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Geothermal Air Conditioning System for Platinum Rated Green Building

HVAC industry has seen so many changes and new ideas in last few decades. Because of the comfort requirement and resources available, day by day new alternatives are also coming to limelight. Geothermal air conditioning system is a new gift to this.

Heat Load
In the initial stages of designing, various options of air conditioning have evaluated along with different options to reduce the building load as well. This 40-year-old ACC building in Thane building was not air conditioned earlier as it was used as a staff quarter. Now when it was converted into a high-end hostel facility, air conditioning options were worked out. As a thump rule, the initial load was coming around 150 square feet/TR which was increasing the supplied electrical load for the building resulting to apply for new electric connection from state electricity board. To reduce the heat gain, first roof insulation was added with U value of 0.21 watt/sqmtr degree Kelvin. After that wall insulation has added in all side of the rooms which could further reduce the load. Then glass was changed to double glazed sandwiched type of U value 2.1 watt/sqmtr degree Kelvin. In each case 3 or 4 types of different materials have checked in thermal simulation software. The electrical fittings have changed by LED type fixtures which further reduced the heat load. Again, fresh intake locations have altered. Instead of directly taking near fan coil units, it has drawn from the window panel, landscape area where the outside temperature is already down because of water bodies in surrounding. CO2 sensors have installed in these rooms to check the fresh air level in these rooms. By doing more than 20 alterations in thermal simulation, 675 sqft/TR load have achieved.

Now, once we achieved, the load, various options of air conditioning also worked out. With the help of ACC staff, and expertise in the industry, this geothermal energy option has worked out.

Working Principle

While doing survey of the project, soil investigation and water taste have carried out and the water temperature in summer has found to be 28C (max). Now, it was almost certain to go for some system utilising the low temperature water below ground. Geothermal heat pump (chiller) collects the Earth’s natural cold through a series of pipes, called a loop, installed below the surface of the ground. Fluid circulates through the loop and carries the heat to the building. There, an electrically driven compressor and a heat exchanger concentrate the Earth’s energy and release it inside the building at a lower temperature. Chilled water is distributed to different rooms in MS pipes. Fan coil units are provided in different rooms which cools those rooms.

The underground loop draws excess heat from the building and allows it to be absorbed by the Earth. The system cools the building in the same way that a refrigerator keeps your food cool – by drawing heat from the interior, not by blowing in cold air.

The geothermal loop that is buried underground is typically made of high-density polyethylene. Open type geothermal system has carried out by drawing the low temperature water from ground by three bore wells in three corners of the building. This water kept in an insulated underground tank connected with a submergible pump. Another MS pipe carries this low temperature water (28C) up to the heat pump (chiller) and cools the condenser. The return line of this loop is connected to one more tank, which further utilised for landscape and water body.

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.

History of Geothermal Air Conditioning System
Geothermal Heating and Cooling uses the Earth’s constant temperature to achieve EER’s (Energy Efficiency Ratings) in the 30C.  The heating COP (Coefficient of Performance) is approaching 5C.  A 5C COP indicates that the Geothermal Systems are producing 5 units of energy for every unit of electricity consumed.  The other 4 come from the Earth.  That’s why this technology is called renewable. It is now widely used in different countries. In India, there is a great scope of this type of system and since last decade, many designers are showing interest in these types of systems.

A heat pump is a device that transfers energy from a low temperature source to a higher temperature sink. It differs from a pure refrigeration cycle in that the end result of the application could be either to heat or cool depending upon the direction that the refrigerant is currently flowing through the system. Figure 1 shows a schematic of heat pump system.

Open Loop Heat Pump: Open loop heat pump system contains three loops; first loop is on the load side (chilled water loop). The second loop is the refrigerant loop inside the heat pump. The third loop in the system is on the source side (Cooling water loop) in which ground water exchanges heat with the refrigerant and to the earth.

As shown in figure 2 in the first loop the chilled water is circulating between AHUs /FCUs coil inside the building and evaporator of heat pump through the chilled water pump; chilled water takes the heat from the building and transferred this heat to the low pressure liquid refrigerant into the evaporator; by absorbing this heat refrigerant changes its state from liquid to vapour phase. Refrigerant vapour then goes in to the compressor; to increase its pressure. High pressure vapour refrigerant which is carrying the heat rejected by chilled water and the heat of compression then entered in to water cooled condenser where this total heat added in to the refrigerant is transferred to the cooling water coming from the underground tank.

As shown in figure 2, the cooling water will be supplied to the underground (UG) tank by three bore wells. UG tank submersible pump (one working and one standby) will supply this low temperature water to the condenser of heat pump. After exchanging heat, the return hot water will be discharged into the ground via two bore wells and one open well or it can be used for any hot water application.

UG tank is having one level controller which will sense high and low water level into the tank to On or Off bore well pump and to regulate the water supply from the bore well to UG tank.

The heat pump machine is having in built control panel with multiple protocol interface board which will allow monitoring various performance parameters at a remote computer. Available protocols are BACnet MS/TP, Modbus, or Johnson Controls N2. The choice of protocol will be field selectable or changeable via the use of a simple selector switch.