Solar-thermal combined energy systems for distributed small-scale applications
Creators: name: Freeman, James
Publisher: name: Imperial College London
Year: 2016

description: Solar power has the potential to provide a significant amount of the global energy requirement for power, heating, cooling and fresh water production. Solar thermal power systems based on steam Rankine cycles tend to be feasible for large-scale centralised power plants, with concentrating collectors generating temperatures of 400 °C or higher. This limits operation to locations that receive focusable direct beam radiation, uninterrupted by cloud cover. On the small scale, photovoltaic (PV) systems dominate the domestic solar market, with solar to electrical efficiencies of around 15%. Compared to PV, solar thermal systems are able to make use of a larger proportion of the solar resource through recovery of useful heat for thermal processes. In this thesis the potential of a small-scale solar combined heat and power (S-CHP) system based on an Organic Rankine Cycle (ORC) is investigated, for the combined provision of heating and power for domestic applications. The system consists of a solar collector array of total area equivalent to that available on the roof of a typical UK home, an ORC engine featuring a generalised positive-displacement expander, and a hot water storage tank. Water heating is provided through the precooling of the superheated exhaust vapour from the ORC expander, and an optional further contribution from the solar collector array. A thermo-economic model is developed in order to evaluate the performance and cost of the S-CHP system under various configurations and climatic conditions. Concentrating and non-concentrating solar collector designs suitable for small-scale applications are compared by way of annual maximum work assessments based on reversible and endoreversible analyses. The thermo-economic model is used to perform a parametric analysis of the S-CHP system to determine the optimum operational settings, and to simulate the system performance over an annual period. For the system simulated with non-concentrating collectors, electrical outputs equivalent to 19% and 33% of the typical household annual demand are predicted in the climates of London (UK) and Larnaca (Cyprus) respectively. In addition, the ability to provide up to 28% (London) and 58% (Larnaca) of annual hot water demand through recovery of rejected heat from the ORC engine is also predicted. A hot-water prioritisation mode is also identified for which hot water output is increased, at the expense of a decrease in electrical output. A cost analysis is performed in order to compare the system with other solar heat and power systems. The whole system capital cost is €9,700-10,500 (£8,000-8,800). The cost per unit average electrical output is higher than that for PV-based systems, however when compared to hybrid PV-thermal (PVT) systems, the total capital cost is found to be lower, and the potential to match local demand by varying the proportional outputs of electricity and hot water is seen as an additional advantage for the S-CHP system. Two experimental apparatus were designed and constructed for the purpose of evaluating the S-CHP system model. The first was an outdoor solar collector testing facility to measure solar collector efficiencies under a range of operating conditions. Steady-state tests were performed on an evacuated tube collector at fluid inlet temperatures up to 112 °C. This exceeds the normal range of operation in water-heating applications but is closer to the optimal temperatures found in the maximum work analysis. The results were used to provide a more reliable model of collector performance at higher temperatures for the purposes of the S-CHP system modelling. Dynamic tests were also performed in which the collector's response to a step change in solar irradiance was measured. The results were used to determine the collector's effective thermal capacity, and to evaluate a range of dynamic solar collector models. The experimentally obtained thermal capacity value was found to provide improved predictions of dynamic collector behaviour when implemented into a lumped dynamic model, resulting in a ~35% reduction in modelling error; while conventional estimation methods for the effective thermal capacity were found to result in significant under-predictions of the collector's thermal response time and an over-prediction of the daily energy yield. An experimental ORC apparatus was also designed and constructed in order to evaluate the performance of small-scale ORC components. The system featured a 1 kWe scroll expander and the working fluid R245fa. Thermal input was provided by an 18 kWth electrical oil heater. Measurements of electrical power output were taken over a range of thermal input settings and working fluid mass flow-rates, while expander rotational speed was also adjusted by varying the electrical load. The experimental results were compared to the predictions from the ORC system model and the performance of the various system components were investigated. The ORC system model was found to over-predict the gross thermal efficiency by 30-90%, attributed to off-design operation of the expander during the experiments and to thermal losses from components such as the heat exchanger, expander and pipework that were not included in the model. A maximum gross thermal efficiency of 5.3% and an expander isentropic efficiency of 49% were obtained during testing.