A Defra funded project at Brunel University could see the use of trigeneration become more widespread in retail applications
Investment in tri-generation plant to provide onsite power heating and cooling is not a decision to take lightly, but supermarkets who take the plunge could benefit from significant gains in emissions and energy reduction, according to research from Brunel University and collaborators. A 15-strong consortium has been undertaking a five year research project, funded by a significant Defra grant, into the economic, engineering and environmental factors involved in integrating a trigeneration system as part of a supermarket.
Whilst the environmental benefits of re-using waste heat from power generation for heating and/or cooling via a sorption system are clear, the project’s conclusion that a payback for an optimised systems can be achieved at only around 3.2 years may be more surprising for those who baulk at the high capital outlay required.
The energy consumption of supermarkets makes such a system pretty compelling - the ten largest retail food chains consume energy that is equivalent to 5 million tonnes of carbon dioxide a year. Brunel has found that around half of the carbon footprint of some chilled products comes from the electricity consumption of the fridge that it is in. The alternative of buying from green electrical sources is a more expensive option and in short supply the consortium contends.
Trigeneration, integrated with CO2, offers a triple benefit of reducing indirect and direct emissions, while improving the efficiency of the refrigeration system.
The configuration that the partnership has come up with claims energy savings of 30 per cent compared to a conventional R404A system in a 5000 m2 sales area supermarket, together with greenhouse gas emissions savings of 43 per cent. This optimised system has a good degree of flexibility, the team says, because it can be tailored to a range of fuel sources or sorption options and can be applied to either transcritical or MT pumped systems (see box).
In this optimised system (see pic), the energy released from a combined heat and power system drives an adsorption or absorption unit, which in turn is used in a cascade arrangement to condense the CO2 refrigerant of a subcritical CO2 refrigeration system. This ensures operation of the CO2 refrigeration system in the subcritical region all the time and at a constant condensing temperature which ensures high energy efficiency throughout the year.
Tests in the Brunel laboratories have shown the LT system to deliver a steady state COP of 4.0 at an evaporating temperature of – 32 deg C and condensing temperature of – 7 deg C. The COP of the MT system is very high, at over 50.0, due to the low power requirement of the pump. System simulation and energy analyses of the integrated CO2-trigeneration system has shown the integrated energy system to produce energy savings of 30 per cent (consumption of 6.7 MWh/yr, against the R404A system’s 9.4 MWh/yr) and greenhouse gas emission savings of 43 per cent (2188 tonnes CO2e against 1247 tonnes CO2e).
The researchers found that the optimum capacity of the trigeneration system was 320-340 kWe, with a 310 kW absorption chiller. A trigen system provides the best payback when electricity prices are at their highest compared to gas, and this ‘spark ratio’ is currently at one of its highest levels, which could make the technology seem even more attractive.
The Optimised Tri-generation System
CHP module: This can be based on an internal combustion engines driven by any fuel such as natural gas, biogas, diesel, or biodiesel or micro gas turbines.
Sorption refrigeration system: Again the thermally driven refrigeration system can employ any type of sorption machine (absorption: ammonia-water; lithium bromide-water or adsorption: silica gel-water) depending on the temperature of the heat available from the CHP plant and the desired condensing temperature for the CO2 refrigeration plant.
CO2 refrigeration system: The refrigeration system uses CO2 as a secondary (volatile) refrigerant for the medium temperature (MT) cabinets and direct expansion (DX) for the low temperature (LT) cabinets. This maximises the system efficiency as the compressor power for MT refrigeration which is the predominant load in a supermarket is replaced by a much lower pump power to pump the liquid refrigerant to the refrigerated cabinets. Further, the team notes that should the trigen system ever fail, the refrigeration system alone can be set to run transcritically, allowing the retailer to continue trading. One of the additional benefits of the research work is that Brunel now has modelling tools for both design and analysis of CO2 systems and complete supermarket energy systems.