Jones, Sean
(2019)
A novel community low-temperature district heat network with distributed generation as part of a multi-vector energy system.
PhD thesis, University of Nottingham.
Abstract
This work was conducted as part of project SCENIC (“Smart Controlled Energy Networks Integrated in Communities“) funded by the Energy Research Accelerator (ERA) and by extension Innovate UK.
There is an immediate need to reduce fossil fuel consumption and improve energy efficiencies in order to tackle climate change. This work investigates an energy system advancement that could contribute to global environment security, in line with the 2050 climate obligations for decarbonisation under the Kyoto Protocol.
This thesis outlines the design, installation, and evaluation of a community-scale low-temperature district heat network (LTDHN) with distributed generation (DG) at the University of Nottingham’s Creative Energy Homes test site. The network links seven residential and office buildings to a double loop network. One loop — the consumer loop — takes heat from the central thermal store to the houses for space heating (SH) and domestic hot water (DHW). The other loop — the prosumer loop — takes heat from the DG situated on each of the houses to the central thermal store. Distributed generators include one flat plate solar collector, two evacuated tube solar collectors, two gas boilers, one biomass boiler and an immersion heater.
The system is the first of its kind to test LTDHN and the prosumer concept — where consumers can buy and sell heat to a network. The system combines the benefits of LTDHN, such as a reduced system design margin, lower heat losses, and improved controllability of heat medium, with DG that aids peak load management and improves system continuity.
A bespoke, flexible building energy management system (BEMS) was designed, built and installed for this work. The BEMS allowed the control of pumps, valves, and energy units over the whole site, and as a result enabled testing of the system’s effectiveness. Results show the network can supply adequate SH and satisfy instantaneous DHW demand. However, prolonged periods of hot water demand cannot be satisfied above the 48 °C comfort requirement for bath and shower water; this is linked to the responsiveness of the consumer supply pipe blending valve and its ability to keep the network supply temperature at 55 °C.
Results showed that heat losses where low from the thermal store and heat distribution pipes. Yet, when a heat interface unit (HIU) was tested there was an average 67 W power output. Over the whole site there is an estimated 704 W constant power output from the HIUs when the system is running. Other key issues included thermal store destratification through mixing and high return temperatures from radiators leading to lower temperature differentials throughout the system.
The system is evaluated through energyPRO modelling of multi-vector energy systems (MES) with the aim of understanding how the SCENIC heat network and future energy systems can best integrate heat and power to facilitate the integration of sustainable energy sources. Modelling of a MES with small-scale DG to a LTDHN had not been done before this research.
MES parametric modelled results found that £2,256/year operational cost and 6,662 kgCO2eq/year emissions saving was possible over the whole site when compared to the current energy system at the CEH. In an idealised MES, where there are fewer design constraints, the CEH site could benefit from an £11,492/year and 13,542 kgCO2eq/year saving compared to the current energy system. Generally, it was found that the MES that had the highest carbon emission and cost savings were those that utilised a high portion of solar thermal heat, backup heating from heat pumps connected to a large thermal store, solar PV and mains electricity from a renewables only tariff, and a community battery. The systems with the worst performance had a high portion of heating from gas or biomass boilers, and immersion heaters supplied by a standard electricity tariff.
Future work, should focus on the testing the SCENIC heat network over a period of one year or more, improving the responsiveness of the network supply pipe blending valve, replacing critical (high-temperature) radiators, preventing mixing of the thermal store, reducing heat losses from HIU, and testing the MES modelled scenarios by installing power-heat interface technologies.
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