Optimisation of a high-efficiency solar-driven organic Rankine cycle for applications in the built environment

Recent years have seen a strong increase in the uptake of solar technologies in the built environment. In combined heat and power (CHP) or cogeneration systems, the thermodynamic and economic ‘value’ of the electrical output is usually considered to be greater than that of (an equivalent) thermal output, and therefore the prioritisation of the electrical output in terms of system-level optimisation has been driving much of the research, innovation and technology development in this area. In this work, the potential of a solar CHP technology based on an organic Rankine cycle (ORC) engine is investigated. We present thermodynamic models developed for different collectors, including flat-plate collectors (FPC) and evacuated-tube collectors (ETC) coupled with a non-recuperative sub-critical ORC architecture to deliver power and hot water by using thermal energy rejected from the engine. Results from dynamic 3-D simulations of the solar collectors together with a thermal energy storage (TES) tank are presented. TES offers an important buffering capability during periods of intermittent solar radiation, as well as the potential for demand-side management (DSM). Results are presented of an optimisation analysis to identify the most suitable working fluids for the ORC unit, in which the configuration and operational constraints of the collector array are taken into account. The most suitable working fluids (R245fa and R1233zd) are then chosen for a whole-system optimisation performed in a southern European climate. The system configuration with an ETC array is found to be best-suited for electricity prioritisation, delivering an electrical output of 3,605 kWh/yr from a 60 m2 array. In addition, the system supplies 13,175 kWh/yr in the form of domestic hot water, which is equivalent to more than 6 times the average annual household demand. A brief cost analysis and comparison with photovoltaic (PV) systems are also performed

Thermodynamic and economic evaluation of trigeneration systems in energy-intensive buildings

Within the building sector, supermarkets are responsible for 3- 5% of the electricity consumed in developed countries. To mitigate the associated environmental impact of this consumption, a growing interest has been developed in local combined heat and power (CHP) systems, due to their higher total efficiencies. However, CHP efficiency is highly dependent on the thermal output utilisation. In food retail buildings, where refrigeration dominates the building energy use, a promising means for utilising the thermal output is by using this to operate absorption chillers. This paper reports on a technical feasibility and financial viability study of an ammoniawater absorption chiller, coupled to a CHP unit, that is also compared to a conventional electrically-driven vapour-compression equivalent. A typical distribution centre located in the UK is selected as a case-study. Three alternative systems are considered: i) a conventional grid connected system; ii) a CHP system; and iii) a trigeneration system. Typical daily cooling, heating and hot-water demand data are provided on an hourly basis, and the system’s ability to cover these loads is assessed. The results indicate that the trigeneration system can reduce the electricity demand by 16% compared to the baseline system, while offering a 48% annual energy cost saving. The system’s primary energy utilisation rate exceeds 60%, while the power-to-heat ratio of the building demand improves from 7.0 to 0.9, thereby more closely matching the CHP system generation profile. Furthermore, the trigeneration system achieves CHPQA rating of 106, and it is qualified for enhanced capital allowance for the CHP plant. The results highlight the great energy and cost savings potentials of integrating trigeneration systems in energy-intensive buildings