ABSTRACT

Over the last few years earth coupled water source heat pump wells for air conditioning systems, both closed and open loop, have been operational in the Texas Gulf Coast area and as a result of rising energy costs, they should be considered as an energy conservation method.  Closed loop systems have been installed in Cretaceous clays and Pleistocene sand and clays.  Open loop systems utilizing shallow ground water (fresh and brackish), although less common, have been installed in Pleistocene sands and clays.  The design criteria for these wells are based on one ton (12,000 BTU hr) (3,024 kg-cal/hr) capacity which, in the Gulf Coast area, requires a four inch (10 cm) diameter 200 foot (61 m) deep well for closed loop and more variable depth well for open loop.  Flow rates average three gpm (11 liters pm) through 1 inch (2.54 cm) diameter piping for each ton (3,024 kg-cal hr).  The major constraint of open loop systems is plugging of the return well and its capacity to accept return flows.  Thermal pollution is one of the main but least understood operational problems with closed loop systems where cooling demand far exceeds heating demand.  Open loop systems do not have the efficiency declines as the closed loop if a suitable distance separates the source and injection well.  However, due to the high carbonate content in ground water, significant chemical reactions occur during heating, cooling, depressurization and pressurization of the source water which may cause incompatibility problems.  Besides requirements of the Texas Department of Water Resources for the injection well, source wells may require incorporating existing recharge well technology and field monitoring of these systems is required to understand the long term effect of operating these systems.

INTRODUCTION

Open loop earth coupled air conditioning heat pump systems require permitting in Texas by the Texas Department of Water Resources prior to installation under regulatory statutes of the underground injection control program.  Texas further requires installation by a licensed water well driller in accordance with the Texas Water Well Driller's Act of 1965.  Additionally, local regulations from ground water conservation districts and municipalities are also imposed for design and installation of these systems.  However, these .are necessary in order to preserve and protect the ground water supplies in a ground water deficient state such as Texas.  Closed loop systems also require state permitting and may fall under municipal regulations.  The need to impose these restrictions on these functionally simple systems is apparent after the concept, mechanics, and operational problems are understood.

CONCEPT

Conceptionally the system is simple and operates under the laws of thermodynamics and specifically heat transfer.  The usual type of air conditioning/heating systems operate by utilizing heat in the winter and extract heat during the summer.  Air, however' is a poor heat transfer medium and these types of systems normally require a significant amount of power in the form of electricity, to operate.  Therefore, in order to make these systems more efficient, water is substituted as the heat transfer medium which is twenty times more efficient than air and the inherent heat within the earth is used as a partial substitution for a significant amount of the power.

In Texas, the average soil/ground water temperature of the upper 100 to 200 feet (30 to 61 m) is equivalent to the average ambient air temperature of 60 to 74° F (15 to 24 ° C).  There are seasonal variations to the soil/ground water temperature of 20° F(7 ° C) approaching a constant temperature at a depth of approximately 30 feet (10 m) where it is equivalent to the earth's normal thermal gradient.  The average soil/ground water temperature is in the range of normally acceptable comfort temperature levels for people by being higher than the air temperature in the winter and lower during the summer. 

It is this constant soil/ground water temperature which earth coupled water source heat pump systems utilize for air conditioning space in residences, offices, schools and institutions.  Even if soil temperatures rise to 90° F (32° C) during the summer or even 50° F (100° C during the winter, these systems can still utilize the 90° F soil temperature to cool air conditioning coils that rise to 180° F (82° C).  During winter, this system also operates when the air temperature is less than the soil/ground water temperature.

SYSTEM MECHANICS

The two types of earth coupled systems in operation are open and closed loop systems.  In the open loop, ground water is withdrawn by a well from an aquifer and is utilized for cooling or heating in a tube in tube heat transfer unit (heat exchanger) where it absorbs or releases heat.  The ground water is then discharged into an injection well back into the aquifer.  In a closed loop system, no ground water is utilized; the constant thermal properties of the soil/ground water are utilized to cool or heat effluent water or a water antifreeze mixture by circulating it in a continuous closed pipe loop from the soil through the heat exchanger and then back into the soil to be cooled or heated.  There are variations to these systems and combinations of both closed and open loop as well as different water sources such as the use of surface water or use of ponds, streams, leachate fields or recharge fields to discharge the used water.  Other alternatives include using horizontal trenches instead of vertical holes for the closed loop systems.  The specific type of system required, is controlled by site specific conditions such as:

·  Hydrogeology at the site,

·  Soil/Ground water thermal range,

·  Heating or cooling demand,

·  Space available for the system,

·  Economics,

·  Government regulations.

Of the several hundred operating systems in Texas, four in two very different geologic provinces are specifically described in this article.  This includes two in the upper Texas Gulf Coast where the soil/ground water temperature averages 720 F (220 C) near the cities of Austin and San Marcos.  In San Marcos, the system is a closed loop. It is installed in low permeability

Cretaceous clays and rated at 165 tons (500,000 kg-cal/hr) capacity.  Another closed loop system is located near Austin that is rated at 50 tons (151,000 kg-cal/hr).  Each of these systems is for air conditioning in schools.  In the lower Texas Gulf Coast at Houston, closed and open loop systems are installed in moderately permeable Pleistocene sediments.  These sediments consist of alternating sand and clay layers and the systems are rated at 30 tons (91,000 kg-cal/hr) for the closed loop and 16 tons (50,000 kg-cal hr) for the open loop.  The closed -loop system is for a single family dwelling of 11,000 square feet (1,000 sq m) and the open loop system is for a strip shopping center.

Closed Loop Systems - The typical design criteria used for the closed lop system requires a four inch (10 cm) diameter, 200 feet (61 m) deep well with an average flow rate of three gpm (11 liters pm) through a one inch (2.54 cm) diameter polyethylene or high density polyethylene piping for each ton of capacity.  The total number of closed loops equals the total cooling capacity in tons (3,024 kg-cal/hr).  Spacing between loops in 10 to 15 feet (3 to 6 m) and each is connected to the other in parallel.  Sand is utilized for back filling the well to enhance heat transfer from the piping to the side walls of the well.  About 50 square feet (4 sq m) per closed loop is required.

Where space is not limited, the piping can be placed in horizontal trenches three to six feet (1 to 2 m) deep and four to six feet (1.5 to 2 m) apart.  This allows for easier access and nominal cost for maintenance and repair, however, longer lengths of piping are required because of the wide variability of soil temperature near the surface.  The system requires deaerated water and water tight sealed piping that is circulated by pumps smaller than that required by open loop systems.  The circulation pumps operate concurrently with the heat pump compressor.  About 500 square feet (40 sq m) is required for trench installation of one ton (3,024 kg-cal/hr) capacity.  

Towards the end of the long Texas cooling season, soil/ground water temperatures adjacent to the closed loop can approach 1000 F (33° C).  This heat build up can be used for heating during the winter.  However, since the heating and cooling demand are not equal, over a period of time, soil/ground water temperatures are expected to rise and the thermal plume expand.

Open Loop System - The design of open loop systems is more variable than closed loop systems and is normally preferred in the rural areas because part of the system, the owner's existing water well, is already installed.  The source well depth will depend on the transmissivity of the sediments and require a pumping rate of about three gpm (11 liters pm) for each ton (3,024 kg-cal/hr) of cooling capacity.  Source wells should be normally over designed by a factor of at least two.  As the water passes through the heat exchanger an increase or decrease of 100 F (0 C) occurs. 

In order to conserve ground water supplies, the discharge water should not be discharged to surface water bodies.  Surface recharge fields or recharge wells (return flow wells or injection wells) are preferred.  Theoretically the same design for the source well can be applied to the recharge well.  However, impractical, this does not work because of inherent problems such as plugging in the recharge wells.  Therefore, recharge wells are normally designed to accept twice the expected flow by having increased well screen diameter, increased length of screen and larger filter packing material around the well.  These precautions in design lessen the friction loss, increases injection capacity and minimizes return well capacity loss problems.  To further avoid return well problems, surface mounted sand separators to remove solids prior to injection and pumps in the return flow wells to back flush the recharge well are recommended.  However, owners rarely want to incur this additional initial cost to reduce future maintenance cost.  

The distance between the source and return flow well is important in that the closer the two are together, the more the system would tend to recirculate the same water.  Therefore, several hundred feet between wells is recommended for 20 to 30 ton (60,000 to 90,000 kg-cal/hr) systems.  For both the return flow well and source well, Texas Water Commission requires compliance with water codes, licensed drillers, seals and reports for aquifer protection and water conservation.  According to Texas State water laws, the land surface owner owns the water under his property and can use it as long as he does not waste it.

CONSTRAINTS AND OPERATIONAL PROBLEMS

The main constraint common to both systems is economics and acceptability.  The capital expenditures are on the order of twice that of normal air conditioning/heating systems with the savings coming in the form of reduced operational expenses.  Approximately three to five years is required to recover the extra capital cost of these types of systems.  The public at large is sometimes leery of new ideas although this is theoretically not new.  The author has operated an open loop system over 25 years.  However, its acceptability is just now beginning because of the rising cost of energy.  As a comparison, an efficient normal air conditioning system for a single family dwelling in Houston could have a $250 operating cost per month, whereas an earth coupled system would operate at $150 for the same month (a 40 percent savings).

Closed Loop Systems - Closed loop systems have the least operational problems and because of this they are preferred.  The initial capital cost is slightly higher than an open loop system but can easily be reconciled by minimal maintenance and repair cost.  It has been alluded to that fluids other than water have better heat exchange properties, however, if leakage in the loops occurs, contamination of the soil/ground water may occur.  Therefore, systems should utilize water and specifically deionized water to inhibit carbonate build up from the ground water in these systems.  This would require 10 gallons (38 1) of water per ton (3,024 kg-cal/hr) of capacity.

The second problem with closed loop systems is the soil/ground water heat build up during the summer, coal down during the winter and gradual rise in temperature over a long period where cooling demand is greater than heating demand.  The exact effect on ground water chemistry and biological activity is not known but believed to be minimal over a limited area around the closed loops.  Other problems which proper design eliminates, is system freezing, surface water contamination of the aquifers and locating systems close to structures which may effect or be affected by the thermal changes.

Open Loop Systems - The open loop system has by far the most operational problems along with social economic and water quality constraints.  These constraints may arise in water deficient areas underlain by shallow fresh water where water use priorities become the controlling factor.  The operational problems are related to the return flow well and include deterioration and clogging.  The controlling factor is ground water chemistry of the source and recharge well.  The temperature change through the heat exchanger does not seem significant enough to cause major precipitation of dissolve constituents in the ground water.  However, pressure changes can alter the equilibrium of dissolved constituents and result in precipitation in piping and in the recharge well.  In the Gulf Coast, the major problems are from the precipitation of calcium and iron from water which is characterized as hard.

The total suspended load of the injected water is also important as just five ppm (5 mg/1) total suspended solids at 20 gallons per minute (75 l/min) is equivalent to injecting five (2 kg) pounds of solids in four days.  This will tend to clog the screen sand pack and even the formation.  Therefore, when total suspended solids are high in the source water, sand separators at the surface or filtering is recommended.  Clogging by accelerated biological activity in the return well happens when the water is heated and depressurized to liberate oxygen.  Therefore, care must be taken to maintain a pressurized system with periodic chlorination to minimize biological activity.

Over a period of time, return well injection rates are expected to decline.  Unless these wells are equipped with pumps to back‑ wash and redevelop them on a periodic basis, conditions can reach a point that even a special work over of the well will not be very effective and a new well would be required. 

Acidic source water and cathodic activity will also cause deterioration of the recharge well system by attacking the metallic parts.  Thus when possible, PVC wells are utilized or cathodic protection is provided.  In addition to these problems, common problems associated with typical water wells also occur for the source well. 

The slow steady build up of heat in the soil/ground water of the return flow well is not as dramatic as with the closed loop system, but it does occur.  The dissipation of the heat is directly related to the velocity and quantity of ground water that flows by the well.  For a system of 20 tons (60 kg-cal/hr), the thermal effects would not be noticeable 100 feet (30 m) away for an indefinite time.

CONCLUSIONS

Earth coupled closed and open loop heat pumps are a viable alternative to existing systems and operates based on the following:

• The heat pump is a cost effective alternative when considering the rising energy costs of today; effectively saving 40% when compared to electric air conditioning methods. 

• The heat pump is effective when there is a significant difference between the soil/ground water temperature and air space to be heated or cooled temperature.