Passive annual heat storage principles in earth sheltered housing, a supplementary energy saving system in residential housing

Earth sheltered homes, refers to structures built within or with the support of earth as superficial material, and mostly built in functional styles devised to meet the needs of common people in their time and place. These structures in the past were built by people not schooled in any kind of formal architectural design or with identifiable building techniques rather they depended on the cover the very structure of the earth could provide them for purposes of shelter, warmth and security. However, recent quest for energy saving and efficiency in buildings has redirected the eyes on the earth material not only as support, but also as massing for passive energy utilization indoors.

Passive annual heat storage principles in earth sheltered housing, a supplementary energy saving system in residential housing Akubue Jideofor Anselm * Green Architecture Department, School of Architecture and Urban Planning, Huazhong University of Science and Technology, Wuhan 430074, China Received 7 July 2007; accepted 3 November 2007 Abstract This paper looks through the many benefits of earth not only as a building element in its natural form but as a building mass, energy pack and spatial enclosure which characterized by location, unique physical terrain and climatic factors can be utilized in developing housing units that will provide the needed benefits of comfort alongside the seasons. Firstly the study identifies existing sunken earth houses in the North-west of China together with identifying the characters that formed the ideas behind the choice of going below the ground. Secondly, the study examines the pattern of heat exchange, heat gains and losses as to identify the principles that makes building in earth significant as an energy conservation system. The objective of this, is to relate the ideas of sunken earth home design with such principles as the passive annual heat storage systems (PAHS) in producing houses that will serve as units used to collect free solar heat all summer and cools passively while heating the earth around it and also keeping warm in winter by retrieving heat from the soil while utilizing the free solar heat stored throughout the summer as a year-round natural thermal resource. # 2007 Elsevier B.V. All rights reserved. Keywords: Passive annual heat storage; Earth shelter; Earth homes; Energy saving design; Building with earth 1. Introduction Earth sheltered homes, refers to structures built within or with the support of earth as superficial material, and mostly built in functional styles devised to meet the needs of common people in their time and place. These structures in the past were built by people not schooled in any kind of formal archit ectural design or with identifiable building techniques rather they depended on the cover the very structure of the earth could provide them for purposes of shelter, warmth and security. However, recent quest for energy saving and efficiency in buildings has redirected the eyes on the earth material not only as support, but also as massing for passive energy utilization indoors. 1.1. Characteristics and significance of earth as resource for energy in buildings Carmody and Sterling [12] suggested that even at very shallow depths and given normal environmental conditions, the ground temperatures seldom reaches the outdo or air tempera- tures in the heat of a normal summer day, thereby conducting less heat into the house due to the reduced temperature differential. Most researchers on earth supported housing are in agreement with the idea that building underground provides energy savings by reducing the yearly heating and cooling loads in comparison with known conventional structures. In his own case, Carpenter [6] views earth sheltered buildings as having the best potential for energy savings in any design. Not only is the temperature difference between the exterior and interior reduced, but the fact yet remains that the building is also protected from the direct solar radiation. In the case of colder climates, according to Kumar et al. [17], it was noticed that during winters, the rate of heat loss in bermed (earth supported) structure was less in comparison to that in on-grade structures, indicating through results that the floor surface temperature increased by 3 8C for a 2.0 m deep bermed structure due to www.elsevier.com/locate/enbuild A vailable online at www.sciencedirect.com Energy and Buildings 40 (2008) 1214–1219 * Correspondence address: Apartment 512, Friendship Apartments, Huaz- hong University of Science and Technology, Hubei Province, Wuhan 430074, China. Tel.: +86 27 87550793; fax: +86 27 87547833. E-mail address: akjideofor@yahoo.com. 0378-7788/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2007.11.002 lower heat transfer from the building components to the ground. Thus indicating the passive heat supply from the ground even at the extreme cold temperatures of winter, hence is a factor for energy saving in earth shelter buildings. Apart from the energy values which the subsurface climate of the earth provides, the other significant characters beneficial to earth shelters includes a major goal of recycling surface space by relocating functions to underground, by this earth shelters liberates valuable surface space for other functional uses and improves ground surface visual environment, open surfaces for landscaping and thus a more greener atmosphere. In order to achieve the maximum benefits from earth sheltered housing, its application could be examined also at an entirely community scale rather than simply at the scale of individual housing. While contemporary use of earth sheltering is confined to individual homes built on single plots of land or a small cluster of houses which will absolutely be affected by surrounding conventional structures around, the traditional use encompassed entire communal design or villages that will stay within the same conditions the micro-environment provides as a few isolated earth sheltered houses do not really reach the scale needed for sustainable development as asserted by Dodd [13],earth sheltered mass-housing may become the general concept for design and building with earth as Moreland [2] envisioned in his book for entire communities enjoying dual land use by locating all housing underground. If a single case of earth sheltering is found to have significant advantages, these advantages can only increase in magnitude if applied to whole communities. 1.2. From the prehistoric to the modern earth shelter principles Earth sheltered ho mes have provided shelter, warmth and security for mankind since the beginning of recorded history. In Japan was discovered the oldest human habitation in a layer of earth about 600,000 years old in Kamitakamori, Miyagi Prefecture (Japan times, 24 October 2000). Archaeologists from the Tohoku Paleolithic Institute, Tohoku Fukushi University and other institutes said they believe that the finding may be one of the oldest in the world. There are only a few remains of human dwelling struct ures from the early paleolithic period in the world, as early humans such as the Peking-man lived in caves. Researchers believed the dwellings were built by primitive man, or homo erectus, who appeared some 1.6 million years ago and likely reached Japan 600,000 years ago at the latest, according to the archaeologists. The buildings could have been used as a place to rest, a lookout for hunting, a place to store hunting tools or to conduct religious rites. In China, however, the modern underground habitats (earth shelters) are commonly called cave dwellings as shown in Fig. 1, even though they are entirely man-made earth sheltered environments and its culture were dated back to before 2000 B.C. It is believed that underground housing preceded above ground housing in this area. From study on the existing Chinese earth habitats were discovered analytical data on the climatic and topographical relations hips to the unique design elements utilized to attain living comfort by the cave shelter dwellers. Such analysis as the rain, wind, sun and seasonal weather conditions that exist in these areas where these dwellings were located according to Golany [8], possibly necessitated the advantage of its existence in these locations. Analysis on each location also provided results and findings in terms of climatic effects, design styles and residential activities of the dwellers. In the North-west of China, variety of structures existed, ranging from striking examples of hidden opulence to humble subterranean cubbyholes where its people immerse themselves in nature’s simplicity. Golany’s [8] Fig. 1. A typical earth shelter plan in North-western China. A.J. Anselm / Energy and Buildings 40 (2008) 1214–1219 1215 research also provided analytical data on climatic and topographical relationships to the structural design styles. With single unit design solution, multi unit designs and finally Urban planning initiatives on how to achieve a sunken city that exists beneath rather than above ground level as seen in Fig. 2 below. Also fascinating in discovery included methods and techniques of ventilating the building units natu rally without the necessary use of mechanical ventilation options. Such natural ventilation alternatives provided a cost efficient and energy efficient value to the whole process. 2. The concept of passive annual heat storage system PAHS a method of collecting heat in the summertime, by cooling the home naturally, storing it in the earth’s soil naturally and then afterwards returning that heat to the contact structure (earth home) in the winter was originally introduced by Hait [4] in his book published in 1983. It includes extensive use of natural heat flow methods, and the arrangement of building materials to direct this passive energy from the earth to the building, all without using machinery. According to this concept, there exists cooling actions when one climbs down basement structures or caves. This cooling action experienced in these enclosed environments is a resu lt of the heat being drawn away from the body to the surrounding air which then transfers this thermal energy into the surrounding structures whose heat content is less than that of the adjacent air mass. The dynamics behind this concept is that heat always flows from a warmer system to a cooler system (as in the case mentioned above with the human body as the warm system and the surrounding air and walls as the cooler system). By this action if you are warmer than the surrounding air, the heat of the body will escape to the surrounding air until temperature equilibrium is attained. Likewise, in the case 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. On the other hand, if the air temperature inside the room is cooler that the surrounding walls, heat will be drawn out of the walls into the air by this warming the air and cooling the walls. Passive annual heat storage (PAHS) uses this thermo- dynamic principal in conjunction with bare earth to aid control the micro-climate within the building, in the case of the earth sheltered dwelling, it utilizes the surr ounding earth to regulate its temperature throughout the year. Globally, the earth receives electromagnetic radiation from the sun which is typically defined as short-wave radiation and emits it at longer wavelengths known typicall y as long-wave radiation. Fig. 3 below shows an analysis of the earth’s short- wave and long-wave energy fluxes produced with details from Bonan [1] . Fig. 2. Aerial view of an earth shelter neighborhood in Lian Jiazhuang, Shanxi Province, North-western China. Fig. 3. Earth’s energy budget diagram showing the short-wave (a) and long-wave (b) energy fluxes. A.J. Anselm / Energy and Buildings 40 (2008) 1214–12191216 This absorption and re-emission of radiation at the earth’s surface level which forms a part of the heat transfer in the earth’s planetary domain yields the idea for the principle of PAHS. When averaged globally and annually, about 49% of the solar radiation striking the earth and its atmosphere is absorbed at the surface (meaning that the atmosphere absorbs 20% of the incoming radiation and the remaining 31% is reflected back to space). 3. Thermal analysis (concept and application in earth shelter design) There exist two major concepts in earth shelter construction; the Bermed shelter and the Envelope or True underground earth shelter. In this case, this study considers the two major styles of Bermed earth shelter construction which are: (a) Elevational or slope design. (b) Atrium or courtyard design. Using PHOENICS-VR fluid- flow simulation environment, the study calculates the thermal flow pattern in the different earth shelter designs whilst identifying the different effects of the earth’s PAHS on the slope and atrium designs as shown in Figs. 4 and 5 below. 3.1. Analysis An architectural 3D model was developed imputing the ordinary concrete wall module for the boundary wall materials, while observing other necessary EARTH Envir onment para- meters. The model was then subjected to two cases of simulation tests; one with the case of the Earth shelter Slope design parameter where only about 50% of the structure’s exterior fac¸ade is in direct contact with the earth mass and the other case of the Earth shelter Atrium design with 80% of the exterior fac¸ade in contact with the earth mass. The temperature ‘Attributes’ assigned to the earth mass was taken from an assumption of winter and summer variations in the annual earth temperature values at below 5–10 m depth and the surface-air temperature likewise which values was assigned as same for the two design cases. 3.2. Results and discussion The simulation experiment expresses the thermal systems of the fluid-flow around and within the indoor environment of an earth shelter structure in context with the PAHS concept. After running the ‘Earth Solver’, the result from the experiment is presented in Figs. 4 and 5 above. The difference in the two cases (the slope design and the atrium design), suggested that the effects of PAHS and the passive cooling effects on the buildings indoor comfort was influenced much by the orientation of the structure (in this case the placement depth of the building below the grade). The Atrium design which has 80% of its exterior fac¸ade in contact with the earth mass presented better indoor conditions (i.e. passive cooling and heating needs) for both the summer and winter temperatures more than the slope design that has just Fig. 4. Effects of PAHS and passive cooling on earth shelter indoor space in summer: (a) elevational or slope design and (b) atrium or courtyard design. A.J. Anselm / Energy and Buildings 40 (2008) 1214–1219 1217 about 50% of its fac¸ade in contact with the earth. From this result, it could be deduced or assumed that the greater the percentage of fac¸ade in contact with the earth the better the passive annual heating and cooling gains. Although this assumption seems rightly beneficial to the energy saving concepts in homes, it is also right to consider other detrimental factors like the normal heat and cooling losses due to thermal transmittance factors. As Klaus [5] stated, that earth shelters are subjected to heat and cooling losses partly via the soil to the external air, via the soil to the groundwater below or directly to the groundwater. Klaus presented the quantity of loss as calculable in this case and the equation is as follows: Q T ¼ A total #i À #OT RAL þ #i À #GW RGW ½W where WOT = mean outside temperature, %0to À5 8C % (We + 15 K), ROT = Ri + RlA+R lB+Re = equiva- lent resistance to thermal transmission room-outside air, RlA = equivalent resistance of the soil to thermal conductivity, RlB = resistance of building component to thermal conductiv- ity, RGW = Ri + RlB+Rls = equivalent resistance to thermal transmission room-groundwater. Rls=T/ls = thermal conduc- tivity resistance of soil to groundwater, D = depth of ground- water, ls = thermal conductivity coefficient of soil, %1.2 W/ mK and WGW = groundwater temperature = 10 8C. Also to further evaluate the performance in the long-term of subsurface environment and accurate environmental informa- tion on the boundary conditions necessary for achieving an efficient design, one of which is the temperature of the surrounding soil, accurate data regarding diurnal and annual variation of soil temperatures at various depths is necessary to accurately predict the thermal performance of earth sheltered structures. Although actual data on soil temperatures is not usually abundant, research has facilitated the evaluation of the underground climate in order to assess the suitability of earth sheltered structures. Algorithms for this calculation of the soil temperatures at various depths have already been developed based on existing field measurements in different regions of the world and by this, the annual pattern of soil temperatures at any depth can be accurately considered as a ‘sine’ wave about the annual average of the ground surface temperature, Labs [15]. Accordingly, a mathematical method was developed by Labs to predict the long-term annual pattern of soil temperature variations as a function of depth and time for different soils and soil properties that are stable over time and depth. This method is sufficiently accurate in the case certain thermal and physical characteristics are accurately estimated. The equation for estimating subsurface temperatures as a function of depth and day of the year is as follows (with the unit of cosine expressed in rad): T ðx;tÞ ¼ T m À A s e Àx ffiffiffiffiffiffiffiffiffiffi p 365a r cos  2p 365  t À t 0 À  x 2  ffiffiffiffiffiffiffiffi 365 pa r  (1) where T (x,t) = subsurface temperature at depth x (m) on day t of the year (8C), T m = mean annual ground temperature (equal to steady state) (8C), as the annual temperature amplitude at the surface (x =0)(8C), x = subsurface depth (m), t = the time of Fig. 5. Effects of PAHS and passive cooling on earth shelter indoor space in winter: (a) elevational or slope design and (b) atrium or courtyard design. A.J. Anselm / Energy and Buildings 40 (2008) 1214–12191218 the year (days) where January 1 = 1 (numbers), t 0 = constant, corresponding to the day of minimum surface temperature (days) and a = the thermal diffusivity of the soil (m 2 /day). Following this assessment of subsurface climate, the calculated soil temperatures can then be used in calculating the heat flux through the building surfaces. The energy efficiency of each wall in contact with the earth at varying depths can thus be investigated for local climatic conditions in the buildings. 4. Conclusion With the information available so far on means of assessing the performance of earth shelters and PAHS effects on these structures, it is then possible for designers and planners in different regions to have access to a simple framework for assessing its efficiency at the initial planning stages. The resulting outputs can then be used for the heat transfer and energy consumption simulations within the building units. Results from these simulations will provides insight into the degree of passive heating and cooling or reduction in heat flow that the soil climate can provide as compared to the surface climate as well as suggesting para meters for dept h placement of earth shelter buildings for more efficient results. Acknowledgements This paper was inspired by the author’s doctorate research work which is based on the use of alternative passive heat storage systems in achieving indoor comfort in hot summer- cold winter regions. Special recognition and appre ciation goes to the Dean of the School of Architecture and Urban planning Prof. Baofeng Li, for his support. The author also wished to acknowledge the support by the National Natural Science Support Fund (Contract No. 50578067) and the Special Research Support Fund for Doctorate Degree research programme (Contract No. 20060487008) for the Changjiang River Districts (Huazhong and Huadong) of China. The author wishes to appreciate the efforts and cooperation received from the Ecology department of the school in providing data and the assurance of assistance on further work. References [1] G. Bonan, Ecological Climatology: Concepts and Applications, Cam- bridge Press, United Kingdom, 2002. [2] F.L. Moreland, An alternative to suburbia, in: Proceedings of the Con- ference on Alternatives in Energy Conservation: The Use of Earth-covered Buildings, National Science Foundation, Fort Worth, TX, 1975. [4] J. Hait, Passive Annual Heat Storage: Improving the Design of Earth Shelters, Rocky Mountain Research Center, 1983. [5] D. Klaus, Advanced Building Systems: A Technical Guide for Architects and Engineers (English Translation), Published for Architecture Basel, Boston, Berlin, 2003, pp. 50. [6] P. Carpenter, Sod It: An Introduction to Earth Sheltered Development in England and Wales, Coventry University, Coventry, 1994. [8] S. Golany, Earth Sheltered Habitat (History, Architecture and Urban Design), Van Nostrand Reinhold Company Inc., New York, 1983. [12] J. Carmody, R. Sterling, Design considerations for underground buildings, Underground Space 8 (1984) 352–362. [13] J. Dodd, Earth sheltered settlements, a sustainable alternative, in: Pro- ceedings of the Earth Shelter Conference, Coventry University, September 3, (1993), pp. 26–36. [15] K. Labs, Underground building climate, Solar Age 4 (10) (1979) 44–50. [17] R. Kumar, S. Sachdevab, S.C. 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