|VoxSolaris: The Voice of the Sun|
More efficient heating
In our page on solar thermal systems we focus on how to capture heat form the sun to heat both water and the space in our buildings. We made the point that heat is very difficult to store without an expensive and super massive, highly lagged tank that would have to find a home somewhere. So here we are, in the middle of winter with very little or no solar thermal heating. We have got to put the heating on. So can we find a way of doing this that is more efficient and more environmentally benign than gas fired central heating or those dreadful off peak electrical radiators?
In many climates, heating a typical house can consume more fossil fuel per year than a full size car doing some 15,000 miles per year. Heating building with gas or oil is very wasteful because we are taking a fuel that is thermodynamically high grade (produces high temperatures) but only using it in a low grade application. The heat energy itself is not wasted but the energy grade is. All devices that consume energy work by lowering the grade of the energy between the input and the output. All the energy we use ends up a low grade heat but normally does useful things before it gets to that point, such as propel a car or generate electricity. With gas or oil heating we are allowing the energy to descend from high grade to low grade without doing anything useful in-between. In our page on CHP (combined heat and power) we describe the benefits of using the heating fuel to first drive a generator, producing both heat and electricity. But such systems on their own can lead to a glut of electrical energy long before our need for heat is met and anyway, what if we don't have a CHP system?
This is where heat pumps come in. These work exactly like refrigerators but in reverse. Instead of refrigerating a confined space and pumping heat out, the heat pump refrigerates the outside and pumps heat in. Heat pumps pump out a lot more heat energy than they take in. This is not a violation of the laws of physics as heat is not being created, it is simply being moved. This is the real value of the energy grade. One unit of high grade energy can improve the grade of several units of energy. The combined grade of all the energy involved will be still be lower. A heat pump has a coefficient of performance which is the ratio between energy moved and the energy expended in the operation. For any given heat pump the coefficient of performance will vary according to the temperature difference between the cold outside and the warm inside. The greater the temperature difference the lower the coefficient of performance. For typical conditions of 0 Celsius (273K) outside and 25 Celsius (298K) inside, many heat pumps have a coefficient of performance of the order of 10.
There are two types of heat pump. Those that have the outside coils in air and geothermal ground source heat pumps. These have a loop of pipes buried two or three meters underground. The pipes carry water mixed with antifreeze and the expansion coils, the cold half of the refrigeration circuit, are located in a tank (heat exchanger) through which the water and antifreeze passes. Cold water and antifreeze is pumped from the tank through the underground loop and is returned to the tank slightly warmer. This is because the ground is at a more constant temperature, typically better than -5C even if the air temperature is -20°C. This type of heat pump is much more expensive because of the cost of the underground pipes, the extra tank, the antifreeze and above all, the labor needed to install it. However the performance more than justifies the extra expense. Heat pumps perform so well that it can be cheaper and more environmentally benign, to run them on grid electricity than it is to heat space using gas - even when that electricity is produced with coal. Of course with a CHP system we can run the heat pumps even cheaper and cleaner, depending on our choice of CHP fuel and the method by which the grid electricity is generated.