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Low & Zero Carbon: No more trouble in store on the heating front

Most renewable systems are delivering far less than the running costs and environmental savings they could, according to Alan Ward. The good news is that he has an answer.
Low & Zero Carbon: No more trouble in store on the heating front
There will be a tenfold increase in renewable heat in the next decade as a result of the Renewable Heat Incentive, according to Greg Barker, Minister at the Department of Energy and Climate Change. This means a huge investment of public funds and an equally large investment from householders, companies and public bodies in the equipment and installation to provide the systems. But will it be worth it?

The answer is 'maybe'. We believe that most renewable systems are delivering far less than the running costs and environmental savings they could; often as low as 50 per cent of those savings. These are not guesses. We are regularly called in to jobs after the first year where the customer has found no improvement in running costs; indeed, sometimes they are higher.

How could this happen? First, many of the owners do not know that the systems are not working to their full potential. The contractor or designer has done his job in putting in the system promised. All the kit is working to the manufacturer's spec. But a system that is only working at 50 or 60 per cent of its capability is making nonsense of the investment.

What is going wrong? A car analogy might help. The customer wants to be green and cut his running costs so he invests £40K in a hybrid car. He then finds the mpg is half what was promised. 'Engine's fine,' says the garage. 'So is the electric drive. Your problem is that the controls, gear box and wheels are not designed to work with a hybrid drive! Sorry.'

Of course, a single automobile design team does work on the entire project. But, what's missing in heating design is a way of integrating the heat inputs from renewable systems, boilers or other back up appliances, to deliver the maximum savings for the customer. Sounds easy, but in practice it's not. The challenge is that each renewable source behaves differently; over time and over the seasons. Overlaying that is the demand pattern the building and its users have for heating, and especially hot water use, through the day. Think of a hotel and leisure centre with underfloor, fan coils and radiator circuits and multiple hot water outlets with high morning and evening demands. How can the designer fit this to, say, solar, heat pumps and condensing boilers?

The conventional answer is a large heat store buffer, since water is an excellent way to store heat. However, this approach can conflict with the optimum performance of the renewable systems. For example, if the buffer has solar panel input, with a condensing boiler backup, a low or zero solar input can trigger the boiler, raising the tank to its highest level - say 60 deg C - and when solar energy is available the store is satisfied. No solar benefit. Challenges galore, but where is the answer?

My company went down the route of applying heat store technology. The system we have used, developed by Ratiotherm in Germany, applies the simple principle we all know from school that water at different temperatures will stratify into layers.

Inhibiting layering

But with most stores the flow and return connections and the coils within the store inhibit layering because the water is mixed and a closer to uniform temperature is the result. Inside the Ratiotherm vessel, known as Oskar, is a five-chamber layering insert. Each layer has a flow discharge from the level and out through the base of the vessel, and therefore the temperature gradient, to suit the water's purpose.
So the DHW flow is drawn from the top level to maximise the water temperature with the transfer to the hot water supply through a plate heat exchanger. The return from the exchanger is connected close to the bottom of the vessel and so finds its own temperature level without diluting the temperatures above.

This means:
• The solar input can run up to the maximum and well above the temperature for DHW supply.
• The DHW water temperature can be safely controlled through the heat exchanger to the required maximum safe level (47 deg C).
• The returned water can, in effect, be used twice as its temperature might suit the heating circuits at medium or lower flow temperatures.
• High temperature water is available for anti legionella purging of the DHW on a preset regime; usually weekly.
• The whole vessel can be safely taken to a maximum of 95 deg C making full use of solar energy when available.

Sensors inside the vessel detect the temperatures at each level and, should the DHW level require top up, the standby boiler is fired and by motorised valve, the flow is directed into the top level only until the required temperature is achieved. This means that gas use is minimised for DHW purposes.

The same principle adds to the benefit of the solar collection. On days with only small amounts of sunshine the flow from the panels might only be 40 deg C, not enough for DHW but fine for the low temperature circuits such as underfloor heating or low temperature radiators.

Heating typically might be provided by a mixture of a condensing boiler alongside a heat pump. Here two varying criteria for energy efficiency are called for.

The condensing boiler needs to run at as low a return temperature as it can to maximise condensing operation.

The heat pump needs to have the highest COP possible to improve its operating economics.

The layering system achieves these objectives by using the lowest temperature layer as the return to the condensing boiler so keeping the exchanger at the flue gas dew point (57 deg C).

A modulating control linked to the Oskar's sensors keeps the boiler delivering heat at the required heat flow temperature without cycling on and off. Say 40 deg C for underfloor or 65 deg C for fan coil or radiators.

The heat pump inputs into the middle of the vessel, but the return can be drawn from a level which has benefited from the heat return from the solar or boiler and so boosts the COP by requiring a lower temperature lift. The heat pump also can benefit from lower electricity tariffs, perhaps at night, raising the temperature of the whole volume of the vessel.

So far we have looked at a common project with just two inputs; solar and heat pumps. Over time and on larger projects further inputs, CHP and biomass might be needed. The system can accommodate these at a later stage without the need to change the vessel.
11 February 2011


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