Yale Carden explains the advantages of using geoexchange technology for temperature control in green buildings In 2007, the Centre for International Economics (CIE) identified that space heating and cooling—that is air conditioning—accounts for 30–40 per cent of power consumption within a typical building. The magnitude of this contribution to global environmental and energy issues has resulted not only in the rapid evolution of the ‘green’ building sector but also the search for high-efficiency heating and cooling systems.
The direct and quantifiable benefits of green buildings and high-efficiency heating and cooling systems include reductions in electricity bills and greenhouse gas emissions. Demandside management benefits such as energy security and reduced reliance on backup power generation, especially during peak air conditioning periods are another, although a less readily quantifiable, benefit. The most interesting and valuable benefits are indirect and often even less quantifiable, at least with the current financial models. They are becoming more apparent as green buildings and energy efficiency become more popular. These include: Reductions in building maintenance; Improved air quality leading to increased workplace productivity Increasing long-term value of the property Hence, the benefits of adopting energy efficiency measures extend beyond simple payback calculations. Lifecycle costing, including some of the less readily quantifiable benefits, is the key to addressing both environmental and energy concerns associated with the built environment.  Background to Geoexchange Geoexchange systems are often referred to as ground heat exchangers, ground-coupled, ground source heat pumps, low temperature geothermal and more. The term ‘geoexchange’ is being increasingly adopted as the systems operate through a heat exchange process with the ground or water bodies that have been warmed by solar radiation (47 per cent of the sun’s energy that reaches the Earth). The systems do not produce electricity from geothermal activity.
In most localities across the globe, depths greater than two to three metres provide a stable temperature environment that is approximatly equivalent to the average annual air temperature for that location. For example, the average ground temperature in Chennai is approximately 29–30°C, in Mumbai it is approximately 28–29°C, in New Delhi it is approximately 25–26°C and in Leh it is approximately 5–6°C. Lund et al (2004) estimated that 1.1 million geoexchange systems have been installed worldwide since the earliest installations in the middle of last century. These systems currently provide 12 GW of thermal capacity and the growth rate of the industry while historically at approximately 10 per cent per annum, has shown a significant increase in the past decade. More recent estimates indicate the presence of over 2 million systems worldwide. How Does it Work? The two main components of a geoexchange system are the ground loop or ground heat exchanger and the Ground Source Heat Pump (GSHP) or geothermal heat pump. The ground loop provides the passive component of the system whereas the GSHP provides the mechanical component and thus the temperature control within the building.
Ground loops can be classified as open or closed. Closed-loop systems use a polyethylene (PE) pipe circuit to circulate water through the ground or water body. Ground loop systems use either vertical U-tube pipes in boreholes that could be up to 200 metres deep or horizontal loops laid out in trenches or pits at several metres deep. Closed water loops submerge the PE pipe in a body of water such as a river, harbour or lake. Open-loop systems utilise a local body of water such as a lake, stream or aquifer to provide almost constant temperature water to the GSHP. Industrial process water and treated effluent have also been adopted in open loop geoexchange systems. Once utilised, the water is returned to its source or used for a secondary application such as irrigation. A hybrid geoexchange system is typically installed at places where there is insufficient land area available for a full-capacity ground loop, in cases of financial constraints or in cases where the building load is heavily cooling-dominant. The two types of hybrid are: A ground or water loop being used in conjunction with a chiller/boiler system; or GSHPs operating in a boiler or cooling tower system with no ground loop. The second component of the Geoexchange system is the GSHP. It receives all the water returning from the loop and transfers heat to either hot/cold air via ducts (water to air GSHP) or as hot/cold water for hydronic heating where it maintains the temperature of pools, spas etc (water to water GSHP). Geoexchange and LEED Geoexchange systems can play an integral part in the achievement of LEED ratings by providing up to 19 points in the Energy and Atmosphere category under Credit 1 - Optimise Energy Performance.
A selection of some case studies and projects identifying the benefits of geoexchange systems in LEED-certified buildings are provided below. These range from government and institutional buildings to commercial and residential applications. A common theme in all these projects is the proper integration of the geoexchange systems into the building. This includes passive design features and the building fabric itself. Another feature of these buildings and the decision to opt for geoexchange in them was the direct financial benefits, which contributed to the final selection of the technology.
Aldo Leopold Legacy Centre The building recognised as the Greenest Building in the world by the US Green Building Council (USGBC) is the Aldo Leopold Legacy Centre in Baraboo, Wisconsin. It has utilised ground-source heat pumps to provide radiant heating and cooling to help it become the highest-rated USGBC LEED Platinum new construction project yet completed.
Utah State Botanical Centre, Wetland Discovery Point Although the LEED Platinum rating of this building at Utah State University was assisted through the adoption of a geoexchange system, the savings on the cost of extending the natural gas service was of equal value to the client.
 Fort Polk, Louisiana: Defence Housing and Barracks Although not LEED-certified, the Fort Polk installation is over 6,000 tonne and provides heating and cooling to 1,300 buildings. It saves the US Department of Defense over $3,000,000 per year and has a simple payback period of 5.5 years. The US Department of Defense is the largest installer of geoexchange systems in the world and is fitting installations on many of its bases both in the US and across the world. It considers the technology to be a key contributor to its long-term energy conservation and energy security goals.
Key Benefits of Geoexchanges Systems As noted above with respect to LEED ratings, there are numerous benefits associated with the technology. Some of the benefits of a GeoCooling geoexchange system are as follows:
Reduced energy costs of between 30 and 70 per cent Reduced CO2 emissions Regulator- and rating-friendly for energy and water for both LEED and GRIHA Reduced maintenance time and costs Longer system life with GSHP replacement anticipated at more than 20 years and the ground loop at more than 100 years No outdoor equipment such as cooling towers or condenser units makes for longer system life, improves building aesthetics and reduces external noise. No need for architectural facades and other structures to hide plant rooms Compatible with Building Management Systems Inbuilt redundancy through modular and staged unit installations Reduced space requirements as some of the plant room is located underground; and Reductions in peak electrical load can reduce power supply mains as well as reduce investment in backup and emergency power supplies. Applying Geoexchange in India There is a broad range of applications for geoexchange systems in India. While many applications in India will be cooling-dominant, the heating component of the system is very important in providing some thermal balance in the ground loop. Fortunately, due to the flexibility of this technology, the heating component can be hot water for domestic use, kitchens, laundries and even pools within the building and room heating.
To accommodate the cooling-dominant nature of most buildings and limited land area available for ground loops, hybrid systems can be widely deployed. The typical hybrid system would require a ground loop that was capable of meeting at least the heating load of the building with fluid/air coolers installed for peak air conditioning periods. In addition to energy and maintenance savings, reductions in peak loads will be of value in areas with a strained energy supply or where backup diesel generators are in common use. Closing the Loop Geoexchange systems can provide significant benefits to the Indian building sector in terms of energy savings, maintenance savings and longer life of systems. The key in the Indian market is to identify those buildings where the benefits of the technology can be optimised from both an energy and a financial perspective. These are likely to be systems up to 500 tonne that have a mix of heating and cooling and have long operational hours. These could be entire buildings or strategic sections within larger buildings that may benefit from the technology.
The author is the Director, GeoCooling Technology Pvt Ltd and GeoExchange Australia Pvt Ltd References Lund, J, Sanner, B, Rybach, L, Curtis, R and Hellström, G (September 2004): “Geothermal (Ground Source) Heat Pumps, A World Overview. Geo-Heat Centre Quarterly Bulletin (Klmath Falls, Oregon: Oregon Institute of Technology) 25 (3): pp. 1-10. ISSN 0276-1084. http://geoheat.oit.edu/ bulletin/bull25-3/art1.pdf. Retrieved 28 June 2010.
The Centre for International Economics, Capitalising on the building sector’s potential to lessen the costs of a broad based GHG emissions cut. 2007, CIE. |
