Tech Directions - February 2009 - (Page 17) Heat Pumps and Geothermal Energy By Harry T. Roman htroman49@aol.com ITH the recent rapid rises in the cost of energy, we’ve heard a lot of talk about alternate energy sources, including considerable discussion of the promise of geothermal energy systems. In this article, I aim to inform technology educators about some of the underlying principles surrounding this interesting and valuable renewable resource. I also suggest some related activities for your students. W Low-Grade Heat Lacking a high temperature does not mean that an object is without heat. Temperature is simply an indicator of the intensity of the heat, not the amount of heat available. By adjusting the volume of a tub of water at 70° F, it is possible to make the amount of heat resident in the tub equal to the amount of heat in a quart-sized teapot boiling on the stove at 212° F. It is true, however, that the higher temperature of the teapot Btus gives them a higher thermodynamic quality, which means that you can do more with them. But the total quantity of heat available is the same. Consider the objects at room temperature that people routinely put into a kitchen refrigerator. A typical “fridge” will cool an object from 70° to 40° F. The heat removed Harry T. Roman is an electrical engineer, inventor, writer, and strong supporter of technology education. must go someplace, and that is into the kitchen space itself. Just place your hand behind the refrigerator’s back panel and you will feel the heat being radiated away. That kitchen refrigerator is actually helping to warm the kitchen. It is a heat source to the room. Pumping the heat outside the cold box is accomplished through the use of electricity and a compressor. This heat pump is what does the work, because the heat would not naturally flow from the food into the room. To get it to climb “uphill,” energy is needed to pump it up the energy hill. It is exactly this heat pump action with moving low temperature Btus around that forms the basis for heating an entire home. Heat Pumps Heat pumps come in a variety of sizes to heat homes and commercial structures. A common example is a heat pump used to harvest the energy in ambient winter air to heat a home. Like the refrigerator action, the heat pump takes the energy in the air and pumps it uphill into a 70° F home. It can do this with the outside air temperature as low as 38°. Below that temperature, electric resistance heating would come on and heat the home, which keeps the heat pump coils from developing an icing condition that could damage them. Many new homes are heated using this combination of a heat pump and electric resistance heating. This operation is very similar to having a giant refrigerator working at refrigerating the outside air, and rejecting the heat into your house to keep you warm in the winter. Since there is so much outside air at relatively low temperatures, there is still a great deal of energy that can be exploited—free energy put there by the sun and the seasons. Heat pumps can also operate using sources of low temperature water like lakes, ponds, rivers, aquifers, wells, or even water stored in tanks. Such systems are often referred to as water source heat pumps. Some solar thermal systems use them as well. Here, solar collectors are used to heat a tank of water, which in turn is a source of heat energy for a water source heat pump. By successively chilling down the water by extracting it’s heat via the heat pump, a house can be heated well into the evening when the sun goes down. Geothermal Systems Since the earth can easily maintain 50°-60° F temperatures several hundred feet into the planet’s crust, it can serve as a huge reservoir of low temperature energy to harness. Because this heat energy comes from the geothermal action of the earth, it is called geothermal energy. It is a wonderfully renewable energy source. To gain access to this energy, water is typically pumped down a well field, where it becomes heated. On its return, one or more heat pumps are used to extract that heat. Such wells may be anywhere from 300'-500' deep. Once the heat is extracted by the heat pumps, it is interfaced to www.techdirections.com POWER & ENERGY 17 http://www.techdirections.com
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.