Using TES with lower temperature heating systems

The UK heating industry is going to be experiencing huge changes during the 20s as we attempt to decarbonise our homes, putting the plumbing and heating installer at the heart of the solution. The government wants to see a mass deployment of heat pumps to the tune of 600,000 annually by 2028, which will require the upskilling of over 30,000 installers. To achieve these challenging targets, the plumbing and heating industry will need to overcome both economic and technical barriers, especially in the early part of the decade when demand is outstripping supply. Following the announcement of the government's 10-point plan, the demand for heat pumps has increased significantly, which has highlighted the real shortage of skilled and trained heat pump installers. This has unfortunately contributed to higher installation costs.

The innovation gap

Whilst the industry tackles the economic impact of a shortage of installers, we cannot ignore the technical challenges and barriers that need to be overcome to achieve the net zero goals. The government has recognised an innovation gap and is launching a fund to help accelerate innovation.

An area which the CIPHE believes will gain traction is the development of new thermal batteries, using materials such as phase change materials (PCM). We also expect to see further innovation into thermal batteries that capture renewable electricity during low heat demand to be used many days later during peak demand. A company called Caldera has developed the Warmstone, which is a patented solid storage material that is made from a combination of recycled and natural materials. The technology uses cheap, green night-time electricity to heat the solid core of the Warmstone. This stored heat then provides a supply of heating and hot water to your home during the days when you need it most, with up to 100kWh of available energy.

What is thermal energy storage (TES)?

Thermal energy storage (TES) is achieved with a wide variety of different technologies. Depending on the type of technology, it allows excess thermal energy to be stored and used hours or days later, such as heating and hot water in domestic homes. The most common type of thermal energy storage is a buffer tank that contains a specific volume of water at a given temperature. Whilst this is relatively cheap to buy, the amount of usable energy available within the tank varies based upon the difference between the maximum tank temperature and the minimum required flow temperature of the heating system. The lower the flow temperature of the heat emitters, the greater the amount of usable energy in the tank at a fixed storage temperature. Figures 1 and 2 illustrate the difference in a TES usable energy with two different flow temperatures, 45°C and 35°C, with the same system ΔT.

The effects on the available energy in the thermal energy store with weather compensation control

The amount of usable energy within the thermal store will vary throughout the year with a low-temperature heating system controlled by weather compensation. Depending upon the outside air temperature, the weather compensation controller adjusts the flow temperature to the heat emitters to meet the required demand, which in turn increases and decreases the available energy within the TES. For example, the radiators might be designed for peak cold periods at a 55°C flow temperature at an external temperature of -3°C, however, the heating system might only require a flow temperature of 45°C at 1°C which would lead to a greater amount of usable energy within the TES. By reducing the flow temperature from 45°C to 35°C with a fixed system ΔT, you can double the amount of usable energy or reduce the tank size.

Why do we use a TES?

There are a variety of reasons for installing a TES in a heating system, such as avoiding the short cycling of heat pumps at low load conditions or adding additional capacity to the heating system during peak heating demand. A TES will also act as a form of hydraulic separation, decoupling the primary circuit from the secondary, however, the focus of this article is to understand the concept of usable energy within a given volume of water at a given minimum and maximum temperature. If in any doubt always consult the TES unit’s manufacturer for guidance and support to validate your assumptions.

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