A reversible Organic Rankine Cycle (rORC) describes one of the two key innovative technologies of the RESTORE solution for integrating more Renewable Energy Sources in District Heating and Cooling (DHC) Systems.
The RESTORE technology cannot effectively integrate Waste Excess Heat (WEH) utilization and Renewable Energy Sources (RES) in DHC without the adoption of a non-conventional power system able to exploit availability of medium temperature heat and electricity-based RES to provide high-temperature heat to the TCES thus boosting the storage of solids on seasonal base and limiting energy waste. This non-conventional power system is based on a thermodynamic cycle that can operate as both a heat pump during charging mode and as a direct power cycle in discharging mode. Most of the pieces of equipment are common to both operation modes and the working fluid is the same in order to get the minimum investment cost of the technology. The most promising working fluids are organic fluids as refrigerants and novel low environmental impact olefins and for this reason, we refer to HP/ORC (Heat Pump/Organic Rankine Cycle) technology.
RESTORE HP/ORC technology gives the unique possibility to unlock the integration of any kind of RES and WEH in DHC networks ensuring their exploitation all year long. This minimizes waste of energy with positive effects on environmental and economic assessment.
A thermodynamic cycle with two evaporation and two condensation levels
The Figure below reports an example of a complex cycle configuration which will be investigated numerically within the project that encompasses a power system with a double evaporator (HP) / double condenser (ORC) configuration. The HP charging cycle has two evaporation levels in order to provide district cooling during summer by contextually boosting energy storage. The thermodynamic cycle when operated as ORC in discharging mode encompasses two condensation levels providing a better match with DH water. This would allow to increase the ORC cycle efficiency and the round-trip efficiency for a fixed DH energy need and supply temperature by further expanding a fraction of the working fluid down to lower operating pressures and integrate DC directly into the cycle.
Optimizing the electrical round-trip efficiency
During the RESTORE project specific activities will determine the actual reversibility of the volumetric machine and the performance penalization due to the adaptation of the intake and discharge ports depending on the type of operation. Furthermore, the research activities will assess the compatibility of materials and geometry with the selected working fluid. This leads to the optimal compromise between electrical performance in charging (HP mode) and discharging (ORC mode) operations.
Moreover, when integrating RESTORE in low-temperature DH networks, medium temperature heat is available during charging mode. This allows for high and competitive electrical round-trip efficiencies and also a tremendous electricity seasonal storage.
By increasing the minimum temperature of the HP, it is possible to reach a very high value of COP (Coefficient of Performance) that more than outweighs the shortcoming related to the limited performance of the Organic Rankine Cycle. Round-trip efficiencies of RESTORE technology coupled with 50°C low supply temperature DH and exploiting around 100°C low grade WEH are around 70%. This value is competitive with other electrical energy storage technologies like Li-ion batteries, CAES (Compressed Air Energy Storage) and LAES (Liquid Air Energy Storage) systems with some additional advantages.