Multi-Variable Control and Optimization Strategy for Domestic Solar-ORC Combined Heat and Power Generation System

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.

Experimental Assessment of a Multi-Variable Control Strategy of a Micro-Cogeneration Solar-ORC Plant for Domestic Application

“…Apart from the satisfying trade-off it assures, between high characteristic efficiencies and mass flowrate variability in low-grade waste-heat recovery, the sliding vane technology is particularly appreciated, since it assures tolerance to dualphase expansions and for smooth operation in off design conditions: the less stringent constraints on the selection of the superheating degree creates conditions for lower losses for irreversibility at the heat exchanger and on a 1-3 % gain on the overall energy efficiency of the system, particularly valuable in small scale systems, usually suffering from a high Back-Work ratio (i.e. ratio between the work for pumping and the work gathered at the expander) [31,34].…”

“…Operation at both design point and under partial load underwent an in-depth investigation: the breakthrough of the plant overall performance was possible by a close-up analysis of the energy performance of each component, particularly the expander and the pump. To the scopes of the experimental characterization, a pair of 12 kW electric heaters provide thermal energy to the water inside the tank and mirror the variability of the solar source: the year-round available solar radiation at the experimental facility (town of L'Aquila, central Italy) on a 15 minutes time-basis is retrieved from [35] and the assessment of the thermal power to the storage unit refers to a 15 m 2 solar thermal collector field, as presented in [34]. As the energy fed to the ORC-unit varies, a different mass-and energy-equilibrium is reached at the evaporator, leading to a different value of the processed mass flowrate of working fluid: a wide range (35 g/s -75 g/s) is considered for the prototype at hand.…”

“…In the present paper, the results of an extensive experimental campaign on a prototype ORC-unit, are presented: based on a preliminary parametric study [34] to account for off-design operation of the plant, the energy performance of the system is assessed. Electrically heated water reproduces the thermal availability from flat plate solar thermal collectors at a 250 L tank.…”