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Are Heat Pumps a ‘Good Thing’?
This question formed the basis of a talk given to the SIRACH network group 2014 (1). In my previous blog Refrigeration: The Unseen Giant I implied that heat pumps could make a contribution to our sustainable energy resources.
Energy reaches the earth from the sun at temperatures very suitable for heating of buildings. Unfortunately it’s mostly wasted, i.e. allowed to dissipate to a lower temperature until it leaves us by radiation to space. This is because it mostly arrives at times and places where it cannot be captured, or if it were captured could not be immediately used. It’s difficult to store high temperature heat. It will always migrate to a lower temperature unless prevented from doing so by insulation. Your cup of coffee always cools down. This is the Second Law of Thermodynamics. I’ve tried to illustrate energy flow to lower levels, down a temperature ladder, in the above picture.
We do manage to capture a little bit; tidal and wind power is the result of solar energy acting on our oceans and atmosphere. Coming back to a local level, we can see that if we could store, say, water, heated up by the sun to a nice warm temperature in summer, it could be used to heat indoor spaces in winter. This has been done, but a very big insulated tanks and heat gathering panels are required, not to mention pumps and controls. Solar energy can be (partially) turned into electricity which can be stored in batteries (expensive) or fed into the grid and put to use.
Heat is plentiful in winter, but at a temperature too low to be useful. A heat pump is a device that can lift it back up to a useful temperature. In attempting to examine whether this is a ‘good thing’ we need to consider several criteria: primary fuel consumption, carbon emissions, and cost. All too often these become confused in people’s minds.
Primary fuel is non-renewable resource; fossil fuel such as gas and oil. Nuclear resources are also limited but difficult to account in terms of cost and carbon. We are going to move now into a world of general assumptions to be explained as we go along. More rigorous analysis of specific systems can be applied with same approach, but simplicity is helpful for quick appreciation. So our first criteria, primary fuel, is illustrated by this slide. We burn primary fuel to release heat at, say, 1000°C subsequently use it to heat air to a comfort temperature of 25°C.
Examples are gas fire or boiler. This is illustrated by the arrow on the left. Heat of combustion at a rate of 1kW potentially gives 1 kW useful heat. In practice, boiler efficiency should be factored in, reducing the useful heat. Using our kW in a power station to generate electricity (red arrow) is the alternative. Generation and distribution losses are conservatively shown as 50%. An electric fire or storage heater would only receive 0.5kW from our fossil fuel kW, so this is a pretty poor choice. Alternatively, a heat pump may be employed to draw heat from the outside air or ground at say 5°C and lift it to 25°C (green arrow). Such a devise would typically achieve a COP of at least 3. This means 3 units of heat for every unit of electricity consumed. The net result is 50% more useful heat for every unit of primary fuel. Sounds good. I made a similar comparison of domestic gas boiler vs. heat pump that led to the conclusion that a heat pump COP >1.7 would result in less primary fuel consumption than direct use of an 85% efficient gas boiler (2). Heat pumps should do far better than that.
Next up is carbon emissions. With a similar approach, and the present mix of fuel for grid electricity generation, the heat pump would result in 0.05kg less carbon emissions than a gas boiler for every kWh of useful heat. As the electricity supply becomes de-carbonised, the HP carbon savings increase.
Cost consists of capital and running/maintenance outlays. There is little doubt that heat pump installation cost will be higher than alternatives. This is being partially addressed by government incentives designed to offset some investment cost on the basis that such investment is contributing to carbon reduction plans. Running costs are very difficult to quantify, partly because of the complexity of energy tariffs. Off peak operation is sometimes possible, and it soon becomes apparent that control systems and user behaviour play an important part. Perceived high cost is a real dis-incentive.
One concluding observation for the refrigeration industry. People buy refrigeration to maintain low temperatures. They buy heat pumps to save money. In other words, dissatisfaction with cooling equipment starts when the temperatures rise but with a heat pump complaints starts when it is found to be more costly than a gas boiler.
News! National Grid say Heat Pumps essential to meet Renewable Energy targets (5th July 2016)
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- “Heat pumps and thermal storage” SIRACH event, February 2014 (via Sirach.org.uk)
- “Pumping heat”, Proceedings of the IOR, 2004
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