Low Temperature Waste Heat to Power Generation

Most of the waste heat (more than 60%) from industrial processes and institutional buildings, is low grade heat which makes its direct use within the facility difficult and also reduces the potential for power generation using Rankine or Organic Rankine Cycles (ORCs). ORCs also require that adequate pressure and consequently temperature differential is maintained between the heat source and heat sink to provide sufficient expansion energy for the turbine. This limits the temperature that the heat source can be cooled down to and the heat recovery and power generation potential.
The Main aim of this project is to overcome the limitations of conventional technologies by developing an innovative low temperature heat to power conversion system that can recover energy from low temperature waste heat streams. The project addresses the energy trilemma through:
i) Reduction of Emissions: by generating electricity from low temperature waste heat and displacing fossil fuel use.
ii) Security of supply: the generation of electricity from a waste heat will reduce the amount of primary fuel required and provides an alternative source of energy supply.
iii) Cost savings: using waste heat will result in significant cost savings for the users.
The system will be optimised for maximum efficiency at heat source temperatures 75 C to 90 C to maximise the range of applications and potential markets. It will operate on what we term 'Controlled Phase Cycle (CPC)'. The CPC is a variant of the Trilateral Flash Cycle (TFC), which is a 3-leg power cycle in which the working fluid expands from saturated liquid state at high pressure to a two-phase state at lower pressure in an expander before rejecting heat and condensing in the condenser. The resulting liquid is then compressed by a feed pump to the higher pressure level before is heated up by the heat source (waste heat stream) and re-expanded. The 'wet' expansion increases significantly the power output compared to the ORC. The TFC has not been commercialised as yet, due to the low efficiency of available expanders and high parasitic losses, particularly pumping power. These shortcomings will be overcome by innovations in this project which include:
i) Control of the quality of the fluid before expansion to a high wetness fraction to increase expansion efficiency;
ii) Thermal instead of mechanical pumping of the liquid from the condenser to the expansion pressure;
iii) Innovative heat exchanger design and controls to maximise heat transfer from the waste heat stream and minimise pressure losses;
iv) Improved screw expander rotor profile to further increase efficiency. The project will lead to a proof of concept system to be installed for demonstration and evaluation in a dairy facility.

The project is of significant interest and importance and addresses national and international energy and decarbonisation priorities. The proposed programme of research, and the strong academic and industry collaborations will make an important contribution to meeting these priorities by investigating and developing a new innovative waste heat to power generation technology that can utilise low temperature waste heat streams.
The potential market for waste heat recovery is expected to reach £35 billion by 2018 with Europe accounting for approximately 40% of this market. For this market to materialise, however, low temperature heat to power systems need to be developed to utilise the vast majority of low grade waste heat for power generation, a market largely untapped. The challenges of conversion of low grade heat (less than 100 C) to power with systems currently on the market such as ORCs are many, and include limited power output and conversion efficiency and high capital cost. This project aims to address these challenges by developing a heat to power system based on the CPC cycle which, unlike the ORC is not limited by constant temperature evaporation of the working fluid but instead uses counter-flow heat addition from the heat source, providing higher pressure differential and up to 3 times higher heat to power conversion, compared to ORC. This is projected to lead to attractive returns on investment ROI of 2-4 years payback.
The main output of the project will be a packaged plug and play low temperature heat to power generation system. A fully working prototype will be developed during the course of the R&D project and tested. The project consortium apart from Brunel involves: Spirax-Sarco Ltd, Howden Compressors; Arctic Circle; IPU Ltd; Dairy Crest and Brunel University. The consortium has the right skills, expertise, knowledge, facilities and extensive network of suppliers and customers to ensure the successful delivery of the project. It also involves a good mix of large organisations and SMEs to ensure cross fertilisation and knowledge transfer.
The market for the CPC system is substantial but initially attention will be placed on selected target markets which include: heat to power generation from cooling towers; incineration plant; and low temperature industry waste streams.
Economic benefits-will start in year 2 from project end, and continue for the foreseeable future from sales of the packaged units by Spirax and indirect benefits from the application of IP generated. Howden will benefit from the development and manufacture of the expander for the unit, IPU from the development and application of control systems and Arctic Circle from the manufacture of the skid and assembly of components. Dairy Crest, will realise reductions in energy costs from the application of the technology. The academic team will benefit from the know how that will be gained, academic impact and IP that will be generated. The technology will offer significant cost savings from the displacement of grid electricity. A 120 kWe unit will displace 1.1 GWhe from the grid annually leading to £87,000 savings and 2 to 4 years ROI. Social benefitsinclude the conversion of waste heat to electrical power, reducing reliance on imported fossil fuels and dependence on the grid and improving sustainability. It will also improve resilience to disruptions in supply. Local manufacturing by the consortium and cost savings by the users will create new employment opportunities will contribute to skills development by the companies and the training of researchers by the university. Environmental benefits will arise from displacement of fossil fuel electricity from central power stations and reduction of temperature of waste heat discharges to air or water. For electricity emission factor of 0.47 kg/kWh (DECC,2013) a 120 kWe unit will save ~500 tCO2e GHG emissions per year.