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Green heat – district heating and energy storage

District heating networks, using gas, waste heat from power stations or heat from biomass combustion, to heat houses and other buildings collectively, are common across much of continental Europe, especially in the North. There are also some large solar-fed heat grids and many heat stores. There are even some inter-seasonal heat stores, which help to deal with variable supplies over the year, and variable demand for heat, e.g. during winter evenings. See my earlier blog.

More district heating projects are proposed. For example, ‘Heat Plan Denmark’ a study financed by the Danish District Heating Association, argues that District heating is the key technology for implementing a CO2 neutral Danish heating sector in a cost effective way. They claim that the Danish heating sector can be CO2 neutral by 2030 by upgrading and expanding the existing system, with, for example, heat pumps being used to upgrade the heat energy currently supplied and more heat stores being added. At present much of the system still uses gas as the main energy input, but they look to the use of more renewables, and more efficient waste-to-energy Combined Heat and Power (CHP) plants with flue-gas condensation. So the emphasis will shift increasingly to using large-scale solar heating, biomass /biogas CHP, geothermal energy and excess wind energy – and more heat storage.

Overall, they see district heating moving up from 46% to 70% of the market share, and suggest that the remaining heat market can be covered by domestic-scale heat pumps and wood pellet boilers in combination with individual solar heating. However, they claim that district heating combined with CHP plants and larger scale renewable energy is more cost effective than domestic-scale solutions based on more investments in the building envelope and/or in individual renewable energy solutions. More at www.danskfjernvarme.dk.

A similar conclusion emerged from a study of district heating in Copenhagen, which has the world’s largest heat network, currently fed mostly by 10 CHP plants with a total of 2 GW of heat capacity. About 45% of the fuel is from renewable sources (biomass/wastes), and that proportion is planned to expand. In addition to using geothermal heat, they are testing a demonstration solar plant to deliver solar heat to the district heating system, with a heat pump being used to raise the temperature of the water from the solar panels, or a linked heat storage tank, before the heat is delivered to the district heating network.

By contrast, we have a long way to go in the UK. Heat accounts for about 44% of UK energy consumption, mostly for heating homes and providing hot water, using individual domestic boilers – 84% of UK homes are heated by gas. This may change as and when the Renewable Heat Incentive (RHI) and the Zero Carbon Houses programmes kick in and domestic-scale solar, biomass micro-CHP and so on are taken up. But what about the larger scale and all of the waste heat from power stations?

The UK’s total demand for heat is about 800 TWh p.a., about the same as that released by all power generation/industrial processes as waste. So far, with gas being relatively cheap, Combined Heat and Power, which can reclaim much of this waste, has not lifted off very significantly in the UK, a few biomass-fired plants aside. Neither has district heating or heat storage. But with gas prices rising and concerns about emissions growing, heat reclamation and storage ideas are now being explored-borrowing from what’s happening elsewhere in the EU. For example, the Energy Technologies Institute is looking at waste heat collection and storage on a large scale. As they note ‘It is technically possible to store very large quantities of heat energy below ground in geological structures such as saline aquifers or disused mines. The heat could even be accumulated through the summer to be used during the winter. Many of the potential heat sources and storage areas are close to centres of population and could be used to support large-scale district heating schemes.

And the recent DECC Microgeneration Strategy Consultation also looks at energy storage, and in particular at Underground Thermal Energy Storage (UTES). It includes a mini case-study of a UTES installation in Sweden, which paid back the additional installation cost (compared to an oil fired system) in under four years and continues to save money, energy and carbon year on year. It reported that, as well a tapping heat from power stations, this approach “can be particularly effective to create energy clusters where excess heat from buildings with a net cooling load can be utilised as a source of heat for others nearby with a net heating load to save carbon and reduce energy consumption”.

However, DECC’s focus seems to be on the smaller scale. It added: “Work is on-going between Sweden and the UK to use similar underground thermal energy storage techniques on a domestic scale.” It commented that in the UK: “In the domestic sector there is scope to store hot water generated by renewable energy through the wider deployment of hot water cylinders.” But, it said: “74% of the circa 1.5 million boilers fitted annually are ‘combination’ boilers, so the opportunity to future-proof homes for renewable-heating technologies, through the provision of hot water cylinders, is limited.” And it noted that: “There are no plans to set mandatory requirements for the provision of hot water cylinders – there is a trade-off between the benefits of large water volumes needed to bank/smooth renewable-energy supplies and the higher standing losses from large volumes at elevated temperatures. There are also design issues to consider. Larger cylinders weigh more and take up more space and there is a greater risk of stratification. There may also be an increased threat of legionella from water storage facilities, unless the appropriate elimination steps are taken.”

So, as seem to be the norm, we are taking it very slowly and cautiously, and focusing mainly (the ETI project apart) on the domestic scale. Ideas like large-scale solar district heating are still evidently heretical. Someone had better tell the citizens of Graz, a city in NE Austria, which has a District Heating network with 6.5 MW of solar thermal input.

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