Search

The function of a building or a house is to provide shelter to its occupants from weather. Since the weather conditions vary widely over the year and in different places, and the humans feel comfortable within certain range of air temperatures and humidities, the houses are made to provide everyday living comfort. The heating of houses in winter to provide comfort by the use of solar energy is an ancient concept and is in use since man started to build habitations. Basically the solar heating systems are divided into two categories: the passive heating and active heating. Passive systems do not need any mechanical system and are designed such that the glazed area, walls and roofs are made use for collecting, storing, and distributing the heat indoors by natural processess of convection, radiation, and conduction. Four basic concepts for passive solar heating are: direct gain, collector-storage wall, sunspace, and collector-storage roof. These concepts will be discussed in a separate chapter in volume 3.

Energy storage provides a means for improving the performance and efficiency of a wide range of energy systems. It also plays an important role in energy conservation. Typically, energy storage is used when there is a time or rate mismatch between energy supply and energy demand or where intermittent energy sources are available, like that of solar energy. This gap in energy demand and supply can be evened out by energy storage with consequent savings in capital costs. Thermal energy storage systems have a high potential to make possible fuel substitution, fuel savings, and special savings in many application areas. Storage of energy, tapped during the period of availability, for times when the availability is not in abundance, is a process found in natural systems which is the fundamental basis for the ecological balance of the nature. The modern amenities enjoyed by human civilization necessitates the storage of energy for the proper functioning of the man made systems during adverse conditions. Thus, the storage of energy, in various forms of it which can be later converted into a useful form is one of the most demanding problems taken up by the technologists.

In many developing countries of the world animate power and muscle power is still used for lifting water for crop irrigation or for drinking purposes. Both these methods are highly inefficient, time consuming and becoming costlier. The energy requirements of rural people in developing countries are quite low and are met largely by non-commercial energy sources like firewood, agricultural waste and cow dung cakes. The agricultural land is either irrigated by natural means like rains or by using animal or muscle power. The use of diesel or petrol operated water pumping sets or electric water pump is also increasing. With the increase in population, more land is to be brought under cultivation to increase crop production for meeting the demand of food for which water is an essential ingradient. Moreover, in developing countries where 40–50 percent GNP (Gross National Product) coming from agricultural sector alone, more energy particularly for pumping of water for irrigation is required. Efforts are being made in developing countries to provide electricity in rural areas but the progress has been slow mainly due to the reason that it is capital intensive and is difficult to connect remote scattered villages with the central electric grid system.

Thermal energy may be stored as sensible heat or latent heat. Sensible heat storage systems utilize the heat capacity and the change in temperature of the material during the process of charging or discharging - temperature of the storage material rises when energy is absorbed and drops when energy is withdrawn. One of the most attractive features of sensible heat storage systems is that charging and discharging operations can be expected to be completely reversible for an unlimited number of cycles, i.e., over the life-span of the storage. Isothermal operation offers a thermodynamic advantage in many applications of thermal energy storage. However, presently most thermal storage devices are based on sensible heat storage inspite of the fact that it is not isothermal: since current technology is adequate for good system design.

It has been estimated that the world population will increase from the present value of 4 billion to 7 billion by the year 2000. It is also estimated that about 600 to 900 million people do not have enough food to eat now and this number is likely to increase with the increase in population, There are obviously two direct ways solving the problem: (1) increase the food production by bringing more area under cultivation and irrigation, using newer mechanised methods of agriculture, and using hybrid seeds and better management and agricultural practices, (2) reduce the food demand by reducing the population growth. The third alternative which is equally important but not given adequate attention is reducing the loss of food during and after harvest. The actual estimate of food loss during post harvest due to spillage; contamination; attack by brids, rodents, and insects; and during storage is difficult to know due to technical and practical reasons[1] but it is so enormous that most conservative estimates were enough in 1975 to result in a resolution by the Seventh Special Session of the United Nations General Assembly to seek 50 percent reduction of post-harvest food losses within a decade.

Flat-plate collectors are generally stationary and their temperature capability is generally below that required for efficient generation of shaft power or electric power. These collectors cannot be used for the supply of heat at temperatures more than 100°C because here the absorbing area is equal to the area by which heat is lost to the outside. The heat lost can be reduced by using a small area which emits the heat (known as receiver) and the temperature of the receiver can be increased by focusing the solar radiations onto the receiver by a large reflector. This combination of reflector-receiver system is called as concentrator.

The oldest and most commonly practiced method to store solar energy is sensible heat storage. The underlying technology is well developed and the basic storage materials, water and rocks, are available abundantly everywhere. In another method, currently receiving considerable world wide attention, the energy is stored as the latent heat of phase change, usually solid-liquid. Although latent heat storage provides higher energy densities than the sensible heat storage, but still these are not sufficient enough to meet the requirements of several purposes like power generation where high temperature steam is required in sufficient quantity. The desirability of high storage density has aroused interest in chemical energy storage (CES). In this concept the energy is stored in the form of heat of chemical reactions which are often of an order of magnitude (Ref.1) larger than the latent heat storage, as seen from Table 4.1.

The most important component of a Solar Water Heater is the Solar Energy Collector whose main function is to absorb radiant energy from the sun as much as possible and convert it into heat and transfer it to the fluid flowing in the channel which is welded on the absorber plate. The most important step to utilize solar energy in an effective way for heating of water is to develop an efficient low cost flat plate solar collector. As discussed earlier a solar energy flat plate collector has five components: absorber plate, glazing, insulation, tubes or channel in the absorbing plate, and tray. There are several advantages of using flat-plate collectors for heating water such as (a) it absorbs direct as well as diffuse radiation, (b) simple to construct, (c) no need of sun following arrangement, (d) low in cost, and (e) the collector can form an integral part of the roof. Since the most expensive part of a solar water heater is the flat plate collector, while designing a collector one should consider the following three factors: 1. Thermal performance 2. Cost of construction 3. Life time and reliability

The importance of high temperature thermal energy storage needs hardly any emphasis. The intermittent nature of sun’s energy, importance to the central receiver solar thermal power system programs, and growing needs of energy in industries have necessiated the development of high temperature thermal storage systems. Development of practical but low cost methods for storing large amounts of high temperature thermal energy for solar thermal conversion to electric power is one of the most important subsystems. Solar thermal energy subsystem is one of the least well-developed system required for a solar electric power plant. Solar collector technology is well developed and is related to antenna technology, receiver technology is also well understood and related to steam boiler technology, and the turbine-generator technology is in use in Industries for the last many years. Since storage subsystem technology has no analogy in industry, therefore, not much work has been done in this direction. Since 1975 when the central receiver solar thermal power system came into operation, Department of Energy (DOE) of USA has given several contracts to industries and institutions for the development of large amount of high temperature energy storage systems. Several review articles (Ref. 1 to 5) are recently written showing the importance, describing various thermal energy high temperature storage systems and the research required in this important area of thermal energy storage.

Solar Energy can be used for producing cold either for cooling of buildings (generally known as air-conditioning) or for refrigeration required for preserving food. Solar cooling appears to be an attractive proposition due to the fact that when the cooling demand is more the sunshine is strongest. This, alongwith the necessity for providing thermal comfort for people in hot areas of the world, and for providing food preservation, may be the motivating factors in continuing research and development in the field of solar cooling systems. Although considerable work on solar cooling system has been done for the last three decades, due to its complexity, both in concept and in construction, the utilization and commercialisation of solar cooling is not as widespread as other solar energy applications like solar water heating and solar space heating. However, if solar cooling of buildings is combined with the solar heating then the combined solar cooling and heating systems can become economical.

It has been estimated that if the present rate of population growth and exploitation of readily available stored energy in fossil fuels continues, then the fossil fuels may be depleted completely in a century or so. The scientists all over the world are in search of new and renewable energy sources. However, developing efficient and inexpensive energy storage devices is as important a field as developing new sources of energy.

The first recorded use of solar energy was made about 2500 years ago in the Greek where people not only worshipped the God‘Sun’ but built their homes facing south, with porticos (or gaps in the roof) so as to heat the indoors during winter. These were the first solar homes, and solar architecture including many other passive features were taken up much later. The great Roman baths were heated by the sun and the invention of glass led to the discovery of the green house. The sun also had more adventurous and romantic roles. In 212 B.C., Archimedes used mirrors to destroy the ships of Roman invaders. Apart from this several peaceful applications of solar energy were also made in the past. However, as ‘Modern’ fuel sources became popular in the middle ages, solar power was almost a forgotten art. It was only another energy crisis (1972) that turned man once again to the sun.

With renewed interest in solar energy utilization and role of thermal energy storage in industrial development in the seventies the need for suitable testing procedure for solar collectors and thermal energy storage systems has been felt. Considerable effort has been made towards developing a suitable procedure for testing and thermal rating of flat plate collectors. The U.S.A., Australia, Germany, Israel, India, U.K., South Africa, France and Japan are some of the countries that have been enthusiastic in developing procedures for thermal rating of solar collectors. The activity towards developing test standards for thermal storages do not appear to be proportionate to the complexity of these systems and their importance. Early work (before 1973) has been reported mainly from Japan (Ref.11–13). Whereas a number of thermal storage devices were incorporated as a part of a larger system, not much effort was made towards their evaluation as a separate component.

The seperation of the collecting and storage units in the conventional solar system permits the efficient design for each unit. Although the efficiency of such systems is quite high so is the unit cost. These systems cannot be scaled up for longterm large energy storage. The large collecting areas and large storage units will be prohibitively expensive. The long term energy storage calls for a different approach. For solar energy to be competitive, the system must be designed where large collecting areas or large storage capacity or both should be cheaply available. In this chapter some such concepts are discussed. In solar pond, both large collecting areas and storage are inexpensive. In other concepts the insulating properties of earth are used to provide large storage space. In this chapter some longterm storage concepts and problems are discussed briefly.

In many parts of the world, temperature, even during 24 hours, varies over a wide range. It is imperative to use artificial sources of energy for keeping temperature f1ucturations within the range of comfortable living. Fossil fuel, oil or electricity were and still are the main source of auxiliary energy. However, the steep rise in oil prices in early seventies focussed the attention on the alternates sources of energy. Solar energy although of low flux can he used effectively to provide comfort year round.

The main drawback of the conventional flat plate collector is its inability to operate with reasonable collection efficiencies at temperature around 80 °C, thus limiting their applications largely for providing hot water and space heating. Therefore, the major concern of solar collector designers is the reduction of heat losses from the solar energy collector to the surroundings. Designers seek economic alternatives to minimize these thermal losses, which result from conductive, convective and thermal radiative heat exchanges and also the reflection losses at the interfaces. For the utilization of solar energy for producing cold, for power generation and for pumping of water, fluid temperatures in the range of 110–150°C are required. The use of moderate vacuum between the cover glass and the absorber plate can suppress the natural convection losses from the absorber plate of the collector and will give the desired temperature and efficiency. The affect of the vacuum environment is especially pronounced when it is used in conjunction with a selective black surface on the absorber plate, making it possible to operate at a temperature of 150°C with a daily energy collection efficiency in excess of 40 percent. Because of the pressure of the atmosphere, however, evacuating the space between flat plates is technically not possible. Honeycombs, or glass supports could offer good physical support if the space were evacuated but many sealing problems remain and this approach is so far impractical. For these reasons, evacuated collectors have generally involved tubular designs, ,which have inherently higher strength to withstand external pressure and are termed as advanced collectors in this chapter. On the other hand there is much technology available in evacuating glass tubes for fluoroscent lighting and TV electron tubes applications, and collectors based on using evacuated tubes have been successfully developed. Therefore, the evacuated tubular collectors which can withstand high vacuum and can be mass produced are preferred. Their high temperature effectiveness is essential for the efficient operation of solar air conditioning systems, power generation and process heat systems. In these tubular or advanced collectors high performance is achieved by the use of the following advanced features: i) Vacuum insulation ii) Selective black absorber coatings iii) Anti reflective coatings films iv) Heat mirror coating v) Highly efficient removal of the absorbed heat from the solar collector by the principle of heat pipe.

Solar energy has been used to heat water for many years, and the design requirements of solar water heating equipment have been studied for more than 90 years. The lack of widespread application has not been due to lack of understanding, but it is simply because that upto this time other sources of energy have been more economical. Interest in solar water heating was limited to those with the understanding and enthusiasm necessary to build their own equipment. Interesting descriptions of the early commercialization efforts are given by Butti and Perlin[1] and Scott[2]. A solar water heater industry in South Florida[3] was started in 1900. It is estimated that about 30000 to 50000 units were installed by 1950, but about that time their popularity began to decline due to readily available cheap energy from fossil fuels. In Southern California, the solar water heater industry started in about 1890, flourished during 1910–1920, and died around 1930.

Adequate quality and reliability of driking water supply is a fundamental need of all people. Without potable or fresh water (less than about 550 parts per million, ppm, of salt) there is no human life. Industries and Agrigculture also need fresh water without which they can not function or thrive. Water is therefore the key to man’s prosperity; it is intimately associated with the evolution, civilisation and destiny; hence it is aptly said that water is everybody’s business. Fresh water which was obtained from rivers, lakes, and ponds is becoming scarce because of idustrialization and population explostion. Moreover these potable water sources are being polluted constantly by industrial wastes and large amounts of sewage. It is said that presently more than 2000 million people are not getting potable water which leads to many deseases and stops the development. In the past, also the shortage of fresh water existed but it was confined to arid and semi—arid regions and regions where only brackish water was available. But now due to industrialization and population rise, the shortage has come into prominence.

Industrial process heat is the thermal energy used directly in the preparation or treatment of materials and items manufactured by an industry. Presently, the heat required by these industries is met by oil, natural gas, coal or electricity, but a large portion of industrial process heat is at sufficiently low temperatures which can easily be supplied by solar energy. The year round need for energy in industries allows a maximum utilization of solar equipment. Some advantage and disadvantage of using solar energy for providing process heat are listed in table 2.1.

For heating the space using solar energy, the solar energy is to be collected, stored, and distributed properly in the space to be heated. In an active solar space heating system, the solar energy is collected using some kind of separate solar energy collectors; solar energy stored in a separate storage unit may be in sensible heat storage materials, latent heat storage materials, or in chemical reactions; and the energy is distributed in the space using electrically operated pumps and fans using radiators etc. While in a passive solar heating system all the three functions of solar energy collection, storage and distribution is done by natural means and generally no electrical or mechanical power and electronic controls are used. In the passive house heating system, various elements of the buildings like walls, roof, windows, partitions, etc. are so selected and so architecturally integrated that they participate in the collection, storage, transport and distribution of thermal energy.