Abstract
Suitability to off-design operation, applicability to combined thermal and electrical generation in a wide range of low temperatures and pressures and compliance with safety and environmental limitations qualify small-scale Organic Rankine Cycle plants as a viable option for combined heat and power generation in the residential sector. As the plants scale down, the electric and thermal output maximization has to account for issues, spanning from high pump power absorption, compared to the electric output of the plant, to intrinsically low plant permeability induced by the expander, to the intermittent availability of thermal power, affected by the heat demand for domestic hot water (DHW) production. The present paper accounts for a flat-plate solar thermal collector array, bottomed by an ORC unit featuring a sliding vane expander and pump and flat-plate heat exchangers. A high-temperature buffer vessel stores artificially heated water – electric heaters, simulating the solar collector - and feeds either the hot water line for domestic use or the ORC evaporator, depending on the instantaneous demand (i.e., domestic hot water or electric power), the temperature conditions inside the tank and the stored mass availability. A low-temperature receiver acts like the heat sink of the ORC unit and harvests the residual thermal power, downstream the expander: a dedicated control, modelled to properly modulate the mass addition/subtraction to this storage unit allows to restore the operating points of the cycle and to limit the incidence of off-design operation, via real-time adjustment of the cycle operating parameters. Indeed, the possibility of continuous ORC generation depends on (i) the nature of the demand and (ii) the amount of hot water withdrawn from the high-temperature buffer vessel. The time-to-temperature for the mass stored inside the buffer affects the amount of ORC unit activations and eventually the maximum attainable generation of electric energy. The plant energy performance is experimentally assessed, and various characteristic operating points are mapped, based on test runs carried out on a real-scale ORC pilot unit.
Highlights
Micro-ORC units, fed by thermal energy from solar thermal collectors, have an enormous potential for distributed electricity generation and contemporary coverage of both thermal and electrical demand in the Residential sector [1,2,3]
The variability, on a seasonal and hourly basis, of the thermal demand for domestic hot water (DHW) production and heating purposes, calls for the integration to the plant of a vessel for thermal energy storage (TES), to serve the dual purpose of (i) decoupling the thermal availability at the evaporator from any environment-induced disturbance [13,14,15,16] and (ii) harvesting thermal energy during no DHW-withdrawals to feed it to the power section for electricity generation
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 still available at the expander outlet: the system integrates 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
Summary
Micro-ORC units, fed by thermal energy from solar thermal collectors, have an enormous potential for distributed electricity generation and contemporary coverage of both thermal and electrical demand in the 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 still available at the expander outlet: the system integrates 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 (i.e. either a flash or a progressive tank discharge) for the storage unit, to maximize, in turn, the electrical and thermal output and to (i) allow the cogeneration unit to operate with little-to-no irradiance, when electricity is typically most required, (ii) allow the instantaneous matching of both electric and thermal demand and (iii) in case of over-production of electric power, to plan the proper management of battery packs or the sale to the grid when it is more valuable, for financial revenue
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