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Technology Focus

Tri-generation plant considerations By Pieter de Bod ( Pr.Eng. Director RPP Consulting Engineers) and Hans Auret (Founding member of Claasen Auret Electrical Consulting Engineers)

In the event that insufficient utility power is available for a proposed project, one could consider the

following as far as distributed power generation by means of high-methane gas is concerned: Distributed power generation is increasingly being considered by government and industry as one of the best medium term mitigation measures to combat the utility power capacity shortages by means of distributed generation. This approach is, however, highly dependent on the availability of sufficient piped gas capacity at realistic tariffs and escalations.

Apart from generating power, the generation efficiency can be greatly improved using tri-generation plants or CCHP (combined cooling, heating and power) methodology to produce electricity, cooling and heating from the same energy source, high-methane gas, at the same time reducing carbon emissions. Tri-generation systems are found in commercial applications typically where there is a need for air conditioning or chilled water by

the customer for a significant hourly usage per day. Owners of commercial buildings and commercial businesses are increasingly seeking ways to use energy more efficiently. This is a direct result of dramatically increasing electric rates, decreased power reliability (blackouts, brownouts, rolling blackouts, and other power interruptions), as well as competitive and economic pressures to cut expenses, increase air quality, and reduce emissions of air pollutants and greenhouse gases.

Co-generation, also known as combined heat and power (CHP), is the simultaneous production of electricity and useful heat, usually in the form of either hot water or steam, from one primary fuel, such as natural gas. While not necessarily always termed correctly, co-generation has also been referred to as district energy, total energy, combined cycle, and simply co-gen. Co-generation has been mostly a technology used in the utilities and industrial marketplace. While cooling can be provided by electric-driven compression chillers, low quality heat

MTN absorbtion chiller plant room

(i.e., low temperature, low pressure) that is not used by the co-generation power plant can be used to drive the absorption or adsorption chillers so that the overall primary energy consumption is reduced.

Tri-generation power plants with absorption chillers have gained acceptance due to their capability of integrating with co-generation systems but also because they can operate with industrial waste heat streams that can be fairly substantial. The benefit of power generation with absorption cooling can be realised through the following example that compares

it with a power generation system with conventional electric-driven compression systems.

Tri-generation, when compared to combined-cycle co-generation, can be up to 50% more efficient, further reducing operating costs, fuel expenses, and environmental pollutants. Tri-generation systems benefit the building owner as well as society in a number of ways, including: • Increasedpowerreliability;• Reducedpowerrequirementsonthe

electricgrid;and• Reduceddependenceoncoalorfuel

oil.

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Technology Focus

The on-site tri-generation system can be economically attractive for many types of buildings, including, but not limited to, the following: • Hospitals• Universities• Manufacturingplants• Datacentres• Hotels• Airports

Except for the tri-generation plant at MTN and Standard Bank, there are no comparative plants in this country, although a number of others are under development. MTN South Africa hosted the inaugural ceremony held on 2 September 2010. The Deputy Minister of Communications, Dina Pule, also attended this festive occasion. The event captured the dream of MTN and (its mechanical and electrical consultants) to promote its mission to use alternative energies as part of the MTN Group (as a GSM member) initiatives.

In principle, absorption chillers operate on ‘free’ thermal energy from high-methane gas powered generators, thermal energy that would otherwise be wasted to the atmosphere. As long as the power generator is operational and there is a cooling demand, chilled water will be produced by the absorption chiller. Absorption chillers use lithium bromide (similar to sea salt) and distilled water as refrigerants, both with an ozone depletion potential (ODP) of zero and a global warming potential (GWP) of zero.

By controlling the temperatures and concentrations of lithium bromide, it is possible to control the leaving chilled water temperature. Hot water from the generator will be pumped to the absorption chillers by means of a pump, situated in the generator plant room.

The tri-generation and absorption chiller technology is well-proven in many American, European, Middle and Far Eastern countries. The advantages of using high-methane gas as an energy source and as a substitute for coal lie in the cleaner burning process of the fuel

and heat energy recovery providing an overall higher efficiency. The currently available gas is natural fossil gas (high methane content +/-91%) from the Mozambique gas fields, piped to South Africa. This gas is currently distributed by Sasol, but in terms of the latest determination by NERSA the gas will be controlled by them from 2014.We are also partly involved with the carbon management registration process, working together with the carbon management firm Promethium. In July 2009 a new CDM (clean development mechanism) registration methodology was written by Promethium and submitted to UNFCCC (United Nations framework convention for Climate change). The methodology was approved in July 2010, and the MTN project is the first of a UNFCCC program of activities (POA) registered by them. The audit and final registration will follow in due course.

Interesting statistics:• Coalemissionfactor(Eskom)-

approximately 96 kg CO2e/GJ (or approx. 1 kg CO2/kWh, and Eskom overall efficiency approximately 34%

• Naturalgasemissionfactor–approximately 56 kg CO2e/GJ (or 0.55 kg CO2/kWh, and estimated tri-generation plant efficiency of approximately 80%

That means at least a 40% reduction in emissions on per GJ basis and efficiency improvement in the order of 40% or more.

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We believe that tri-generation technology is here to stay and we will see more and more similar plants erected in near future. Since the inauguration event we have presented many tri-generation proposals to clients. The only challenge is the availability of high-methane gas

The integration of the tri-generation plant with the site infra-structure to has to be carefully considered when sizing the system to ensure that the most efficient solution is achieved.

Technology Focus

By correctly sizing the plant, it will not only increase the efficiency of the building but it will also be the most cost effective to maintain and provide years of reliable service. System utilisation isofutmostimportance;thereforeanyexcess hot water (water not used by the absorption chillers) will be used for space heating (or other heating needs). Hot water will be supplied to fan coil units by means of piping and plate heat exchangers. A+

Why we should build wooden skyscrapers Building a skyscraper? Forget about steel and concrete, says architect Michael Green, and build it out of … wood. As he details in this intriguing talk, it’s not only possible to build safe wooden structures up to 30 sto-ries tall (and, he hopes, higher), it’s necessary.

Michael Green wants to solve architecture’s biggest challenge - meet-ing worldwide housing demand without increasing carbon emissions - by building with carbon-sequestering wood instead of concrete and steel.

Courtesy of TED Talk

Time: 12:22

Technology Focus