Assessment of CCHP systems based on biomass combustion for small-scale applications through a review of the technology and analysis of energy efficiency parameters

Abstract

An alternative way of increasing and improving the use of biomass resources and distributed generation is by using combined cooling, heating, and power (CCHP) small-scale plants. The first step in designing a CCHP plant is to determine the best configuration in terms of the thermodynamic integration of all the subsystems and the optimization of the overall energy efficiency.In this work, small-scale biomass CCHP systems are evaluated to provide a basis for studies on their thermodynamic feasibility and energy efficiency in two main sections. First of all, a state of the art review is presented regarding the technologies involved in CCHP systems based on biomass combustion. The conclusion drawn might initially be considered obvious: the best prime mover (PM) technology to develop this type of plant is the Stirling engine (SE) due to its higher electric efficiency, but its low market availability and operational problems currently limit its use in commercial plants. The organic Rankine cycle (ORC) is a very widespread technology but its use in CCHP systems in the power range between 1 and 200kWe is limited due to their low sink temperatures, which prevent its direct cascade integration with thermally activated cooling (TAC) for refrigeration. However, it is possible to integrate these technologies in an energy efficient way (achieving primary energy savings) if the plant is designed according to some guidelines regarding the heating and cooling production. The second section of this work demonstrates this idea by analyzing different possible integrations of the prime movers and TAC units currently available on the market. The overall performance and the energy feasibility, in terms of the relation between the heating and cooling loads, are evaluated through a parameter analysis, using the Artificial Thermal Efficiency (ATE) and the Primary Energy Saving Ratio (PESR). The evaluation of the configurations proposed shows that direct coupling of the PM and TAC units is highly important, more so than high efficiency and coefficient of performance (COP) of the technologies, for achieving primary energy savings in almost every possible relation between the cooling and heating loads generated by the CCHP plant. As a result, some guidelines are proposed to develop small-scale CCHP plants based on biomass combustion in an energy efficient way.