Thoughts of a Lapsed Physicist

Part 2 of 2: ground source (‘geothermal’) heat pumps and geothermal’s potential

Air source heat pumps, well known and widely used, deliver heat to a house or other building that is drawn from outside air. The problem with such heat pumps is threefold: they sit outside and are exposed to weather elements, they use electric-powered air conditioning, and when heat is required and the outside air gets cold enough the heat pumps are effectively electric heaters. This increases electricity demand on the grid and can cause brownouts, as happened during a cold winter snap on the U.S. East Coast a number of years ago.

Ground source heat pumps, in contrast, exchange energy (heat and cool) with the ground (or underground aquifers), taking advantage of the fact that once one gets at least 10-12 feet below ground the ground’s temperature in most locations is reasonably constant at about 55F. Thus, a ground source heat pump can cool in the summer and heat in the winter (basically a reversible refrigeration cycle) and can be located inside a building where it is shielded from weather effects and more rapid aging.

Other advantages of such heat pumps are reduced peak demands on utility grids and reduced consumer energy costs. The disadvantage for consumers is the need to drill (and pay for) holes for the heat exchange with the ground, either deep vertical holes with heat exchanger tubing or more shallow holes with more numerous heat exchange loops of tubing. The tubing, filled with heat exchange fluid, is grouted to the earth to enhance heat exchange.

Another problem is that many heating and cooling contractors are still not familiar with ground source heat pumps and do not bring them to consumers’ attention. This was particularly true in the mid 1990’s when I had responsibility for DOE’s ground source heat pump program. There was some history – a ground source heat pump had been installed in a hotel in Kentucky in the 1980’s and its energy demand was shown to be less than that of an identical hotel in the same location outfitted with a traditional air exchange heat pump – but no commercialization took place. My response, once I understood the benefits of ground source heat pumps, was to initiate a joint program with the electric utility industry to educate people about ground source heat pumps and facilitate their deployment. This led to establishment of the Geothermal Heat Pump Consortium, now known as GeoExchange. One important utility response was to advance the money to homeowners for drilling the heat exchange holes, a major barrier to heat pump deployment, recognizing the utility benefits from peak power reductions. Consumers repaid the loans from the savings on reduced energy bills.

Today more than 600,000 ground source heat pumps are installed in the U.S., with new installations occurring at about 60,000 per year. The U.S. Defense Department is an increasingly heavy user of ground source heat pumps and pioneered in their use in the late 1990’s by installing 4,003 ground source heat pumps in military housing at a base in Louisiana. A side effect of the U.S. program was the introduction of ground source heat pumps to China in 1998 when I first went to China on a government-to-government visit. Today China is incorporating ground source heat pumps into many if not most of its new buildings.

Geothermal’s potential: hydrogeothermal’s potential is geographically constrained (conditions that make water circulate to the surface are found in less than 10 percent of the earth’s land area) but further growth is anticipated. One example is co-production of geothermal electricity in oil and gas wells that also produce lots of hot water.

EGS’s (enhanced geothermal systems) potential is huge, given that hot rock underlies all parts of the earth, but deep drilling is expensive and risky. At present the U.S., Japan, France, Germany and Australia are supporting EGS R&D programs.

Familiarity with ground source heat pumps is increasing and most new buildings, as we progress toward zero energy buildings, will probably incorporate them.

Direct uses of geothermal heat in space heating (and even cooling), agriculture and desalination are also increasing as people get familiar with their advantages and wide range of applications.

To close this discussion I would like to speculate on one other way to possibly use geothermal heat – tapping the hot fluids being expelled from hydrothermal vents (also called ‘Black Smokers’) at spreading fridges on the ocean floor. They are the result of cold seawater leaking through fissures in the ocean crust into hot magma below the crust, being heated and reemerging as hot water vents enriched with dissolved minerals (sulfur, copper, zinc, gold, iron). These minerals deposit out on the ocean floor when the heated water (some of these vents reach temperatures of over 700F) hits the cold seawater, creating massive deposits which obviously will attract commercial attention. The energy content of this hot water is also immense and represents one way of tapping the heat energy in magma. Needless to say, tapping this heat energy requires operating at great depths in oceans under extreme conditions, possibly bringing the hot water to the surface, and trying to do all this in a reliable and cost-effective manner. Intriguing, given the amount of energy potentially available, but obviously not on any anyone’s short- or mid-term agendas.