Thermal stores – the answer to getting the most from renewable energy

Thermal stores are the key to getting the best from renewable energy. Here Mark Wozencroft, Managing Director of Minus 7, explains how they work and the benefits.

Energy storage

A crucial part of getting the most from renewable energy is the correct storage of the energy you have harvested. This is affected by many factors including the storage medium, the size and shape of the storage vessel, and insulation properties.

There is much industry chatter about electrical direct current (DC) storage and inter-seasonal stores. As everyday solutions DC storage, although practical, is still in development and may be so for some time. Inter-seasonal storage, which would theoretically store heat from summer to winter, is currently impractical due to the inability to minimise heat loss over time.

Water way to go

There are various ways to store heat, but water – as well as being the most economical – also has extraordinary properties of thermal storage. It is energy dense, and can store large amounts of energy by volume – more than granite or concrete, for example – and can do so at any temperature, as opposed to phase-change materials that can only store at a set temperature.

So it is strange that most of the industry believes that using water as a medium for heat storage is not the answer. They believe that incorporating a large thermal store into their system is a disadvantage, and would rather find another way to do it.

A big enough store

However, for thermal storage to work effectively, what we must have is an adequately-sized store. There needs to be a balance between its ability to hold a large enough amount of energy, and being practically sized.

If well designed and produced, a tank not much bigger than a Smart car, with dimensions of 2900 mm (L) × 1700 mm (W) × 1600 mm (D), could easily hold up to 42 kWh. If constructed from fibre-glass with an inner and outer skin and insulated with 200 mm of polyurethane, it would only lose around 3 kWh in the worst-case scenario of midwinter.

Connected to a solar thermal system offering up to 14 kWh per hour in summer, or around 6 kWh per hour in winter, this would be sufficient to heat an average family home and supply adequate hot water.

Size matters

When sizing a thermal store, the heat requirement of the building and its occupants should be considered. This can be correctly calculated and specified by an mechanical & electrical (M&E) consultant. To give a couple of generic examples: an average row of family houses might require a room temperature of 21°C, whereas sheltered housing might require a room temperature of 23.5°C, so the size of thermal store would vary for this.

The key to calculating the correct size of a thermal store is to work out the amount of heat required to heat a building and provide hot water, then balance this with the next-best opportunity to replenish the thermal store, and what are the costs per kWh.

The optimal shape for a thermal store would be spherical, as this shape offers the minimum surface area for heat loss. However, as thermal stores are most often buried, a cube-shaped thermal store is generally the most practical solution.

Excellent economy

Thermal stores really are a fantastic implement for future-proofing against the rising costs of home heating and hot water. And as they need no ongoing maintenance, through-life costs are also excellent. If you are able to collect heat energy economically, then storing it in a thermal store will allow you to remove the peaks and troughs of usage. Thermal stores enable renewable energy systems to deliver affordable heat and hot water, heavily reducing the carbon footprint.

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