Geo - Exchange HVAC Systems

This article examines the various technologies developed for reaping the benefits of natural thermal properties of the Earth and look at ways to integrate with the regular HVAC solutions to get better energy efficiencies.
During seasonal changes the temperature of air changes from one extremity to the other and the variation is over a time period of day or gradually over the season. Due to variation in temperature there is increase or decrease in running efficiency of all HVAC systems, be it the ones dependent on air cooled condensers or water cooled systems with cooling towers. Geo-exchange systems on the other hand use a constant thermal body Earth to give you higher energy efficiency levels.
Earth with its huge mass is a neutral source of thermal energy for cooling or heating purposes. During summers as the temperature of earth is lower than the atmospheric temperature it can be used for cooling purposes where as during winters the earth temperature is higher than the atmospheric temperatures, it can be used for heating purposes.
The temperatures with in earth are negligibly affected by variation in atmospheric temperatures. While going down into earth the upper 5 to 8 feet is effected by the seasonal changes with a time lag of a few days to months the variations are from 3 to 6°C, but as we progress down with in the range of 8 to 20 feet this variation reduces down to a change of 2 to 4 °C over a time period of a few months based on lag in transfer of thermal gradient through earth. After this the temperature remains nearly stable with in a variation of 1 Degree as we progress in to the earth and nearly constant after a depth of around 30 feet.
Thus in most parts of India requiring cooling the average temperature in earth below 8 feet depth is around 25 °C with a variation of around two degrees either way with respect to geographical conditions. For checking the temperature of Earth the simplest procedure is to check the temperature of water coming out of the bore well in your local area.

GEO EXCHANGE HEAT EXCHANGERS

As in the case of all HVAC systems one requires to make a heat exchanger to extract the thermal temperature from ground. geo-thermal heat exchangers can be broadly divided into two categories, first using water circulation as the thermal heat exchange medium and second using air as the thermal heat exchange medium. The same have been briefly discussed below-

WATER BASED GEO-THERMAL HEAT EXCHANGERS

Ground Water:

The simplest method is making a bore up to the deep water table source and extracting water which is at the temperature of earth and then passing it through a heat exchanger for removing heat from a refrigerant, water or air as may be the case. Generally depending on the efficiency of the heat exchanger the temperature of refrigerant, water or air can be brought down with in a variation of 2 to 3 °C of the ground water temperature. This option involves extraction of water, circulation through a heat exchanger and re-injecting it back through an injection well. This system could require governmental approvals since in some places there are restrictions with respect to extraction of water through borings, This system is most efficient and gives the best results at lowest initial investments.

Vertical Closed Loops:

The process involves making bores within the ground and lowering of two tubes jointed at the bottom with a U - bend into the bore and then refilling it with the grout / slurry of the soil extracted and plugged with slurry at top. The bore has to be without a casing so that once filled back the pipes surface now comes in contact with earth through out its depth and works as a heat exchanger. Water or a solvent based solution is filled in these tubes and circulated in a closed loop interconnecting it with a heat exchanger connected to a chiller or a heat pump. Irrespective of the fluctuations of the day and air temperatures during the peak season this gives a stable returning water temperature with in a variation of 2 to 3 °C of the Earth temperature. For instance, if the temperature of earth is 25°C then the returning water temperature of 28 °C can be achieved by proper designing of the earth heat exchanger and this would remain nearly constant and not vary with the increase in summers or day temperatures. As this option has a closed loop, negligible requirement of refilling of water is required. Further, since the bore is not used for extraction of water there is no impact on the water table. It is highly efficient since the temperature of earth below 30 feet remains stable and boring can go down to depths of 100 to 400 feet. The vertical loops are interconnected in parallel to reduce pumping loads. A closed loop system provides a uniform stable thermal exchange option and gives maximum efficiency. It also involves minimal amount of excavation. The costs of execution shall vary depending on the costs of boring in different earth strata. Since water moves in a closed loop there is no refilling required once air is purged from the closed loop. Again since the system is underground there is no effect of high humidity during rainy seasons on the efficiency as in the case of cooling towers.

Horizontal Closed Loops:

This option is to excavate land to a depth of 10 to 12 feet and lay the pipes horizontally. In this system various configurations of pipe layout are available where it can be placed as straight pipes or in circular roles placed in a slinky form on the ground, forming a heat exchanger and the same is then interconnected with the condenser of a chiller or a heat pump. The pipes are connected in parallel through headers to reduce pumping loads, and water is circulated in the closed loop. The earth is then filled back with proper compaction and the land can be used for horticulture purposes or for parking, roads etc as the need may be. In this case also if the earth temperature is say 25 °C a constant returning water temperature of 28 to 29 °C is achievable irrespective of changing extreme summer temperatures. This also holds true for winters and for locations such as hill stations where heating is the major requirement now the returning water temperature is higher and as such we get nearly free heating.
In locations such as hill stations where heating is the dominant requirement with cooling required for a limited time period in a year the same pipes can be placed at a depth between 4 to 6 feet since the average earth temperature is lower than the plains. Thus, the storage of heat in the top layers of earth during summers is extracted during the colder climate to give heating to the buildings.

Integration of Water Based Geo-Exchangers with HVAC Systems.

The water coming out of a geo-exchange system is circulated through a water cooled condenser to remove heat during summers since the water is at a constant temperature of earth which is generally lesser than the atmospheric temperature and the wet bulb temperature during day. The efficiency of the condenser is improved which in turn improves the efficiency of the complete HVAC system. This results in more BTU's of thermal cooling delivered for the energy consumed. This additional amount of BTU's delivered can be termed as the cooling provided by Earth.
During winters this water is at a higher temperature it provides heat thus when circulated through the heat exchanger it now adds heat energy to it.
Generally it is seen that the efficiency of the HVAC system coupled to a geo-exchange heat exchanger improves from 10 to 20 % over a time period of one season depending on manner in which it is used.
Locations where Cooling Tower is more beneficial during certain time periods of the day i.e. evening and night time the geo-exchange system can be used in series or parallel with a cooling tower to take advantage of both the systems to get maximum efficiency levels.
These underground heat exchangers are best suited to be used with Ground-Source Heat Pumps since they are designed to provide cooling during summers and work in reverse during winters thus taking the maximum benefits of the earths thermal properties. Heat pumps when coupled with geo-exchange systems are very energy efficient with Energy Efficiency Ratio of 13 to 22. Climate Master one of the major manufacturers of heat pumps in the world has come out with water-source heat pumps giving EER of up to 27 at part load when coupled with geo-exchange systems.

AIR BASED GEO-THERMAL HEAT EXCHANGERS

We shall now examine the use geo - exchange systems with the option of just running air through the earth in larger diameter tubes.
The process involves laying pipes varying in diameters from a few inches to a feet or more and then circulating air through it this air when it comes in contact with the internal surface of the pipes attains a temperature with in the range of 2 to 3 °C of the earth temperature. The Pipes layout and length is again designed after calculating the full day load the properties of the material used in pipes and the nature of the surrounding soil. These pipes are placed at an depth of 10 to 20 feet below ground in trenches or open excavations which are then filled back here provision is made for cleaning these pipes and manholes are provided accordingly. Provision is also made for removal of any water accumulation caused due to humidity in air.

Geo- Exchange Space Cooling / Heating:

The system is used for directly conditioning the internal temperature of the building space. In this the pipes are laid in a closed loop with a circulating fan placed with in the loop, the internal air is re-circulated through the ground loop and the temperature slowly reduced over a number of circulations. Again the principle is same - temperatures variation of 3 °C from the earth temperature is achievable with in the closed environs. This is very useful in areas where the ground temperature is low and can thus provide HVAC solutions at a very low running cost. Generally people make the mistake of not designing the system or the air circulation system properly or use it in locations where the earth temperatures are high and then are not satisfied. Thus these systems should be properly designed for the best results.
For hilly regions circulation of air through earth tubes provides heating and is very effective.

Fresh Air Pre-Cooling:

The second use of this system is for pre-cooling of fresh air which is a major energy consumer in commercial and office buildings. In this case instead of a closed loop system fresh air is forced in at one end and then circulated through the earth pipes and allowed to mix with internal air at a mixing point on the other end of the earth tube. Again in this case the length and diameter of earth tube should be based on material of pipe and soil conditions and should be properly calculated for the total load over the time period of the day. Provisions for moistening the soil around the pipes should be made for in dry soils to expedite the heat exchange. This process can again reduce fresh air temperatures to with in 3 to 5 °C of the earth temperature and thus give substantial energy savings.

Conclusion

As discussed above one feels that using of Geo-Exchange system can be of immense benefit in reducing the running energy loads in HVAC systems. This reduction is going to increase over the years as the cost of power supplied increases. Since there are no moving mechanical parts in the ground heat exchangers, no maintenance cost is involved. Normally it has a lifetime span of 50 years and more.
Geo- Exchange systems in association with proper planning of the building for thermal heat gain control and insulation of the building can reduce the total running energy demands by 40 to 60%.
It shall also be worthwhile to note that today Carbon Credit options are available for the use of such systems where the energy saved is certified against a bench mark and the corresponding carbon dioxide reduction is calculated this then is calculated as carbon credits which can then be sold at lucrative prices. To sum up, a geo-exchange system can give benefits in two manners, firstly reduce running costs and secondly help you make earnings through sale of carbon credits year after year.


Ishvinder Singh Gill
BSc. B.E. Civil.
Consultant- Energy Efficient Construction Technologies
Building Insulation Systems
Geo - Exchange HVAC Solutions.
Geo-Exchange based Fresh Air Pre-Cooling Solutions.
Address: G -108, Phase -1, Ashok Vihar, Delhi -110052
Contact: 9811274373; E. Mail: insulbuild@yahoo.co.in