What is District Heating?
District heating is a system that distributes heat that is generated centrally by a power plant around a network of pipes to a multitude of buildings. The heat is circulated across the networks through insulated pipes, this heat delivers space heating and hot water to the connected buildings. Some positives of the system are:
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Lower carbon emissions
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More energy efficient
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Economic Benefits
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Reliable
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In line with governments interests
District heating networks have been in use since 1880s, the system was named the first generation of district heating. The system used high steam temperatures as the heat carrier which resulted in low energy efficiencies, due to high heat loss.
Since the 1880s district heating has progressed through two stages of development. The first was the introduction of second-generation district heating in the 1930s and was used for all new networks until the 1970s. The thermal energy changed from steam to pressurised water, which reduced the distribution temperature, but it was still more than 100°C. The second development was named third generation district heating and it has been in use since the 1970s. The heat carrier is still pressurised water, although the temperature had decreased to below 100°C. The decrease in temperature increased the energy efficiency of the system. The incentive of the development was to move away from oil to more reliable fuels such as coal and biomass.
The fourth generation of district heating is the next development, which aims to employ a sustainable energy system that incorporates renewable energy systems and energy efficient buildings. The new generation would follow the previous incarnations of decreasing the distribution temperatures, which in turn increases the energy efficiency. The fourth generation would integrate renewable energy systems such as heat pumps, hydrogen and biomass as seen in figure 1.
Figure 1- The four generations of District Heatinng. Source: (Lund et al., 2014)
At the center of most district heating networks is an energy-efficient technology called Combined Heat and Power (CHP). A CHP system generates electricity and harnesses the heat that this process creates which would normally be wasted for use as thermal energy. The thermal energy can then be used for cooling, space heating or hot water.
The CHP can achieve efficiencies of over 80% due to capturing waste heat, this makes it more efficient than other technologies (Combined Heat and Power (CHP) Partnership, 2021). CHP engines are usually powered by fossil fuels.
Strathclydes Current System
The University of Strathclyde’s district heating network was completed in 2018. As of 2019, there are 18 buildings connected to the system. At the centre of the heating network is a 3.3MW CHP, shown in figure 2 and three 8 MW boilers, shown in figure 3 that create a heating capacity of 27.3MW, located within the Energy centre- shown by the blue triangle in figure 3. The CHP is nearly constantly generating electricity which in turn generates heat, the boilers are used when the CHP heat cannot meet the demand of the system.
The system is oversized for the current demand as the university hope to expand to more buildings within and out-with the campus. The 18 buildings connected to the current systems described as phase one -shown in red in figure 4- of the project by the university, including connections to the TIC, James Weir, Thomas Graham, Graham Hills and Lord Hope Buildings. Moving into Phase two - shown in purple in figure 4- they have planned to add student residents to the system. The hope in future Phases is to eventually add surrounding buildings that are not owned by the university, sites such as the Royal Infirmary and the Glasgow City Chambers for the third phase of the expansion.
The current CHP and Boilers all rely on natural gas to produce heat, this is not in line with the universities plan of achieving net zero emissions by 2040 (Climate Change, 2021).
Figure 2- 3.3 MW Natural Gas fuelled Combined Heat and Power (CHP) engine. Source: Nick Kelly
Figure 3 - The 3 X 8 MW Natural Gas fuelled Boilers within the Energy Centre. Source: Strath.ac.uk
Figure 4- Map Detailing the District Heat Pipe Network, with current connections shown in red and future connections shown in blue
From analysis of meter readings from the Energy Centre (Charles, 2021) and invoices (Vital Energi, 2020) the demand of the current system is found to be;
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CHP + Boiler 1-3 Half Hourly Energy Demand for 2020= 38,548 MWh
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Peak Demand of CHP + Boiler 1-3 for 2020 = 15.7 MWh
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Average Demand for 2020= 4.4 MWh
The current system had to be modelled to determine the size needed for the new system, it has a capacity of 27.3MW with peak demand of 15.7MW and an average demand of 4.4MW. Using the data, the two technologies could be sized to accommodate the demand, the new system must be able to supply the average demand and have the capabilities to supply the peak demand. The demand data showed the current system was oversized; it was decided the new system would be sized to accommodate the current demand rather than replacing the oversized system. A combination of energy conversion and storage technologies will be utilised to create a system that can supply the fluctuating demand.