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British scientists have proposed a way to combine photovoltaic thermal energy with Stirling engines and battery storage in residential buildings. Although the initial costs are high, they claim that the cost-effective hybrid cogeneration system could significantly reduce CO2 emissions.
Researchers at Durham University (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 satisfy 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 a grid-based electricity baseline 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 heat-driven fluid compression and expansion, using a heat transfer fluid to meet demand.
“In this work, the Baxi Ecogen business unit is analyzed,” the researchers explain. “It is an SE-based free-piston micro cogeneration unit capable of generating up to 1 kW of alternating current 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 mixed with the hot water obtained from the outlet of the PV-T collector and stored in a DHW tank,” the group explains. “Subsequently, hot water stored at a temperature of 60 C is easily supplied to homes, to meet the demand for heating and DHW.”
The system also generates electricity in alternating current through the SE units alternator and in direct current through the PVT collectors. Both are used to satisfy the homes electrical demand through an inverter, and excess energy is stored in the battery. Electricity from the grid can be used when both energy sources do not cover demand and excess energy can be injected into the grid when the battery is fully charged.
The scientists tested the proposed system configuration on three types of residential buildings: single-family houses, semi-detached houses, and intermediate semi-detached houses. They found that the DH configuration achieves the greatest overall reduction in CO2 emissions compared to the SDH and MTH configurations, which depends on the larger size of the PV-T system used for the DH architecture.
“However, in terms of the CO2 emissions reduction rates achieved, there are no major differences in the different types of houses, all within the range of 30% to 45%,” the researchers say. “This implies that the carbon emission reduction rate of the proposed hybrid cogeneration system is not sensitive to the type of house, to some extent.”
The DH typology was also found to have the highest exergetic efficiency.
“Buildings of this type have the lowest levelized cost of electricity (LCOE), at 0.622 pounds ($0.78)/kWh, the lowest levelized cost of heat (LCOEth), at 0.147 pounds/kWh, and the lowest cost leveling of the total energy (LCOEeq,el), with 0.205 pounds/kWh”, explain the researchers, who point out that the variations observed in these values ??should be attributed to the different exergetic efficiencies achieved by the three types of housing.
“However, the initial capital costs of such a system are high, especially those derived from the SE unit and the photovoltaic-thermal (PVT) collector assembly, which acts as a barrier to widespread penetration today. ”Zhou added. If the initial investment can be reduced even further, especially that of the PVT collectors and the SE unit, the payback time will be significantly reduced and the widespread implementation of this technology will be attracted.”
Details of the system are available in “ Techno-economic and environmental analyzes of a solar-assisted Stirling engine cogeneration system for different dwelling types in the United Kingdom “, recently published in Energy Conversion and Management.
Another research group at Durham University recently proposed a new design for thermoelectric heat pumps (TeHP) that reportedly takes advantage of all the benefits that heat pump technology offers, especially when applied in residential buildings. |
