Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar...

Principles of Solar Engineering

D. Y. Goswami, F. Kreith, J. F. Kreider Principles of Solar Engineering

Principles of Solar EngineeringChapter 5: Solar Heating Systems

Part I

D. Yogi Goswami, Frank Kreith, Jan F. Kreider







Outline

Introduction

Calculations of Heating and Hot Water Load in Buildings. Calculation of Heat Loss Internal Heat Sources in Buildings The Degree-day Method Service Hot-Water Load Calculation

Solar Water Heating Systems Natural Circulation Systems Forced-Circulation Systems Industrial Process Heat Systems

Liquid-Based Solar-Heating Systems for Buildings Physical Configurations of Active Solar Heating Systems3 Solar Collector Orientation Fluid Flow Rates Thermal Storage Other Mechanical Components Controls in Liquid Systems Load Devices in Liquid Solar-Heating Systems

Chapter 5: Solar Heating Systems





Introduction

Historically, methods used for collecting and transferring solar heat were passive methods, that is, without active means such as pumps, fans and heat exchangers.

Passive solar heating methods utilize natural means such as radiation, natural convection, thermosyphon flow and thermal properties of materials for collection and transfer of heat.

Active solar heating methods, on the other hand, use pumps and fans to enhance the rate of fluid flow and heat transfer.

This chapter describes in detail the function and design of active systems for heating buildings and service water.

Energy for heating buildings and hot water consumes about one-fourth of the annual energy production in the United States.

In many areas of the United States and the world, solar heating can compete economically with other types of fuel for heating, without even considering the environmental benefits.






Energy requirements for space heating or service water heating can be calculated from basic conservation of energy principles.

For example, the heat required to maintain the interior of a building at a specific temperature is the total of all heat transmission losses from the structure and heat required to warm and humidify the air exchange with the environment by infiltration and ventilation.

ASHRAE has developed extensive heat load calculation procedures embodied in the ASHRAE Handbook of Fundamentals.

Chapter 5: Solar Heating SystemsCalculations of Heating and Hot Water Load in Buildings

5.1 Calculations of Heating and Hot Water Load in Buildings





Figure on the right shows the combinations of temperature and humidity that are required for human comfort. The shaded area is the standard U.S. comfort level for sedentary persons.

Heat loss calculations for buildings

Chapter 5: Solar Heating SystemsCalculations of Heating and Hot Water Load in Buildings





The components of heat loss calculations of a building are given below.

Heating load calculations for buildings

Heating load component Equations Descriptions/References

Walls, Roof, Ceilings, Glass

are inside and outside air temperature, respectively. values of composite section are calculated from the thermal properties of components given in Appendix 5.

Basement floors and walls below ground levelhas special units of . Values of for various ground water temperatures are given in Appendix 5.

Concrete floors on ground is the perimeter of the slab. values are given in Appendix 5.

Infiltration and ventilation air

or Watts

or Watts

is volume of air flow in and are density and specific heat of air. is the latent heat of water at room temperature. is humidity ratio difference between inside and outside air.

Assuming ,

Chapter 5: Solar Heating SystemsCalculations of Heat Loss





Transmission heat losses through attics, unheated basements, and the like are buffered by the thermal resistance of the unheated space. For example, the temperature of an unheated attic lies between that of the heated space and that of the environment.

As a result, the ceiling of a room below an attic is exposed to a smaller temperature difference and consequent lower heat loss than the same ceiling without the attic would be.

The effective conductance of thermal buffer spaces can easily be calculated by forming an energy balance on such spaces.






Example. Calculate the heat load on a house for which the wall area is 200 m2, the floor area is 600 m2, the roof area is 690 m2, and the window area totals 100 m2. Inside wall height is 3 m. The construction of the wall and the roof is shown below.

Cross-sections of the wall and the roof for the Example






Selective SurfacesExample. The thermal resistance of the wall can be found by the electrical resistance analogy as:

Rwa = Routside air + Rwood siding + Rsheathing + Rcomb + RWall board + Rinside air

Combined thermal resistance for the studs and insulation (Rcomb) is found as:

Assuming that the studs occupy 15% of the wall area,

or






Reflecting SurfacesSpecular reflectance values

Therefore the wall resistance, Rwa, can be found as:

The heat loss through the windows depends on whether they are single-glazed or double-glazed. In this example, single-glazed windows are installed, and a U factor equal to 4.7 W/m2 oC is used.

𝑈𝑤𝑎=1𝑅𝑤𝑎

=1

2.179=0.46𝑊 /𝑚2℃






Transparent Materials

If the respective areas and U factors are known, the rate of heat loss per hour forthe walls, windows, and roof can be calculated. Walls: qwa=(200 m2) x 0.46 = 92 Windows: qwi=(100 m2) x 4.7 = 470 Walls: qrf=(690 m2) x 0.32 = 220 +____________ Total qtr =782

The roof is constructed of 12.7 mm gypsum wall board, 51 mm foam insulation board, 38 mm still air, 12.7 mm plywood, and asphalt shingles (wooden beams and roofing paper are neglected for the simplified calculations here). Therefore,

𝑈 𝑟𝑓=1

0.030+0.077+0.11+0.17+2.53+0.079+0.1=0.32𝑊 /𝑚2℃

Outside air Shingles Plywood Air Gap Foam Wallboard Inside air


If double-glazed windows were used, the heat loss would be reduced to 552 .






The infiltration and ventilation rate Q for this building is assumed to be 0.5 ACH(Air Changes per hour).

𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒=𝑄𝜌𝑎𝐶𝑝𝑎(𝑇 𝑖−𝑇𝑜) 𝑞𝑙𝑎𝑡𝑒𝑛𝑡=𝑄𝜌𝑎h 𝑓𝑔∆𝑊

volume

Sensible heat load Latent heat load

In residential buildings, humidification of the infiltration air is rarely done, so latent heat load is neglected.






𝑞𝑡𝑜𝑡=𝑞𝑠𝑒𝑛𝑠𝑖𝑏𝑙𝑒+𝑞𝑡𝑟=(782+300 )=1082𝑊 /℃

Therefore,

This calculation is simplified for purposes of illustration. Heat losses through the slab surface and edges have been neglected, for example.

More refined methods of calculating energy requirements on buildings do not use the steady-state assumption used above [23].

The thermal inertia of buildings may be expressly used as a load-leveling device. If so, the steady-state assumption is not met and the energy capacitance of the structure must be considered for accurate results.

Many adobe structures in the U.S. Southwest are built intentionally to use daytime sun absorbed by 1-ft-thick walls for nighttime heating for example.





Internal Heat Sources in BuildingsChapter 5: Solar Heating Systems

Heat supplied to a building to offset energy losses is derived from both the heating system and internal heat sources.

Some common internal sensible heat gains that tend to offset the heating requirements of buildingsa

aFor more data see [2].bShading factor is the amount of a window not in a shadow expressed as a decimal between 1.0 and 0.0.





Internal Heat Sources in BuildingsChapter 5: Solar Heating Systems

Commercial buildings such as hospitals, computer facilities, or supermarkets will have large internal gains specific to their function.

Internal heat gains tend to offset heat losses from a building but will add to the cooling load of an air-conditioning system.

The magnitude of the reduction in heating system operation will be described in the next section.





The Degree-day MethodChapter 5: Solar Heating Systems

The preceding analysis of heat loss from buildings expresses the loss on a per unit temperature difference basis (except for unexposed floor slabs).

In order to calculate the peak load and total annual load for a building, appropriate design temperatures must be defined for each.

The outdoor design temperature is usually defined statistically, such that the actual outdoor temperature will exceed the design temperature 97.5 % or 99 % of the time

over a long period.

The design temperature difference () is then

The design is used for rating non-solar heating systems, but is not useful for selection of solar systems, since solar systems rarely provide 100 % of the energy demand of a building at peak conditions.






A more useful index of heating energy demand is the total annual energy requirement for a building. This quantity is somewhat more difficult to calculate than the peak load.

It requires a knowledge of day-to-day variations in ambient temperature during the heating season and the corresponding building heat load for each day.

Building heat loads vary with ambient temperatures as shown at the figure.

Building load profile versus ambient temperature showing no-load temperature Tnl and desired interior temperature Ti.






The environmental temperature Tnl, above which no heat need be supplied to the building, is a few degrees below the required interior temperature Ti because of internal heat-generation effects.

The no-load temperature at which internal source generation qi just balances transmission and infiltration losses can be determined from the energy balance

where is the overall loss coefficient for the building (W/oC).

Then,

The total annual heat load on the building, QT, can be expressed as

+ indicates that only positive values are considered.






In practice, it is difficult to evaluate this integral, therefore, three simplifying assumptions are made:

1. is independent of time. 2. is independent of time. 3. The integral can be expressed by the sum.

Thus,

Where n is the day number, and the daily average temperature can be approximated by

in which Ta,max and Ta,min are the daily maximum and minimum temperatures, respectively.






The quantity is called the degree-day unit.

For example, if the average ambient temperature for a days is 5oC and the no-load temperature is 20oC, 15 degree C-days are said to exist for that day.

However, if the ambient temperature is 20oC or higher, 0 degree-days exist, indicating 0 demand for heating that day.

Degree-day totals for monthly and annual periods can be used in to calculate the monthly and annual heating energy requirements.

In the past, a single value of temperature has been used throughout the United States as a universal degree-day base, 65.0°F or 19°C, however, since many homeowners and commercial building operators have lowered their thermostat settings in response to increased heating fuel costs, thereby lowering Tnl, this practice is now outdated.

Therefore, a more generalized database of degree-days to several bases (values of Tnl) has been created by the U.S. National Weather Service (NWS).






Example. A building located in Denver, CO, has a heat-loss coefficient of 1000 kJ/hr°C and internal heat sources of 4440 kJ/hr. If the interior temperature is 20°C (68°F), what are the monthly and annual heating energy requirements?

A gas furnace with 65 % efficiency is used to heat the building.

Solution. In order to determine the monthly degree-day totals, the no-load temperature (degree-day basis) must be evaluated,

The monthly degree C-days for Denver are taken from the U.S. National Weather Service and given in the table.

Monthly and annual energy demands






The energy demand is calculated as,

The annual energy demand of 62.9 GJ is delivered by a 65% efficient device.

Therefore,

Monthly and annual energy demands





Service Hot-Water Load CalculationChapter 5: Solar Heating Systems

Service hot-water loads can be calculated precisely with the knowledge of only a few variables.

The data required for calculation of hot-water demand are, Water source temperature (Ts) Water delivery temperature (Td) Volumetric demand rate (Q)

The energy requirement for service water heating qhw is given by,

Where is the water density and is its specific heat.

The demand rate, Q(t), varies in general with time of day and time of year; likewise, the source temperature varies seasonally.

Source temperature data are not compiled in a single reference; local water authorities are the source of such temperature data.






Example. Calculate the monthly energy required to heat water for a family of four in Nashville, TN.

Monthly source temperatures for Nashville are shown in Table 5.5, and the water delivery temperature is 60°C (140 °F).

Solution. For a family of four, the demand rate Q may be found using a demand recommended from the table.

Approximate service hot-water demand rates






Approximate service hot-water demand rates

The density of water can be taken as 1000 kg/m3 and the specific heat as 4.18 kJ/kg °C.

Monthly demands are given by,

The monthly energy demands calculated from the equation above with these data are tabulated in the next slide.






Water heating energy demands for the example





Solar Water Heating SystemsChapter 5: Solar Heating Systems

Solar Water Heating Systems

Solar water-heating systems represent the most common application of solar energy at the present time.

There are basically two types of water-heating systems: natural circulation or passive solar system (thermosyphon) Forced circulation or active solar system

Natural circulation solar water heaters are simple in design and low cost.

Their application is usually limited to nonfreezing climates, although they may also be designed with heat exchangers for mild freezing climates.

Forced circulation water heaters are used in freezing climates and for commercial and industrial process heat.





Natural Circulation SystemsChapter 5: Solar Heating Systems

The natural tendency of a less dense fluid to rise above a denser fluid can be used in a simple solar water heater to cause fluid motion through a collector.

The density difference is created within the solar collector where heat is added to the liquid.

Schematic diagram of thermosyphon loop used in a natural circulation






As water gets heated in the collector, it rises to the tank, and the cooler water from the tank moves to the bottom of the collector, setting up a natural circulation loop. It is also called a thermosyphon loop.

For the thermo syphon to work, the storage tank must be located higher than the collector.

The flow pressure drop in the fluid loop () must equal the bouyant force "pressure difference" () caused by the differing densities in the hot and cold legs of the fluid loop.

where H is the height of the legs and L is the height of the collector, is the local collector fluid density, is the tank fluid density, and is the collector outlet fluid density, the latter two densities assumed uniform.






The flow pressure term, , is related to the flow loop system head loss, which is in turn directly connected to friction and fitting losses and the loop flow rate:

where , with K being the sum of the component loss velocity factors and V the flow velocity.

Since the driving force in a thermosyphon system is only a small density difference and not a pump, larger-than-normal plumbing fixtures must be used to reduce pipe friction losses.

Since the hot-water system loads vary little during a year, the angle of tilt is that equal to the latitude, that is, .

The temperature difference between the collector inlet water and the collector outlet water is usually 8-11°C during the middle of a sunny day.






Passive solar water heaters

compact model using combined collector and storage section view of the compact model

tank and collector assembly






After sunset, a thermosyphon system can reverse its flow direction and lose heat to the environment during the night.

To avoid reverse flow, the top header of the absorber should be at least 30 cm below the cold leg fitting on the storage tank, as shown, otherwise a check valve would be needed.

To provide heat during long cloudy periods, an electrical immersion heater can be used as a backup for the solar system.

The immersion heater is located near the top of the tank to enhance stratification and so that the heated fluid is at the required delivery temperature.






Several features inherent in the thermo syphon design limit its utility.

If it is to be operated in a freezing climate, a nonfreezing fluid must be used, which in tum requires a heat exchanger between collector and potable water storage.

Heat exchangers of either the shell-and-tube type or the immersion-coil type require higher flow rates for efficient operation than a thermo syphon can provide.

Therefore, the thermo syphon is usually limited to nonfreezing climates.

For mild freeze climates, a heat exchanger coil welded to the outer surface of the tank and filled with an antifreeze may work well.






Example 5.4. Determine the "pressure difference" available for a thermosyphon system with 1 meter high collector and 2 meter high legs. The water temperature input to the collector is 25°C and the collector output temperature is 35°C. If the overall system loss velocity factor (K) is 15.6, estimate the system flow velocity.

Solution. is solved with the water densities being found from the steam tables (see Appendix 3 and Tables 3.8 and 3.9).






The system flow velocity is estimated from the system K given, the pressure difference calculated above, taking the average density of the water around the loop (at 30°C), and substituting into,





Forced-Circulation Systems


Open loop system

In an open loop system the solar loop is at atmospheric pressure, therefore, the collectors are empty when they are not providing useful heat.

A disadvantage of this system is the high pumping power required to pump the water to the collectors every time the collectors become hot.





Closed loop system

Forced-Circulation SystemsChapter 5: Solar Heating Systems






Closed loop drain-back systemThis disadvantage can be overcome in a closed loop drain-back system which is not pressurized.

In this system, when the pump shuts off, the water in the collectors drains back into a small holding tank while the air in the holding tank goes up to fill the collectors.

The holding tank can be located where freezing does not occur, but still at a high level to reduce pumping power.






In all three configurations, a differential controller measures the temperature differential between the solar collector and the storage, and turns the circulation pump on when the differential is more than a set limit (usually 5°C) and turns it off when the differential goes below a set limit (usually 2°C).

Alternatively, a photovoltaic (PV) panel and a DC pump may be used.

The PV panel will turn on the pump only when solar radiation is above a minimum level.

Therefore, the differential controller and the temperature sensors may be eliminated.





Industrial Process Heat SystemsChapter 5: Solar Heating Systems

For temperatures of up to about 100°C, required for many industrial process heat applications, forced circulation water-heating systems described above can be used.

The systems, however, will require a large collector area, storage and pumps, etc.

For higher temperatures, evacuated tube collectors or concentrating collectors must be used.

Industrial process heat systems are described in more detail in Chapter 8.





Solar Water Heating SystemsChapter 5: Solar Heating Systems

Liquid-Based Solar Heating Systems for Buildings

The earliest active solar space-heating systems were constructed from enlarged water-heating components.

Solar space-heating systems can be classified as active or passive depending on the method utilized for heat transfer

A system that uses pumps and/or blowers for fluid flow in order to transfer heat is called an active system

On the other hand, a system that utilizes natural phenomena for heat transfer is called a passive system

Passive solar heating systems are described in Chapter 7.

In this section, configurations, design methods, and control strategies for active solar-heating systems are described.





Solar Collector OrientationChapter 5: Solar Heating Systems

Typical solar-thermal system for space heating and hot-water heating showing fluid transport loops and pumps.






The collector fluid loop contains fluid manifolds, the collectors, the collector pump and heat exchanger, an expansion tank, and other subsidiary components.

The storage loop contains the storage tank and pump as well as the tube side of the collector heat exchanger.

To capitalize on whatever stratification may exist in the storage tank, fluid entering the collector heat exchanger is generally removed from the bottom of storage.







The best solar collector orientation is such that the average solar incidence angle is smallest during the heating season.

For tracking collectors this objective is automatically realized.

For fixed collectors in the northern hemisphere the best orientation is due south (due north in the southern hemisphere), tilted up from the horizon at an angle of about 15° greater than the local latitude.






Fluid Flow RatesChapter 5: Solar Heating Systems

Although high flows maximize energy collection, practical and economic constraints put an upper limit on useful flow rates.

Very high flows require large pumps and excessive power consumption and lead to fluid conduit erosion.

In practice, liquid flows in the range of 50-75 kg/hr mc

2 (10-15 Ib/hr ftc2) D

of water equivalent are the best compromise among collector heat-transfer coefficient, fluid pressure drop and energy delivery.

Effect of fluid flow rate on collector performance as measured by the heat-removal factor FR; F‘ is the plate efficiency factor.





Thermal StorageChapter 5: Solar Heating Systems

Experience has shown that liquid storage amounts of 50-75 kg H2O/mc2 (10-15 lb/ft2)

are the best compromise between storage tank cost and useful energy delivery. Effect of liquid storage capacity on liquid-based solar-heating system energy delivery





Controls in Liquid SystemsChapter 5: Solar Heating Systems

Control strategies and hardware used in current solar system designs are quite simple and are similar in several respects to those used in conventional systems.

The single fundamental difference lies in the requirement for differential temperature measurement instead of simple temperature sensing.






The collector-storage difference is sensed by two thermistors or thermocouples, the difference being determined by a solid-state comparator, which is a part of the control device.

Room temperature is sensed by a conventional dual contact thermostat.

The control system operates as follows;

If the first room thermostat contact closes, the mode selector valve and distribution pump are activated in an attempt to deliver the thermal demand from solar-thermal storage.






If room temperature continues to drop, indicating inadequate solar availability, the mode selector diverts flow through the backup system instead of the solar system, and the backup is activated until the load is satisfied.

The collector-storage control subsystem operates independently of the heating subsystem described above.

If collector temperature exceeds the temperature in the bottom of the storage tank by 5-10oC, the collector pump and heat-exchanger pump are activated and continue to run until the collector and storage temperature are within about 1-2oC.





Load Devices in Liquid Solar-Heating SystemsChapter 5: Solar Heating Systems

A heating load device transfers heat from the solar storage to the air in the space.

Therefore, a liquid-to-air heat exchanger is sized based on the energy demand of abuilding.

Several generic types of load devices are in common use;

1. Forced-air systems-tube-and-fin coil located in the main distribution duct of abuilding or zone of a building.

2. Baseboard convection systems-tube-and-fin coils located near the floor on external walls. These operate by natural convection from the convectors to the room air.

3. Heated floors or ceilings-water coils. These transfer heat to large thermal masses that in tum radiate or convect into the space. This heating method is usually called radiant heating.






Forced-air heating system load device location upstream of non-solar heat exchanger or furnace.

Load Devices in Liquid Solar-Heating Systems

Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar...

Documents

Transcript of Principles of Solar Engineering D. Y. Goswami, F. Kreith, J. F. KreiderPrinciples of Solar...