Assisting one of our energyPRO users modelling a newbuild multi-residential district heating project where the scheme includes PV, a ground source heat pump, electric batteries and a thermal store brought into sharp focus for me the relative cost benefits of thermal storage vs electric storage.
Thermal storage on a district heating network typically costs around £1000 per m3 – with a delta T (the difference between the flow and return temperatures of the heating system) of say 20 degrees and say 90% of the water content practically useable that’s £50/kWh of storage. A delta T of 40 would be more typical in a Danish district heating system halving the cost for the same volume of water.
Turning to batteries, a 13.5kWh Tesla Powerwall costs £5,900 equating to around £435/kWh of storage. Looking at the large-scale 200kWh Tesla Powerpack surprisingly doesn’t seem to reduce prices that much. Then when we look at lifespan the picture gets even worse for batteries. A thermal store could last 30 years or more. A Tesla Powerwall battery comes with a 10 year unlimited cycle warranty – when used in “self-powered” mode.
Obviously a battery provides a great deal more versatility than a thermal store. We can’t run our televisions and computers from a thermal store – we’re limited to space heating and hot water uses. But where in my particular newbuild modelling case above the aim of the storage is ensure that the heat pump can take electricity either from the PV or from the grid during cheap off-peak periods – it doesn’t matter whether the storage sits on the input to or output from the heat pump – the impact is the same. A store allows you to operate independently of the actual requirement for heat and so follow the electricity supply. A battery means you can operate independently of the electricity supply and follow the heat demand.
Of course whether you opt for the battery or the thermal store or both you’d need some software to control your scheme optimally. You need to be able to predict the output from your PV and the requirement for heat both based on weather data, predict electricity prices, check the status of your plant, crunch the numbers and then tell battery the heat pump and the store what to do. You might even want to bid into the electricity markets to buy or sell electricity. At this point I could introduce you to some software that can do all that and we happen to sell in the UK called energyTRADE…
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