Analyzing Geothermal Closed Loop & Open Loop Systems

Geothermal Closed Loop & Open Loop Systems The concept of GHPs (Geothermal Heat Pumps) was first used during the latter part of the 1940s. The pumps employ the earth's constant temperature as their medium for exchange, rather than the temperature of outside air. Ground temperature will be more than that of air over it in winter, whereas in summer, the ground will be cooler. GHPs make use of this phenomenon -- they exchange heat energy with the ground using ground heat exchangers.

A ground loop system can be either open loop or closed loop. The former has one or more wells, while the latter system comes in three forms -- pond/lake, horizontal, and vertical. It is a lot easier to affix an open loop system. Designing of closed loops necessitate substantial installer knowledge about the system. An open loop system has the advantages of steady EWT (entering water temperature); therefore, the temperature of well water remains fairly constant throughout the year. The system's output heat capacity depends on EWT.

The system extracts heat energy from the water that enters. Hence, if EWT is high, it will have greater capacity of heating up one's home (and as a result, will run more efficiently). A closed loop system, on the other hand, will forever circulate the very same brine (freeze-protected liquid). This liquid extracts the earth's heat; in the late winter season, EWTs are able to reach temperatures of thirty degrees.

This paper will be in the form of a study of the specifications, showing efficiency and heating capacity of a GHP unit of four tons. Introduction GHP, sometimes called GeoExchanges, are earth-coupled, water- or ground- source pumps that have been utilized ever since the latter part of the 1940s. They employ the earth's constant temperature, rather than external air temperature, as their medium for heat exchange.

This enables the systems to attain quite high efficiencies (between 300 and 600%) on extremely cold winter nights, as compared with air-source pumps (175-250%) during winter (Energy.Gov, n.d.). While several parts experience extreme seasonal temperature -- from sweltering heat during the summer months to below-freezing temperatures in winter -- some feet underneath the surface of the earth, the ground temperature will be fairly unchanging. Based on latitude, the temperature of the ground will range between 7°C (45°F) and 21°C (75°F).

Just like in caves, ground temperature will be more compared to the air over it in winter months, and cooler compared to the air during the summer season (Kavanaugh & Rafferty, 1997). GHPs make use of this phenomenon -- they exchange heat energy with the ground using ground heat exchangers. Despite high installation costs linked to geothermal systems, as compared to air-source systems having identical cooling and heating capacity, the extra costs return to the buyer as energy savings during the course of the next 5-10 years.

The lifespan of the system is estimated to be more than half a century for ground loops and about half that time for internal components. A ground loop system can be either open loop or closed loop. The former has one or more wells, while the latter system comes in three forms -- pond/lake, horizontal, and vertical. Which of the two proves most effective depends on soil conditions, climate, local on-site installation expenses, and available land.

All the aforementioned approaches may be applied in commercial and residential buildings (Energy.Gov, n.d.). Closed-loop Systems Design A majority of closed-loop GHPs circulate some 'antifreeze' solution along a plastic closed loop, immersed in water or buried underground. The role of heat exchangers is heat transference between the closed loop's antifreeze solution and the heat pump's refrigerant. The loop may be in any one of three configurations: pond/lake, horizontal, or vertical (Goldscheider & Bechtel, 2009).

A variant of the above approach -- direct exchange -- pumps the coolant via copper tubing buried underground in a vertical or horizontal configuration, rather than using heat exchangers. Such systems need a bigger compressor and are most effective in moist soil conditions (at times, additional irrigation is required for maintaining soil moisture). However, one needs to avoid its installation in soils that can corrode copper tubes. As such systems circulate the coolant via the ground, their use may be prohibited by some local environmental regulatory authorities.

Horizontal Such an installation is often most economical for residential complexes, especially new constructions in which ample land is available. It entails digging of 4-foot-deep trenches. The most commonly used layouts have two pipes, of which one is buried six feet deep, while the other is buried four feet deep. Another common layout has two pipes that are placed one beside the other five feet deep, within a trench of two feet width.

The Slinky pipe-looping method enables incorporation of more pipe-length into a trench of lesser width (Energy.Gov, n.d.), lowering installation expenses and making horizontal installation feasible in places where traditional horizontal applications won't work. Figure 1: Horizontal Closed loop system (adopted from Energy.Gov, n.d.) Vertical Schools and big commercial buildings typically make use of vertical systems, owing to the prohibitive land area that would be needed for horizontal loop installation.

Vertical loop systems are also applied in places where the ground is extremely shallow and disallows trenching; these systems will minimize disturbances to the existing landscaping. In case of vertical structures, roughly 4-inch-diameter holes will be drilled at a depth of 100-400 feet, and around 20 feet from one another. Two pipes are lowered into the holes; the pipes are linked together at the base using a U-bend. This forms a loop. Vertical loops are connected using manifold horizontal piping set in trenches.

This will be attached to the building's heat pump (Energy.Gov, n.d.; Kavanaugh & Rafferty, 1997). Figure 2: Vertical Closed loop system (adopted from Energy.Gov, n.d.) Open-loop System Design Such a system makes use of surface or well water as its heat exchange liquid, which directly circulates throughout the system. After one circulation, the water is returned to ground via the well, surface discharge, or recharge well. This alternative is clearly feasible only if one has ample supply of fairly clean water.

Further, all local regulations and codes pertaining to groundwater discharge must be abided by (Energy.Gov, n.d.). Figure 3: Multi-well open loop system (adopted from Energy.Gov, n.d.) Open-loop GHPs can employ various techniques to get rid of used water. One way is using surface drainage; in this technique, water is deposited to some river, pond, or other low area. Re-injection is another way of disposing of water. In this technique, water gets pumped back to its source via an independent discharge well.

During the return of water to the ground (earth), one should take care not to generate any pollution. The sole difference in original and disposed water from the GHP must be a slight temperature variation (Lund, 2001; Energy.Gov, n.d.). Prior to open loop installation, it is essential for one to ascertain if the water source contains sufficient water for powering the GHP. While wells normally do have water in requisite amounts, they may end up depleting the well source of a neighbor.

Thus, one must check with local contractors to determine if ample water exists for installation of open-loop GHP. Open Loop System This system has the simplest configuration of all. Successfully employed for many decades, the system works as follows: ground water obtained from aquifers passes through the heat exchanger of the pump, and is subsequently discharged. After the water exits the building, one can dispose of it using any one method mentioned below (Ohio Water Resources Council, 2012).

One must remember that local regulations and codes can impose restrictions on discharge techniques. 1. Surface drainage -- Water is directed to a pond, stream, lake, river or other low area. 1. Re-injection -- The water gets pumped back to the very aquifer it was derived from. 1. Sub-surface -- Water is directed to a special drain field whose size depends on the quantity of water the pump requires.

Multiple Well Open Loop An open loop system normally includes at least one supply well, together with a minimum of one discharge, diffusion, return, injection, or recharge well. In such systems, the production/supply well draws groundwater out of an aquifer and pumps it to a heat pump in which it plays the role of heat sink or source, in the process of cooling or heating. After it traverses the pump, the groundwater is returned, via the injection well, to its source (aquifer).

The sole difference between original and returned water will be a temperature variation (Kavanaugh & Rafferty, 1997). Usually, a capacity of between two and three gallons/minute/ton is needed for efficient heat exchange. As groundwater temperature is almost constant all year round, open loops have remained a commonly adopted option in places that permit their use. The above systems are not used as often as closed loop GHPs; however, if ample groundwater is available, one can employ them cost-effectively.

Local environmental authorities ought to be consulted if one wishes to install such a system. In some areas, the installation might partly or completely be governed by local regulations, agreements, licensing conditions or rules. Poor quality of water can cause serious issues in an open-loop system (Idaho Geothermal, n.d.). Iron content, hardness, and acidity of water ought to be tested prior to heat pump installation. Poor quality of water may lead to accumulation of mineral deposits inside the pump's heat exchanger; this may require periodic cleaning.

An open-loop GHP does no harm to the environment; the sole difference between supplied and re-injected water is a mild temperature increase. One key design consideration is the distance of the production well from the injection well. Complete prevention of water flow between injection and production wells is not essential; however, one needs to ensure all flow between these wells is low enough for discharged water to reach the supply well at nearly identical temperatures as the aquifer's temperature (Idaho Geothermal, n.d.; Rafferty, 1995).

Typically, the wells are spaced in a 200-600 feet range. This depends on natural aquifer flow rate and thickness, maximum system heating or cooling load, and standard maximum load duration. If one fails to pay proper attention to this key design factor, undesirable temperature rise may occur within the aquifer, leading to development of unwanted organisms that may increase incrustation and bio-fouling. Standing Water Column Open Loop This system is normally one deep well that is made in bedrock. It consists of a casing fixed from grade to bedrock.

From here, the well is essentially an open well of rock walls. This arrangement is most efficient when the water available is non-scaling and non-corrosive, since such water can directly be utilized in heat pumps. Here, geothermal water gets circulated in one well only. Water that is taken from the well's bottom returns to the top; it will be able to cool or heat as it moves back down to the place of withdrawal (Orio et al., 2004).

Such a heat exchanger configuration with vertical motion of water is known as standing column or turbulent well. It offers an effective and convenient method of heat transfer. Based on the Water and Energy Systems Corporation's knowledge, a water column of 50-60 feet is required per ton (nominal cooling of 12,000 British thermal unit/hour) of the building load. These wells are the customary technology in specific areas, particularly northeastern U.S. (Orio et al., 2004). Their standard dimensions are: 1500 feet depth and a diameter of six inches.

Ground water has to be plentiful if such a system must operate effectively. Installation of these wells in places with extremely deep water gives rise to excessively high pumping costs. Under ordinary circumstances, water directed towards potable building use will be replaced by groundwater of constant temperature, making the system function similar to open loop systems. If the temperature of well-water becomes overly low or high, water may be "bled" to enable restoration of the well water's temperature to usual operating range.

Conditions for bleed water discharge are different for different localities; however, they are easy, as water is never treated using chemicals and its quantity is small (Rafferty, 2004). Closed Loop Systems These systems come in various types. All types make use of an ongoing loop in which circulation of heat transfer liquid takes place. Underground geothermal loops are normally made up of tough plastic -- high-density polyethylene (HDPE) -- which is amazingly durable, yet enables efficient heat transfer.

Installers heat-fuse joints, and make connections stronger compared to the pipes themselves, when connecting pipe sections (Rafferty, 2008). An eco-friendly antifreeze solution or water is used as liquid in loops; these flow in closed systems through pipes. Horizontal Closed Loop Earth Connection This is generally the most economical when one has sufficient yard space and trenches can be easily dug. Backhoes or trenchers are used to dig 3-6 feet deeps trenches; this is followed by positioning a sequence of parallel pipes made of plastic. The trench will then be back filled.

At this point, one must be careful and steer clear of debris or sharp rocks capable of damaging pipes. Standard horizontal loops have between 400 and 600 ft. of pipe/ton. Land area needed for these horizontal loops will be between 1500 to 3000 sq. ft./ton of cooling/heating based on earth temperatures and soil properties (Rafferty, 2008; Rezaei et al., 2012). Open Loop vs. Closed Loop Systems A large number of individuals consider open loop layouts to be better than those that are closed. This contention is deceptive and not applicable to operating expenses.

Broadly speaking, one may consider both systems to be equally effective. The basic factor to take into consideration during purchase is which system suits one best. "Closed" and "open" loops refer to system source. This is where heat is rejected or extracted. Load side refers to the home or office one attempts to cool or heat. The plus-points and drawbacks of both are tabulated below.

It must be borne in mind that, for heat pumps of every water source, efficiencies exceed those of all other present-day heating systems (Rezaei et al., 2012).

Source Pros Cons Open Loop ("once-through" or "pump & dump"): Local well water is circulated across the system, followed by discharge elsewhere Economical installation (In a majority of cases) Consistent temperature of entering water Marginally more capacity needed in the late winter months Increased usage of well pump Concerns regarding water quality Extra value for water control needed Discharge water set-up / location Closed Loop (Trench or horizontal pit, horizontal bore, vertical bore,): The fluid is circulated out via ground; it returns via the GHP in an unbroken loop with HDPE pipe Control over brine/water / quality No concern regarding accumulation or scaling Less maintenance (heating) No well-water consumption No consumption of energy from the well pump Initial installation expenses usually higher Lower temperature of entering water Needs yard space Assembling a unit and directing water towards it using pipes is considered a lot easier; hence, a number of contractors prefer open loops and deem them to be more efficient.

Also, everybody does not know the installation procedures of closed loops. To design such loops, installers have to possess knowledge regarding what they are.