In his new book for the Institute of Physics, power engineer Paul Woods offers an ‘Introduction to District Heating and Cooling Low carbon energy for buildings’. He says that, for the UK, ‘a number of studies have estimated the economic potential for heat networks to be between 18% and 43% of the total heat demand’ and adds ‘whichever figure proves to be correct this still represents a huge increase in capacity from the current market share of around 3%’.
District Heating (DH) is not a new idea. There are already many heat networks around the world, including large whole-city systems in Moscow, Copenhagen, Helsinki and Stockholm. While the earliest district heating schemes used steam as the heat distribution medium, for example those in Copenhagen and New York, Woods says ‘it is now universally preferred to use hot water’.
He notes that heat nets are not a power source as such, just a way of transmitting energy, and use can be made of many types of heat inputs, including heat derived from renewable power. That includes electricity from wind and solar (possibly used to drive a heat pump). Other options include direct solar heat (possibly with a large solar heat store), geothermal heat and heat from burning biomass, plus of course heat from burning fossil fuels like gas and even possibly, Woods says, direct heat from prospective local small modular nuclear reactors. But not for a while! Waste heat can also be extracted from various industrial processes and Woods also looks to ‘opportunities for heat extraction from gas CCGT, energy from waste, or biomass power stations fitted with CCUS’(i.e. with carbon capture)
He looks in detail at the basic engineering options for district heating and cooling (DH/C) and at many examples of current projects of all sorts. District cooling systems are few and far between and a relatively new thing so far, whereas he says that ‘the development of DH in Europe was given fresh impetus in the 1970s as a result of the two world oil price rises. New sources of heat had to be found urgently’. But he says ‘in the UK, we were more fortunate as we had access to North Sea gas which has met most of our heating needs for almost 50 years. We had little reason to develop DH when fuel was plentiful & cheap. As a result, our energy system continues to be inefficient. Despite the rise in wind & solar energy, thermal power stations (gas, coal, biomass, waste and nuclear) still account for 63% of our electricity supply & the heat lost in power generation is equivalent to the heat required by all of the domestic buildings in the UK.’
There is an alternative to the wasteful way in which fossil fuel have been converted into electricity. As Woods explains, Combined Heat and Power (CHP) ‘co-generation’ plants make use of some of this waste heat to feed DH nets, so, depending on how you do the z factor CHP power-conversion loss sums, it can actually be a more efficient way to use fuel to generate heat than heat pumps. But the economic and practical viability for DH depends on the output heat temperature you need and, as far as climate issues are concerned, the carbon intensity of the input fuel. Heat pumps do need a reasonable input ambient temperature differential, but they can make very efficient use of zero carbon green electricity, and large heat pumps are even more efficient. Heat pumps can also be used reversibly for cooling.
Heat pumps are nowadays certainly seen as preferable for most home heating with many possible applications including for flats, although it can depend on the location: if there is large source of waste heat nearby, e.g. from a power station, it make sense to use it. Woods notes that ‘in the past, gas-fired CHP was of benefit because it displaced both coal and gas power stations so CO2 reductions were obtained from both the efficiency of CHP and fuel switching from coal to gas’. However, he says, while ‘in recent years, CHP has been displacing gas-fired power stations... in the future, as the grid decarbonises, the benefits of CHP will decline further as the hours in the year when its operation will lead to CO2 savings will be curtailed. As a result, Government policies such as the Green Heat Network Fund are not supporting small-scale gas-fired CHP. Nevertheless, a number of existing DH schemes use gas-fired CHP as their primary heat source so it is important to understand this technology and how to assess its benefits.’ He does indicate that biomass is also now quite widely used as fuel for CHP/DH projects, and they can be near carbon neutral, or with CCS, carbon negative, so CHP is far from dead- it’s important to capture the waste heat too.
He looks briefly at the hydrogen home heating option and adopts the now fairly standard view that heat pumps are a better bet: ‘About six times as much electricity is required if hydrogen boilers are used compared to electricity and heat pumps’. But he does note that ‘when comparing DH for a large city area with individual building heat pumps or hydrogen it is worth noting that there may be costs for upgrading of the electricity and gas distribution networks if these options are to be pursued across the area, even if it is difficult to assess’.
Looking some more at large complete systems, he says it can make sense to have large heat stores linked to big CHP and DH projects, and that, whatever the heat source, energy storage (e.g. via Pumped Hydro, hydrogen, as well as heat) can help sustain power grid stability. He suggests that DH/CHP, especially with storage, could play a flexible role ‘helping to manage the challenge of increasing amounts of variable renewable electricity generation’ e.g. with varying inputs from heat pumps and CHP, balancing variable green power inputs and also matching variable demands. He also notes a UCL study that suggested that, in a district heating system heavily reliant on heat pumps, storage become very valuable, so as to avoid the high cost of having a system able to meet the occasional peak heat demands- see UCL’s recent study for more.
Overall, he backs DH strongly, with heat pumps seen as helping to maintain flexibility, as well as, in some contexts, cooling. But, in addition to backing energy storage, he also sees CHP as having a role. Thus he says that, to maintain flexibility, ‘there is a case for increased amounts of thermal energy storage’ and concludes that, given its efficiency and system stabilisation potential ‘gas-fired CHP may still have a valid role into the 2030s’.
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