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If you've ever experienced the coolness of caves, cellars or even large above-ground stone structures such as European cathedrals during the hot summer months, you've exposed yourself to the dynamics underlying the concepts behind passive annual climate control systems. The cooling action you experience in these enclosed environments is a result of the heat being drawn away from your body to the surrounding air which transfers this thermal energy into the surrounding structures whose heat content is less than that of the adjacent air mass. The key concept to remember is that heat always flows from a warmer system (your body in the aforementioned example) to a cooler system (the surrounding air and walls). So if you are "warmer" than the surrounding air, the heat of your body will escape to the surrounding air until a temperature equilibrium is reached. Likewise, if the air inside the room is warmer than the surrounding walls, heat will be drawn out of the air into the walls, thus cooling the air (and warming the walls). Conversely, if the air inside the room is cooler than the surrounding walls, heat will be drawn out of the walls into the air thus warming the air (and cooling the walls). Passive Annual Heat Storage, PAHS for short, (term I believe was originally coined or at least rendered mainstream by John Hait) uses this thermodynamic principal in conjunction with bare earth to help control the climate within a man-made structure. For example, an earth sheltered dwelling will use the surrounding earth to regulate its temperature throughout the year. Such passive systems require an effective heat storage mechanism. Typically, materials with a high heat capacity such as earth or granite are effective at storing heat for a relatively long period of time. Heat capacity is a measure of a material's ability to store heat. This may be a misnomer since materials do not actually "store" heat like a 55 gallon drum stores water. Instead, heat is constantly flowing within the material from a hot region to a cold region until it reaches a seldom reachable equilibrium. What gives the material the apparent ability to store heat is the rate of heat flow. The longer it takes for heat to flow through a material, the longer this energy remains within that material. So in essence, stone walls of the cathedrals in Europe or the earth from bermed homes, slow the transfer of heat to a crawl thus foregoing the release of that energy at a later time. The key in effectively utilizing the storage mechanism of these dense materials is timing. If properly designed, a structure can be made to absorb the heat during the day and released at night for diurnal heat exchange, or, for long term use, heat can be stored during the summer months and released during the winter months. The same concept can be applied to warm regions of the planet whereby the stone walls or soil can be cooled during the night (or winter months) then used to extract heat from the room during the day (or summer months) A good example of the diurnal use of such system is the Iranian wind tower.

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Online resources

Glossary useful for this site
Bo Atkinson's PAHS research (external link)
Earthshelters website (external link)
Basics of earth's radiation budget
Tom's PAHS home
Aerogel: future of glazing?
Passive Cooling Systems in Iranian Architecture

Some articles of interest
  • Yogi, G.D. and A.S. Dhaliwal, "Heat Transfer Analysis in Environmental Control Using an Underground Air Tunnel." Journal of Solar Energy Engineering, Vol. 107, pp. 141-145, May 1985.
    This paper investigates some exergoeconomic parameters for an underground air tunnel system based upon some operating conditions. The ratio of exergy loss rate to capital cost (Rex) changes between 0.052 and 0.552. The total exergy losses values are obtained to be from 0.26 kW to 2.50 kW for the system. The daily average maximum cooling coefficient of performances (COP) values for the system are also obtained to be 11.96 for experimental period, while the total average COP is found to be 5.89. The overall exergy efficiency value for the system on a product/fuel basis is found to be 56.9%.
  • Yogi, G.D. and S. Ileslamlou, "Performance Analysis of a Closed-Loop Climate Control System Using Underground Air Tunnel," Journal of Solar Energy Engineering, Vol. 112, pp. 76-81, May 1990. ABSTRACT
    A passive summer cooling technique that utilizes the underground soil temperature has application in climate control of residential as well as agricultural buildings. The soil temperature stays fairly constant at a depth of eight feet or more. Earlier studies have shown the usefulness of this technique for an open-loop system. However, the previous analyses in the literature did not evaluate the usefulness and limitations of this method for closed-loop air conditioning. In this study an analysis of the "Coefficient of Performance"(COP) of a closed-loop system, based on the above technique, in combination with a conventional air conditioner, has been done. In this system, the cooling needed to neutralize the heat gain of the conditioned space is provided by the air cooled in an underground air pipe in combination with an air conditioner. The underground air tunnel is used for hot parts of days and is off for cooler parts of days and nights. The analysis has been done by a computer model solution, using central finite difference method. When the system is on, the air temperature and the soil temperature are calculated. When the system is off, the heat is transferred within the soil and a new set of soil temperatures around the pipe are calculated for the next day. As the soil temperatures around the pipe increase, the COP of the system decreases. The COP is calculated for each hour until it decreases to the COP of an air conditioner. This shows us the length of time for which the underground cooling method will be useful. Since the knowledge of soil properties is very important, a computer model solution has been developed to predict the soil thermal properties by using an approximate analytic method based on simple temperature measurements.
  • Perrill, C.V. "Performance of Passive Cooled Hospital Complexes in India over a Period of 55 Years." Proceedings of American Solar Energy Society's Solar 99 Conference, and the Proceedings of the 23rd National Passive Solar Conference, Portland, Maine, June 1999.
    This paper reports the Thoburn-Perrill research on climate-oriented buildings and their walk-through air-conditioning tunnels, along with a record of their living and working in these houses and hospitals over a period of 55 years. Also, it provides a short description of much larger, radioactive fall-out shelter tunnels constructed as refuges for patients and hospital workers. The research is based on the Heat-Decay curve developed by Professor W.C. Thoburn in 1940. The tunnel system allowed the conventional air-conditioning system to be downsized by 80%. The operating costs were 20% of the conventional equipment.
  • Ozgener, L. and Ozgener, O. "Energetic performance test of an underground air tunnel system for greenhouse heating" Energy, Vol. 35, pp. 4079-4085, Oct 2010.
    The main objective of the present study is to investigate the performance characteristics of an underground air tunnel (UAT) for greenhouse heating with a 47 m horizontal, 56 cm nominal diameter U-bend buried galvanized ground heat exchanger. This system was installed in the Solar Energy Institute, Ege University, Izmir, Turkey. Based upon the measurements made in the heating mode, the average heat extraction rate to the soil is found to be 3.77 kW, or 80.21 W/m of tunnel length, while the required tunnel length in meters per kW of heating capacity is obtained as 12.46. The entering air temperature to the tunnel ranges from 14.3 to 21.5°C, with an average value of 15.5°C. When the system operates, the greenhouse air is at a minimum day temperature of 13.1°C with a relative humidity of 32%. The maximum heating coefficient of performance of the UAT system is about 6.42, while its minimum value is about 0.98 at the end of a cloudy and cold day and fluctuates between these values at other times. The daily average maximum COP values for the system are also obtained to be 6.42. The total average COP in the heating season is found to be 5.16.

Last Modified on Tuesday, January 27, 2015