Abstract
The feasibility of a solar-ORC system for domestic combined heat and power generation (CHP) is deeply affected by both the time-varying ambient conditions (e.g. solar irradiance, temperature, wind speed) and the thermal and electrical load profiles variability of the final application. The definition of a proper control strategy is proven to be a major design-challenge for successful operation of solar-ORC systems, with the main goal of assuring that the thermal power demand for space heating and Domestic Hot Water (DHW) production and the electricity needs are simultaneously satisfied. The rising demand for energy-autonomous systems also calls for the inclusion of a storage system within the base-layout, that could assure the electricity demand is properly matched after sunset or in very-low irradiance conditions, such as cloudy days. A comprehensive model accounts for the dynamic of the plant-integrated unit, featuring an ORC-based plant that bottoms a flat plate solar thermal collector: a parametric study is presented, and an off-design analysis is performed to properly assess the energy performance of the system. The heat availability to the ORC heat exchanger is evaluated, based on solar availability, thermal losses in the pipes and plant requirements, in terms of operating temperature and pressures and organic fluid mass flowrate. R245fa is selected as working fluid in the ORC-section. Sliding vanes machines expander and pump – are considered as rotary equipment. Flat plate heat exchangers complete the base layout, the analysis accounts for. Due to the need for DHW production, a storage unit for hot water is present, upstream the recovery branch: dependently on the ability the fluid at the collector outlet has to meet the ORC requirements for proper operation (about 110°C), the ORC evaporator is fed and the recovery section enabled. Both continuous and unsteady operation underwent an in-depth analysis, as well as the benefits associated with different discharge times for the storage unit. A dedicated control strategy is defined, dependently on whether the electrical output or the thermal one need to be maximized, and accounts for either a flash or a progressive tank discharge. A virtual platform allowed the setting-up of a pilot plant, for direct performance assessment, in presence of different amounts of tank discharges per day and different lower temperatures at the storage tank.
Highlights
A significant interest has risen in the last years towards low-temperature heat conversion into electric power, via Organic Rankine Cycle-based plants (ORC), in the Building – namely, Residential - sector [1,2,3]
A comprehensive model of control strategy is defined for a domestic solar-ORC combined heat and power generation (CHP) plant
Since the working fluid mass flowrate processed in the ORC section depends on the thermal energy provided at the evaporator, the splitting of the mass flowrate of collector fluid at the hot side of the evaporation section represents an interesting option to fulfill better energy performances and a higher net electric power generation
Summary
A significant interest has risen in the last years towards low-temperature heat conversion into electric power, via Organic Rankine Cycle-based plants (ORC), in the Building – namely, Residential - sector [1,2,3]. In light of the typically very low characteristic conversion efficiency of ORC-based plants, a large share of the thermal power provided to the organic fluid at the evaporation section is expected to be still available at the expander outlet: the model accounts, for the possibility to gather this thermal energy in a low-temperature buffer vessel to assist the system in the primary purpose of DHW production and increase the amount of thermal energy potentially available to electric generation Both continuous and unsteady operation underwent an in-depth analysis, as well as the benefits associated with different discharge times for the storage unit. A virtual platform allowed the setting-up of a pilot plant, for direct performance assessment, in presence of different amounts of tank discharges per day and different lower temperatures at the storage tank
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