Thursday, October 11, 2012

How does geothermal heat pump work?

Unlike what many people think, geothermal heat pumps aren't brand new technology as they have been now used for almost seven decades, starting from 1940s. Geothermal heat pumps can be used for both heating as well as cooling purposes. This is because the temperature below the earth's surface remains at a relatively constant temperature throughout the whole year, in average ranging between 45°F(7°C) to 75°F(21°C).
This relatively constant temperature means that during the winter months ground temperature is higher than the temperature of the air, making it ideal for heating purposes while during the summer months ground temperature is lower compared to air, and is therefore used for cooling.

The geothermal heat pump working principle is pretty simple, much simpler than many people think it is; in fact geothermal heat pumps basically use the same working principle like air conditioners or refrigerators, the one based on the transfer of the heat. 

In simple terms, this means that geothermal heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cooler space. This will of course depend on whether they are used for heating (during the winter) or for the cooling purposes (during the summer).

Geothermal heat pump transfer heat from the ground

There are also some notable differences between geothermal heat pumps and air-source heat pumps (such as compressors). Compressors transfer heat to or from the outside air, while a geothermal heat pump exchanges heat with the ground. This makes the work of geothermal heat pumps more efficient because underground temperatures are much more constant compared to air temperatures that usually significantly vary throughout the year.

Let us also say few words about the types of geothermal heat pumps. Geothermal heat pumps can be either open-loop or closed-loop systems. Open-loop systems draw well water for use as the heat source or heat sink, and after the use, return this well water to a drainage field or another well. Closed-loop systems use a water and antifreeze solution, circulated in a ground loop of pipe in order to extract heat from the earth. Ground loops can be installed in a vertical well or a horizontal loop. Horizontal loop is usually the most cost effective, and vertical loop is primarily used where there is little yard space or when surface rocks make drilling difficult because of high drilling costs.

Here are also few words about the geothermal heat pump economics. The installation price of a typical geothermal system is in many cases at least double the price of conventional system. In average, a geothermal heat pump system costs about $2,500 per ton of capacity, and the average household would require a three-ton unit costing around $7,500. This is not the final cost, as you also have to add the costs of drilling, and these costs can significantly vary depending on the area you live in and the quality of terrain. If for instance your system needs to be drilled vertically deep underground then we are talking about major drilling costs, in range from $10,000 to $30,000, or even more.

Installation costs are relatively high but you can very soon return your investment, in energy savings, in return period from 3 to 10 years. This return period doesn't sound bad, particularly when you consider that system life is estimated at 25 years for inside components and 50+ years for the ground loop. The additional advantages of these systems are that components inside the house are small and easily accessible which doesn’t mean difficult maintenance, and the entire system works silently because unlike air conditioners these systems have no outside condensing units.

The popularity of geothermal heat pumps continues to grow as each year more than 50,000 new geothermal heat pumps gets installed across the United States. On global level there are already more than a million installed units providing around 12 GW of thermal capacity, with an annual growth rate of 10%.