Heat pumps, powered by the sun, have proved to be more efficient than their electric counterparts. Bob Harper explains.

I grew up in a time when energy was plentiful. In fact, my bed was about 120m above the coal mine in which my grandfather worked.

The theory back then was simple; if you need heat, burn some coal. If the coal wasn’t convenient enough, let the power station down the road burn coal and deliver the electricity to your house, where a simple resistor converted the electricity back into heat.

Very nearly 100% of the electricity would be converted to heat, but unfortunately only around 33% of the coal energy was converted into electricity. Between the powerhouse and home there was between 5% and 15% loss in the line and distribution system.

I don’t intend going into the ecological argument around burning coal. That’s a whole other discussion.

So, what was wrong with electric heaters if they were so efficient? Well the problem is we didn’t need to generate the heat because it was already there, all around us. What we needed to do was move the heat from one place to another.

Think of heat energy as sand. It already exists, so when we need sand we scoop it up in one place to pile it up where we want it. And so it should be with heat.

I was recently told that an electric hot water system (HWS) is around 70% efficient, but by comparison, a heat pump is 280% efficient! Where does this apparently free 210% come from?

Remember, the efficiency of the electric HWS is based on converting electricity into heat. The heat pump on the other hand uses the heat already delivered from the sun. It takes a lot less energy to move what already exists, around a quarter in fact.

Homes have had refrigerators for a hundred years; the world’s first gas refrigeration plant was invented by James Harrison of Melbourne in 1850.

Most of us believe we know the basics of how refrigeration works. Take a gas and compress it into a liquid. Remove the heat from the compression, allowing the gas to condense into a liquid using a condenser. Transfer the liquid to where you want it, and allow the liquid to evaporate back into a gas in an evaporator. In evaporating and expanding, the gas absorbs energy, heat energy, making the space around it colder. From there it goes back to the compressor and the cycle is repeated until we have pumped as much heat energy out of the refrigerator as we needed to.

Remember that heat is transferred from a hot body to a cold body. Therefore to collect heat, the collector must be colder than the heat source, and to impart heat, the radiator must be hotter.

More recently, air conditioners have become common and most are now reverse-cycle, meaning that they cool the house in summer and heat the house in winter. To achieve this simple miracle, the system uses condensers and evaporators that are interchangeable, a changeover valve that swaps the condenser and evaporator, and an expansion valve designed to act in the appropriate direction, by various means.

Even when the temperature outside is below 0ºC, the outside air can provide usable heat as long as the air is dry. Wet air will simply freeze to the evaporator, as the air is drawn below zero.

There is a popular misconception regarding the meaning of zero degrees (0º). The term ‘zero degrees’ (0ºC or 0ºF) is a remnant from old measurement systems based on water or alcohol thermometers. Once scientists realised that heat is energy, they began to plot the temperature at which there is no energy in a material.  All materials seem to have zero energy at a temperature Lord Kelvin called ‘absolute zero’ or zero Kelvin (0K), which is -273.16ºC or −459.67° F.

In fact, scientists originally used refrigeration techniques to get down to the coldest temperatures they could, but had not reached that magic 0K (-273.16ºC) until recently. A report came out in Nature (3 January 2013) that physicists had used lasers on potassium atoms to cause the atoms to effectively go below 0K. Negative temperature? Negative energy? No doubt there will be more said about that as time goes by. Maybe absolute zero is no longer absolute.

For energy harvesting purposes, even frozen materials can be made colder, and therefore heat energy can be drawn off, but of course it’s easier if there is a lot of heat to be harvested. Higher temperatures generally mean that there is more heat available. Remember that heat and temperature are not the same parameters. Heat is a measure of energy as is electric charge, whereas temperature is a measure of thermal pressure more alike to voltage.

The concept of a heat pump has been around for a long time, but has only recently become recognised as an energy efficient method of producing home heating, cooling or hot water.  Common technologies currently use the simple reversed refrigeration cycle as shown in Figure 4 to draw the heat from the outside air and thus heat the inside air, or hot water system. The layout of typical heat pump HWS unit is drawn in Figure 5.

Heat pumps for an HWS are just starting to become known and accepted, but other similar technologies have been common place overseas. In Arizona for example, home heating systems incorporating the air conditioner, HWS, swimming pool and geothermal, or ground heat reserves, are all combined to shift heat from one area to another, removing excess heat from inside the home to be used to heat the HWS or pool water.

Heat management is essentially energy management, and it is more efficient to manage heat energy than to discard excess, and then make more when required. Industrial applications are also becoming more apparent as industry strives to conserve energy, and thus save Carbon Tax. While it is the engineers designing energy management systems, electricians need to prepare for even more applications of the simple heat pump.