Geothermal Heat Pump Systems and Earthtube Systems
Geothermal heat pumps (GHPs), sometimes referred to as GeoExchange, earth-coupled, ground-source, or water-source heat pumps, have been in use since the late 1940s. They use the relatively constant temperature of the earth as the exchange medium instead of the outside air temperature.
Although many parts of the country experience seasonal temperature extremes — from scorching heat in the summer to sub-zero cold in the winter—a few feet below the earth’s surface the ground remains at a relatively constant temperature. Depending on latitude, ground temperatures range from 45°F (7°C) to 75°F (21°C). Like a cave, this ground temperature is warmer than the air above it during the winter and cooler than the air in the summer. The GHP takes advantage of these more favorable temperatures to become high efficient by exchanging heat with the earth through a ground heat exchanger.
As with any heat pump, geothermal and water-source heat pumps are able to heat, cool, and, if so equipped, supply the house with hot water. Some models of geothermal systems are available with two-speed compressors and variable fans for more comfort and energy savings. Relative to air-source heat pumps, they are quieter, last longer, need little maintenance, and do not depend on the temperature of the outside air.
A dual-source heat pump combines an air-source heat pump with a geothermal heat pump. These appliances combine the best of both systems. Dual-source heat pumps have higher efficiency ratings than air-source units, but are not as efficient as geothermal units. The main advantage of dual-source systems is that they cost much less to install than a single geothermal unit, and work almost as well.
Earthtubes, earth–air heat exchanger (EAHE), is another promising technique which can effectively be used to reduce the heating/cooling load of a building by preheating the air in winter and vice versa in summer. In the last two decades, a lot of research has been done to develop analytical and numerical models for the analysis of EAHE systems. The method to calculate the earth’s undisturbed temperature (EUT) and more recently developed correlations for friction factor and Nusselt number are used to ensure higher accuracy in the calculation of heat transfer. The developed equations enable designers to calculate heat transfer, convective heat transfer coefficient, pressure drop, and length of pipe of the EAHE system. A longer pipe of smaller diameter buried at a greater depth and having lower air flow velocity results in an increase in performance of the EAHE systems. (https://geothermal-energy-journal.springeropen.com/articles/10.1186/s40517-015-0036-2)
Earthtubes (EAHE systems) are a highly recommended low-tech, sustainable, non-electric, zero-energy, geothermal passive solar heating and solar cooling systems. Earthtubing utilizes conventional, thin wall plastic sewer drain vent pipe to passively pre-heat your home’s fresh air intake with zero-energy consumption. Filtered fresh air enters a series of non-porous pipes embedded around the interior of your home’s foundation, absorbing energy from the surrounding soil, moderating the temperature of fresh air intake. When done correctly, air drafts naturally through your earthtubes for a truly sustainable, non-electric, zero-energy, passive geothermal system that perfectly supplements a high thermal mass HTM home’s back-up heating and cooling system. (https://www.thenaturalhome.com/earthtube/)