mounted microgeneration technologies is typically covered underpermitted development rights and planning permission is notgenerally required. However advice should be sought for listedbuildings or those in conservation areas.

Blocks of flats have a relatively small roof area availablecompared to houses and will be correspondingly less effective per

dwelling. PV does, however, complement CHP in that it generatesmost electricity during the summer when heat demand is low andCHP systems may be generating less electricity or not in operation.

Where possible, panels should be installed at a pitch of greaterthan 15 to enable self-cleaning of dust and debris by the rain.In urban areas, dust accumulation can cause a power reductionup to 10%.

13.2 WindWind turbines are generally much better suited to rural locations,since wind speeds are significantly higher and less turbulent.There is also a greater opportunity for pole-mounting turbines,further increasing the available resource with height. Building-mounted turbines may be an option in suburban areas, althoughcare should be taken with older buildings, as vibration of theturbine in operation can lead to structural issues. Community-scale turbines are often the best and most cost effective option.

Urban locations are not well suited, with the possible exception oftower blocks.

13.3 Micro-hydroHydro power is almost always a competitive option if the resourceis available. However this is the case in a minority of rural areasonly. An environmental impact assessment should be carriedout and planning authorities consulted to ensure the relevantregulations and appropriate permissions are obtained. TheEnvironment Agency normally requires an abstraction licence.

13.4 BiomassBiomass boilers on the individual dwelling level are best suited to

refurbishments in rural areas. A large volume of fuel is requiredand availability of storage space, ease of delivery and potentialfor local supply are all factors to consider. Additionally, theequipment itself requires a relatively large footprint withinthe dwelling.

Biomass appliances are not widely available in very lowcapacities, so tend to be better suited to larger rural properties orrefurbished houses, which are more likely to have greater thermallosses. A biomass system can be easily incorporated within anexisting wet heating system. It is an excellent option for propertiesthat are not on the gas grid.

Smoke Control Areas in many urban locations require theinstallation of appliances from an approved list. There is currently

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14. Capital and maintenance costs

Where applicable, installed costs are calculated by addingtogether costs per installation and per kW. For example, an 8kWground source heat pump (bore holes) for a newbuild dwellingwould be expected to cost approximately:

3,170 + (790 x 8kW) = 9,490

14.1 Individual dwellings

Table 2: Capital and maintenance costs for individual dwellings

Technology Fixed cost Marginal cost Annual(/installation) (/kW) maintenance cost

Newbuild Retrofit Newbuild Retrofit (% of capex)

Solar thermal* 1,420 2,000 820 950 1.5%Solar PV 4,000 5,250 1.0%Wind (


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Domestic Low and Zero Carbon Technologies 2010 editionCapital and maintenance costs

14.2 Community scale

Table 3: Capital and maintenance costs for community-scale dwellings

Technology Fixed cost Marginal cost Annual(/installation) (/kW) maintenance

cost

Newbuild Newbuild (% of capex)

Wind (50-1000kWe, community scale) 2,475 1.0%Biomass boiler (


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15. Sources of grant funding

15.1 Feed-in tariff and renewable heat incentiveThe Government introduced a feed in tariff in April 2010 so thatpeople and businesses who generate their own electricity fromlow carbon sources will be paid for doing so. A similar schemefor renewable heat is planned to follow in April 2011.energysavingtrust.org.uk/generate-your-own-energy/sell-your-own-energy/Feed-in-tariff-scheme

15.2 Bio-energy Capital Grants schemeThe Bio-energy Capital Grants scheme is aimed at businesses,organisations and charities in the commercial, industrial andcommunity sectors that are considering investing in biomass-fuelled heat and/or CHP projects. Grants of up to 500,000 wereavailable, although restricted to England only. 38

15.3 Scottish Biomass Heat SchemeThe Scottish Biomass Heat Scheme provides grants for installationof biomass heating systems in small-medium scale enterprises(SMEs) across Scotland. The Scottish Government is keen toencourage the development of district heating, and wouldparticularly welcome applications for district heatingdemonstrators from private developers.

Up to 50% funding of the additional capital costs of a biomassheating system compared to an equivalent fossil-fuel system canbe supported under the scheme, subject to a maximum of 100,000.

15.4 Community Sustainable Energy ProgrammeThe Community Sustainable Energy Programme (CSEP) providesgrants to community-based organisations for the installation ofmicrogeneration technologies and energy efficiency measures. Grantsof up to 50,000 or 50% of the project value (whichever is lower)are available. Products and installers must be certified under MCS.

It also provides project development grants that help communityorganisations assess the feasibility of installing any combination ofthe microgeneration technologies supported. Grants of up to5,000 or 75% of the feasibility study cost (whichever is lower)are available. Consultants registered with CSEP must be used 39.

15.5 Scottish Community and Householder RenewablesInitiativeThe Scottish Community and Householder Renewables Initiative(SCHRI) provides funding to householders and advice and fundingto communities.

Grants of up to 30% of installation costs to a maximum of 4,000are available to householders. Grants of up to 100,000 areavailable for community projects in addition to free support andadvice throughout the installation process. The amount of fundingawarded is determined on a case-by-case basis, with the averagegrant being in the region of 50%. 40

15.6 Renewable Obligation CertificatesThe Renewables Obligation is described in section 1.3.2.Microgeneration installations generating 50KWe or less may beentitled to claim support under the Renewables Obligation (RO).

Eligible renewable sources are: landfill gas, sewage gas, hydroover 20MW (commissioned after 1st April 2002), hydro under 20MW, onshore and offshore wind, co-firing of biomass, otherbiomass, geothermal power, tidal and tidal stream power. Someenergy from waste is also eligible and specific details can befound on the BERR website41 or from Ofgem who administerthe Obligation42.

15.7 Reduced rate of VATVAT on many energy saving materials and heating systems ispayable at the reduced rate of 5%. This is most applicable torefurbishments, since all VAT can be recovered on newbuilddevelopments. Further information is available at:hmrc.gov.uk/VAT/sectors/consumers/energy-saving.htm

15.8 Energy Saving Trust databaseHouseholders can search the Energy Saving Trust database forgrants available for LZC installations and energy efficiencymeasures in their area. Seeenergysavingtrust.org.uk/gid

38. See bioenergycapitalgrants.org.uk/39. See communitysustainable.org.uk/40. See energysavingtrust.org.uk/scotland/Scotland/Scottish-Community-and-Householder-Renewables-Initiative-SCHRI41. See berr.gov.uk/energy/sources/renewables/policy/renewables-obligation/microgeneration/page39851.html42. See ofgem.gov.uk/Sustainability/Environment/RenewablObl/Pages/RenewablObl.aspx

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16. Developing a renewable energy strategy

The approach is broadly similar regardless of the type of systeminvolved. The use of renewable energy technology should alwaysbe combined with energy efficiency. The carbon reductionsachieved from energy efficiency measures will generally begreater than those from LZCs. The following step-by-step approachshould follow a consideration of energy efficiency measures.

Step 1: Assess the potential Consider LZC technologies during feasibility stages (RIBA stage

B). If refurbishing, consider whether additional works(e.g. re-roofing) will facilitate the installation of LZC systems.

Identify the current and/or projected energy demand. Evaluatespace heating, water heating and electric power requirements(for appliances and lighting). Estimate the contributions thatmay be made by renewable energy.

Identify the appropriate options different technologies lendthemselves to different types of buildings and sites.

Compare the carbon emissions reductions achievable from thevarious renewable energy technology options, taking intoaccount the type of fuel or energy to be offset.

Step 2: Assess the required investment

Consider what can be afforded, evaluate residents benefits andidentify the available grant funding, loans and other supportmechanisms.

Complete a feasibility study including whole-life costing, toestablish the net present value (NPV) of the proposal.

Step 3: Consult with tenants, local residents and contractors

Some renewable energy measures may have an impact onlocal residents.

If refurbishing, tenant approval for the scheme is important asit is required by most grant funding schemes.

Tenants and prospective residents will need to be informed andeducated about the benefits of renewable energy technology.

Step 4: Develop a specification

Specify the technology and the required performance of thedwellings, in terms of fuel use, fuel costs and carbonemissions.

Step 5: Apply for funding

Review and select from the many grant funding schemesdesigned to promote the development of LZC energy.

Consider seeking non-government grant finance. Examine the potential for Renewable Obligation Certificates

(ROCs).

Step 6: Implementation

Apply for planning permission, obtain competitive tenders andimplement the scheme.


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Appendix A

CE317_Domestic Low & Zero Carbon Technologies 2010

Documents

Transcript of CE317_Domestic Low & Zero Carbon Technologies 2010