Three popular earth-coupling methods are most commonly employed throughout the geothermal industry. All three options take advantage of the virtually limitless renewable energy that the earth provides. Each of these three methods have advantages and disadvantages, they are discussed below. To best understand theses options first consider the general approach to taking energy from OR returning energy to the earth. During the winter months we extract stored solar energy that resides in the earth. Approximately 50% of the solar energy that strikes the earth is stored in the waters of the earth. In the summer the relatively cool earth serves as a convenient sink for the excess energy from our homes and buildings.
Closed Loops, either horizontal or vertical (as shown) depend upon an antifreeze solution being returned to the earth and exchanging energy from the average New England earth temperatures of 50°F – in winter, a cold 30°F return solution will make the earth energy flow towards the well bore (CONDUCTIVE FLOW), warming the solution. The opposite energy flow occurs in the summer.
Standing Columns, likewise, take advantage of the conductive heat transfer, but also augment their energy transfer capability by simply moving small amounts of stable earth temperature (50°F) water, “ADVECTIVE” flow. This stable temperature water is, typically located only 40-50 feet away from the bore.
Open to Re-circulation earth coupling methods simply take advantage of the stable ground water temperatures, within rock fractures or in porous earth and depend solely upon an ADVECTIVE flow into the borehole. High yield wells and a responsible method of retuning the water to the earth is required. This method is also known as a “doublet” earth coupling. As geothermal heat pump application grows each of these generalized methods have developed variants. This technical bulletin reviews the industry validated methods evaluated by third party agencies. These methods are available through the established Heating Ventilating & Air Conditioning (HVAC) infrastructure in the US and internationally. The International Standards Organization (ISO) has taken responsibility to evaluate geothermal heat pumps in its “standard ISO 13256”. Previously, the Air conditioning & Refrigeration Institute ( ARI), in the USA, had that responsibility. Today, ISO and ARI are overlapping with ISO taking the lead.
Geology
An Open to Earth recycling system requires large amount of water and a responsible return of the water to the earth where its energy is renewed by solar contribution. (Every 55 days, the waters of the earth absorb as much solar energy as all of the know oil and gas reserves in the world). Wells in the northeast are not often able to provide the high flow rates as required to utilize this beneficial method – and often a return or diffusion well is not practical. Under local or State environmental controls, return has been allowed to surface waters owned by the user. The geology south of Plymouth MA and Long Island NY can lend itself to this advantageous method.
A Standing Column Well system depends upon near-surface bedrock, this is defined as bedrock being within 150-200 feet of the surface. The bedrock provides enhanced heat transfer and requirement for costly steel/plastic casing to keep the borehole open in the “overburden” above the stable bed rock. Approximately 65% of the U.S. meets this criteria and approximately 80% of the northeast qualifies. The geothermal designer must know the rock types and densities and heat transfer characteristics of the rock.
Closed Loops, in a likewise manner, the designer must know the earth or rock types, moisture content and thermal characteristics of each to design an effective earth coupling method. Non-biodegradable plastic piping and earth coupling grouts are utilized in this earth coupling method. Non-toxic antifreezes and antifreeze additives must be employed in the plastic loops to prevent heat pump heat exchanger or loop freezing during winter operation.
First Costs
Most often, the Open/Recycle method is realized in areas where there is abundant nearsurface water. Wells are not deep and pumping is achieved with modest pumping costs. If employed return wells are generally similar shallow depths and slightly larger, but also at low costs. Open wells are often in “unconsolidated” aquifers, loose gravel and sand, and require steel or plastic casing to maintain bore hole integrity. Casing is typically required in “overburden” and terminal moraine (where the glacier stopped)
Standing Column Wells are employed when there is near-surface bedrock and require depths of 50 -100 feet of water column per ton (12,000 btu/hr) of heating of cooling requirement. (The typical home maybe in the 3 to 5 ton range). Well depths are deeper, consequent costs are higher. However, the cost of a return or diffusion well is not required as the return water to the earth is returned back to the same borehole. Typically, domestic water needs are met by the same well. This can positively effect “first cost” as the well can provide for both needs.
Closed loops are the most costly as bore depths are typically 150-200 linear feet of bore per ton for a heating dominated geothermal application (vertical application). For a cooling dominated, typical of a very large home or commercial, bore depths are in the 220 – 280 linear feet of bore per ton. Horizontal loops, including slinky, straight horizontal, and pond applications typically require 1,000ft or more of pipe per ton.
Maintenance
Each of the three methods should check their “liquids” every 1 ½ to 2 years.
The Open Systems and Standing Column Well systems are often employed in conjunction with domestic water systems and should be maintained in a sterile state at all times. Any open well system, whether used for a heat pump or only domestic, should be free of harmful bacteria (e.g. fecal coliform) should be checked periodically. Iron bacterial (e.g. ganlionella) is not harmful to humans and is often man-induced. The iron bacterium causes “red-brown” deposits in toilet bowels and in pipes. If the deposits are not controlled, pipes can eventually become occluded. Geothermal heat pump heat exchangers are not effected, as they are copper-nickel alloy and are heated well above bacterial killing temperatures (>130°F) during air conditioning periods.
Closed Loop systems can develop oxygen entrainment and can react with the antifreeze solutions and their additives to develop and acid concentration. These are easily checked with litmus or other pH evaluation methods. Some closed loop systems have automatic make-up water devices in the event the loop should develop a leak. These systems must be periodically verified for proper antifreeze concentration and pH (Acid activity).