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UK scientists have proposed a way to combine photovoltaic-thermal energy with Stirling engines and battery storage in residential buildings. Despite the high upfront costs, they said the cost-effective hybrid co-generation system could significantly reduce CO2 emissions.
Researchers from Durham University in the United Kingdom have developed a hybrid cogeneration system that combines photovoltaic-thermal (PVT) collectors with a Stirling engine (SE) and battery storage. The system is designed to meet the demand for electricity and domestic hot water (DHW).
“The advantages of such a hybrid system are that it can greatly reduce primary energy consumption and household energy bills directly,” researcher Shunmin Zhou told pv magazine. “It also reduces carbon emissions compared to a gas boiler and grid-based electricity reference system.”
A Stirling engine is a closed-cycle regenerative heat engine with a permanent gaseous working fluid such as gas or air. It generates mechanical motion from the heat-driven compression and expansion of the fluid – using a heat transfer fluid to meet demand.
“The commercial Baxi Ecogen unit is considered in this work,” the researchers said. “It is a free-piston SE-based micro-CHP unit that is capable of generating up to 1 kW of AC electricity and 7.7 kW of heat simultaneously.”
The hybrid system consists of 28 m2 of PVT collectors, the Stirling engine, a DHW storage tank, and a lead-carbon battery pack. It uses a primary pump to supply cold water to the PVT system and a secondary pump to supply cold water to the Stirling engine.
“The hot water discharged from the SE unit is then mixed with hot water obtained from the outlet of the PV-T collector and stored in a DHW storage tank,” the group said. “Subsequently, the stored hot water at a temperature of 60 C is readily supplied to the residential houses, to meet the space heating and DHW demand.”
The system also generates AC electricity via the alternator of the SE unit and DC power through the PVT collectors. Both are used to meet the electrical demand load of the household via an inverter, with excess power being stored in the battery. Grid electricity can be used when both sources of power do not meet demand and surplus power can be injected into the grid when the battery is fully charged.
The scientists tested the proposed system configuration for three different residential building typologies – detached houses (DHs), semi-detached houses (SDHs), and mid-terraced houses (MTHs). They found that the DH configuration achieves the highest overall reduction in CO2 emissions compared to SDH and MTH configurations, which depends on the larger PV-T system sizes employed for the DH architecture.
“However, in terms of the achieved CO2 emissions reduction rates, there is no big difference in different house types, all within the range of 30 % to 45 %,” the researchers said. “This implies the carbon emission reduction rate of the proposed hybrid cogeneration system is not sensitive to the house type, up to a point.”
The DH typology was also found to have the highest exergy efficiency.
“DHs boast the lowest levelized cost of electricity (LCOE) at GBP0.622 £ ($0.78)/kWh, the lowest levelized cost of heat (LCOEth) at 0.147 £/kWh, and the lowest levelized cost of total energy (LCOEeq,el) at 0.205 £/kWh,” the academics said, noting that the observed variations in these values must be attributed to the different exergy efficiencies achieved by the three housing types.
“However, the initial capital costs of such a system are high, especially arising from the SE unit and the photovoltaic-thermal (PVT) collector array, which acts as a barrier to widespread penetration at present,” Zhou added. If a further reduction of the initial investment, especially the PVT collectors and the SE unit, can be achieved, this will significantly reduce the payback time and attract the widespread deployment of this technology.”
Their details of the system are available in “Techno-economic and environmental analyses of a solar-assisted Stirling engine cogeneration system for different dwelling types in the United Kingdom,” which was recently published in Energy Conversion and Management.
Another research group at Durham University recently proposed a new design for thermoelectric heat pumps (TeHPs) that reportedly exploits all the advantages that heat pump technology offers, especially when applied in residential buildings. |
