Geothermal air conditioning comprise water-cooled package DX unit or water-cooled chiller, condenser pump and condenser piping, and finally, heat rejection well to dis sipate the heat in the ground. The first two are well-known in the air-conditioning industry, while the last one is a puzzle and a big unknown for our industry that has forced most of us to shy away from this excellent power-saving and water-saving air-conditioning application.
We are blessed in the gulf as well as in the Red Sea coastal areas with high water tables that are few meters deep (2 to 7m). This by itself is the key enabler for geothermal air conditioning.
We have three types of geothermal wells: closed loop well, standing column well, and open loop well.
Closed loop wells comprise a series of wells, typically 6-inch in diameter spaced 6m apart and can go as deep as 150m with a limited heat rejection capacity per well of 2 to 4 Tonne of cooling. In other words, each 100m of pipe is capable of rejecting heat of 1 Tonne of cooling or around 1.25 Tonne (4.4 kW) of heat rejection.
Closed loop wells do not require to hit ground water and it may be recommended for small applications such as individual homes in inland areas such as Al Ain, Riyadh, Madinah Al Monawarh, and Qassim.
The standing column well comprise a single well around 250m and up to 450m deep with pump installed at the bottom of the well and its return at top of the well. It has higher heat rejection capacity of 20 and 27 Tonne of cooling for 6-inch and 8-inch pipe, respectively. Ground water is not a necessity but if available it will improve capacity. It will require to bleed 10% of the water to avoid heat buildup and that can be used for irrigation if it is not too salty.
Standing column well is often referred as a hybrid between closed loop and open loop system. Because of the extreme depth of the well, the drilling technology is highly specialised and expensive.
The open loop system comprise two wells one for intake with submerged pump at the bottom and another well to return the water to the same aquifer. The well requires to hit ground water and have the highest heat rejection capacity. In the gulf, the depth can vary as shallow as 10m to as deep of 150m. The 6-inch borehole can have 60 Tonne capacity and larger bore holes of 10-inch or 12-inch has been used with up to 400 Tonne cooling capacity.
It is ideal for the gulf where we have shallow water table and which requires large cooling capacity.
Hydro-geological research data and thermal testing data is where future investment is required in a knowledge-based economy that puts energy-efficiency improvement as a priority.
Air conditioning dominates the electric power consumption in the gulf and why geothermal is needed is because it reduces the lift of the compressor and reduces the power from a typical 1.4 KW/Tonne for air-cooled system down to 0.71 KW/Tonne for geothermal cooled system. Radiant cooling that utilises 14 degree C water under tiles and behind walls require high-density polyethylene (HDPE) plastic piping that can further reduce compressor power to 0.58 KW/Tonne and eliminate fan power estimated at 0.1 KW/Tonne.
The best thing to share about geothermal is a home-grown success story.
Three luxury towers, Al Rashid, situated on Jeddah corniche next to Lamar Towers (under construction) where they have a shallow geothermal system of three intake wells of 10-inch and one discharge well of 14-inch handling 3 x 400 Tonne of air conditioning via indirect heat exchangers located at the base of the tower. In the building, recirculating closed heat rejection circuit circulate between the water-cooled packages located in each floor and the ground level heat exchanger. Inside the residential tower the air distribution is like any other package A/C with typical duct work and grilles.
The writer is George J. Berbari, the CEO of DC PRO Engineering – a leading electro-mechanical consultancy firm specialised in District Energy Services and Green Buildings MEP design. He is also the author of the book, The Energy Budget.