Optimisation of high-efficiency combined heat and power systems for distributed generation
Creators: name: Chatzopoulou, Maria Anna
Publisher: name: Imperial College London
Year: 2018

description: Distributed combined heat and power (CHP) systems have the potential to cover a significant amount of the global energy requirements for power, and heating. Small-to-medium scale CHP systems, in the built environment and in the industry (up to a few MWs), are typically driven by internal combustion engines (ICE). In CHP-ICE systems, more than 55% of the energy input is transferred as heat in the exhaust-gas stream and the jacket water cooling circuit. Unless these thermal outputs are utilised, the energy will be released to the atmosphere as waste heat, deteriorating the system’s efficiency. Organic Rankine cycle (ORC) engines are a promising heat-to-power technology, for converting waste heat into power. Therefore, coupling ORC engines as bottoming cycles to CHP-ICEs can maximise overall system efficiency, and reduce energy costs. In this thesis, the design of ICE-ORC CHP systems is investigated from thermodynamic, operating and economic perspectives, aiming to fully unlock the potential of such advanced high-efficiency cogeneration systems. An integrated ICE-ORC CHP optimisation tool is developed, which, unlike previous studies, captures the performance trade-offs between the two interacting engines, to optimise the combined system performance. A dynamic ICE model is developed and validated, along with a steady-state model of subcritical recuperative ORC engines. Multiple working fluids are investigated, along with naturally aspirated and turbocharged ICEs. By optimising the combined ICE-ORC CHP system simultaneously: i) the total power output increases by up to 30%, in comparison to the conventional approach where the two engines are optimised separately; ii) the electrical efficiency increases by up to 21%, in comparison to the stand-alone ICE; and iii) in the integrated system the ICE operation is adjusted to promote the ORC power output, which generates up to 15% of the total power, improving fuel efficiency. When focusing on maximising power output only, this comes at the cost of higher fuel consumption. In contrast, when optimising the integrated ICE-ORC CHP system for specific fuel consumption (SFC), the fuel consumption decreases by up to 17%. These findings prove that by taking a holistic approach in the design of ICE-ORC CHP systems, considering the combined system interactions, these can generate more power, with lower fuel consumption and costs. ORC engines in ICE-ORC CHP systems will experience variable heat-source conditions (temperature and mass flow are), while the ICE load fluctuates. To maximise the running hours of ORC engines, and improve their economic proposition, the system should maintain high efficiency, not only at the design point, but also at off-design operation. An off-design optimisation tool is therefore developed to generate optimised off-design operation maps. This work differs from previous studies in that the tool considers explicitly the time-varying operational characteristics and interactions of the ORC engine components in the integrated system. Double-pipe (DPHEX) and plate-frame heat exchanger (PHEX) models are used for sizing the ORC evaporator and condenser, and piston and screw expander models for sizing the expander. The ORC system is first sized for full-load ICE operation (design point). Then, ICE part-load (PL) conditions are obtained and new ORC operating points are optimised. Results reveal that the ORC engine power output is underestimated by up to 17%, when the off-design operational characteristics of the components are not considered. The piston expander efficiency increases by up to 16% at PL operation, while the ORC thermal efficiency increases by up to 7% at off-design operation. Optimised ORC engines with screw expanders operate always with two expansion stages. Although the latter generate slightly more power at their design point than when using piston machines, pistons perform better at off-design conditions. ORC engines with PHEXs generate 5-12% more power than DPHEX designs, while having U-values almost double that of DPHEXs. Although, heat transfer coefficients decrease by 25-30% at off-design, the HEX effectiveness increases, by up to 15%. By considering the time-varying characteristics of the ORC components, as the ICE PL reduces, the optimised ORC engine power output decreases at a lower rate: at ICE PL of 60%, the optimised ORC engine with fluids, such as R1233zd, operates at 77% of its nominal capacity (with piston expanders). An ORC thermoeconomic optimisation tool is then developed. Unlike other studies where the component types are predefined, the tool scans alternative components’ typologies, sizes, and configurations, to obtain the best-performing design. New cost correlations are presented to assist with predicting ORC costs. ORC engines optimised for maximum power output have the highest SICs, falling in the range of 1,000-7,500 GBP/kW for piston expanders, and 1,500-5,500 GBP/kW for screw expanders, due to high HEX areas, and high volumetric flow rates. In contrast, when minimising SIC, power output reduces by 15-50%, but the cost also reduces by up to 35%. In the optimised designs, the evaporator is selected to be a PHEX, the condenser is a DPHEX, whilst ORC engines with screw machines operate with two-stage expansion. Multi-objective optimisation reveals optimal ORC systems with a range of power outputs between 70-100 kW, for which increasing power by 33%, results in an increase of SIC of less than 10%. This indicates a promising range of capacities for next-generation ORC engines. ORC engines optimised for high power output result in high net present value (NPV), but also high discounted payback period (DPP). In contrast, engines optimised for low SIC achieve DPPs of 2.8-6 years, making them an attractive investment. Overall, these findings can be used by ICE and ORC engine manufacturers, and integrators, to inform component design decisions, and by ORC plant operators to maximise their system performance, under variable operating conditions.