Effectiveness Of Using Solar Power In Providing Energy To Space Air Conditioning - Essay Example

1 The study seeks to investigate the effectiveness of using solar power in providing energy to space air conditioning through absorption cooling systems as an alternative to thermodynamic electricity which greatly contributes to global warming through emission of green house gasses from the thermal power stations. It examines the technology involved in the absorption and cooling process, its applications and operation as well as advantages and disadvantages of using solar power over conventional cooling methods which greatly rely on thermal energy. 2 Introduction Basically space conditioning refers to the control of temperature, moisture content, air quality and air circulation as required by the occupants, a process or a product in the space. This process requires the use of electric power to aid the heating and cooling processes involved. However, previous research has shown that space conditioning contributes approximately 6.5% of the total greenhouse gas emissions globally through the use of thermodynamic electricity to provide heating and cooling. In 2004, the use of space conditioning in residential buildings and commercial buildings contributed about 57% and 35% respectively to greenhouse gas emissions worldwide. This means that approximately 6.5% of global greenhouse gas emissions can be attributed to space conditioning. On the other hand, research has also shown that space conditioning basically consumes lower capacity of energy which can even be sustained by the effective use of solar energy. It is against this background that the study seeks to investigate how the widespread use of solar energy in space conditioning would act as a remedy in reducing the disastrous consequences of greenhouse gas emissions globally. Currently, thermodynamic electricity with higher voltages is used to provide energy for space conditioning in many buildings across the whole world. To a certain extent, this is also costly as another relatively cheaper source of energy in the form of solar energy which also has the capacity to provide enough power in the process of space conditioning can be effectively used as an alternative to the use of thermal electricity. The sun basically provides enough energy daily that can be stored using specifically designed gadgets for the purpose. Solar energy can also be directly used in space heating or cooling through the use of appropriate devices. Thus, this paper seeks to investigate the technology involved in the process of absorption and cooling involved in space conditioning as well as its application, advantages and disadvantages. The study will also give recommendations for future developments and research in the area. 3 System fundamentals In any air conditioning system, different configurations can be used to provide space conditioning. The process can be illustrated diagrammatically as shown below. Figure 1 Heat is removed from the space by a chilled water, air, or refrigerant loop. In the case of using a water or refrigerant loop for cooling delivery, space air is cooled by the fluid via a heat exchanger. The cooling fluid picks up heat from the space and is returned to the chiller to be re-cooled. In any refrigeration cycle, there is a heat rejection stage. This occurs at the condenser. Often condensers are water cooled in which case a cooling water loop is run past the condenser. Cooling water picks up heat from the condenser and rejects it to the atmosphere. In larger systems this is often achieved through the use of cooling towers. Basically, as illustrated above, in the process of cooling the space, the air supplied must have sufficiently low temperature and moisture content to absorb the total cooling load of the space. As the air flows through the space, it is heated and humidified then sent to the conditioning equipment where it is cooled and dehumidified thus rejecting heat and supplying cooled air to the space again. In a conventional air conditioning system, chilling is achieved through the use of an electrically powered compressor in conjunction with an evaporator, expander and condenser. An absorption cooling system also works with similar mechanical system of compressing vapor. It also has an evaporator located in the conditioned space that takes in heat from the internal environment and a condenser located outside the conditioning space that rejects heat. An absorption chiller differs from a vapor compression chiller mechanically in that instead of using a compressor; an absorption chiller uses a generator and an absorber. A diagrammatic illustration showing the use of an absorption chiller for producing chilled water is shown in figure 1 above. The basic principle behind absorption cooling is that mixtures have different boiling points at different concentrations. Heat is required to be supplied to the generator and it is derived from many sources including the direct combustion of fossil fuels such as gas which leads to global warming as a result of gases such as carbon monoxide being emitted into the space. In the case of solar air-conditioning, this heat is supplied by the solar power absorbed by a special device on top of the roof and converted to heat water. Cooling often requires the absorber and the condenser which cools the air or the water. This is achieved by sending water for cooling to the conditioning equipment. Thus, the most common source of cooling for absorption chillers is water driven often via a cooling tower as illustrated in the diagram above. Cooling process of the space is as follows: Liquid refrigerant moves from the condenser to the evaporator through an expansion valve. This valve lowers the pressure and facilitates vaporization of the refrigerant in the evaporator. Heat is then drawn from the space to aid evaporation of the refrigerant. This is how the useful cooling effect is achieved. The refrigerant vapor then flows into the absorber where it is absorbed or condenses and forms a concentrated solution. A pump is used to increase the pressure of the highly concentrated liquid from the absorber to the generator pressure. In the generator, heat is added to the mixture which drives some of the refrigerant out of the solution. This lowers the concentration of the solution. This hot solution is passed to the condenser at high temperature and pressure where heat is extracted until the mixture condenses. This liquid can then be passed through the expander to the evaporator where space cooling is provided and the cycle begins again. There is a threshold amount of the generator temperature that must not be exceeded for the chiller to operate. This is because the concentration of refrigerant must be lower in the generator than the absorber for the cycle to continue. This process of absorption chillers limits the application of solar energy to some cooling systems. This is further discussed in Section 4. 4 Types of Absorption Chillers Available The two main refrigerant or absorbent mixtures currently on the market are Water/Lithium-Bromide chillers and Water/Ammonia chillers. 4.1 Water/Lithium-Bromide Chillers In this type of system, water acts as the refrigerant and Lithium-Bromide (Li-Br) as the absorbent. The evaporation temperature in the Water/Lithium-Bromide system is approximately about 10°C. This makes the Lithium-Bromide system suitable for air conditioning in buildings. Water cooling is required for absorber and condenser of this kind of machine. The generator temperature of Li-Br chillers is in the range of 75-95°C which makes it suitable for use with evacuated tube solar collectors. The use of flat plate collectors to supply this temperature would be unreliable because fluctuations in solar radiation may mean that the generator supplied water temperature drops at times. If this occurs, the generator temperature may drop below the cut-off temperature and the system would not operate. The pressure difference between the high and low pressure side of this system is low enough such that a vapor lift pump and gravity return can operate alone putting aside the need for a mechanical pump. 4.2 Water/Ammonia Chillers In the Water/Ammonia (NH3) system, water acts as the refrigerant and ammonia as the absorbent. The evaporator temperature of ammonia/water system operating at just above the generator threshold temperature like a system powered by solar heated water does, has a co-efficient of performance (COP) of around 0.7. The COP is about 0.6 if the generator (supply water) temperature is increased to around 130°C. The pressure difference between the low and high pressure sides of an ammonia/water system are much higher than in the water/Li-Br system, so a mechanical pump is required to return the solution from the absorber to the generator. This means that some electricity is required for an ammonia chiller to work. Standard water/ammonia absorption chillers generally use air-cooling for the condenser and absorber. The amount of heat removed from the system dictates the operating temperature of the generator and for air cooling systems. The generator works in the temperature range of 125°C - 170°C. This means that it is much more difficult to power air cooled ammonia/water absorption chillers with solar thermal power as the collectors operate at a lower efficiency in this temperature range. However, if water cooling is applied, the generator temperature is required to be around 95°C - 125°C, so evacuated tube solar collectors can be utilized to heat the generator. 4.3 Electrolux Refrigerator A special application of the ammonia/water absorption chiller is the Platen-Munters, or Electrolux Refrigerator. Unlike standard ammonia absorption chillers that require a pump, this system requires no electrical or mechanical power. It operates from direct heating using a fuel such as natural gas, liquid petroleum or kerosene. Circulation of ammonia is maintained by gravity. The system is physically configured such that when the ammonia is in gaseous form, it rises and when it is in liquid form, it falls. The main benefit of the Electrolux Refrigerator over other types of refrigeration is that no electricity is required. The fuel used to power the refrigerator is portable and for this reason, the Electrolux Refrigerator is common in applications such as off grid housing and caravans. The Electrolux refrigerator can still be found today in some caravans. 5 Properties of Absorption Chillers 5.1 Advantages The main advantage of solar powered absorption cooling is that the peak temperatures characteristic of solar thermal power corresponds almost exactly with the peak cooling temperature demands for most applications. One obvious advantage of solar energy is that it is free compared to the use of thermal electricity to run absorption cooling systems which offers supplementary benefits by reducing electricity demand on the grid. It is extremely expensive for energy utilities to meet the demands required for consumption as it requires more gas for the turbines. At times when demand out-strips base load supply, the spot price of electricity can peak to around hundred times the average spot price. Thus, meeting peak demand has significant monetary impacts for electricity retailers. Other benefits associated with absorption cooling include a reduction in environmental impacts in the form of reduced greenhouse gas production and ozone depletion potential. Vapor compression chillers require a large amount of shaft work for the compressor because the vapor undergoes a large change in volume. Absorption chillers offer a means of raising the pressure of the refrigerant without greatly altering its volume. It follows that the amount of input work required is greatly reduced. As mentioned previously, space conditioning contributes up to around 6.5% of global greenhouse gas emissions from the use of thermal power. The replacement of an electric compressor system with an absorption cooling system can reduce the high demand of electricity required for cooling. The only electric energy used by an absorption cooling system is required to pump from the generator to the absorber, to convey cooling fluids and operate fans and controls. It can thus be noted that this greatly helps reduce the amount of greenhouse gas emission associated with the running of chillers powered by energy generated using thermal power stations. Both Ammonia and Lithium-Bromide have zero ozone-depleting and global warming potential. This makes them desirable to use from an environmental point of view as they do not contribute to global warming or ozone depletion. From an environmental standpoint, ammonia is quite attractive as a refrigerant as it also has a relatively high COP of 1.033 as compared to R-134a. 5.2 Disadvantages The greatest deterrent for the implementation of cooling system is cost. The cost of an absorption chiller is slightly more than that of a standard compression chiller and as system capacity increases, the premium associated with absorption cooling is less than at lower capacities. On average, the capital cost of an absorption chiller is approximately 10-15% greater than that of a standard vapor compressor chiller. If solar thermal energy is to supply power for the absorption chiller then the cost of solar collectors must also be considered. In order to develop a comprehensive life cycle costing for absorption cooling compared to vapor compression cooling, electricity savings and maintenance costs must also be considered. Without considering the costs associated with reduction in peak electricity demand and limited greenhouse gas reduction, it is expected that over the life cycle of a system, the cost of an absorption cooling system is slightly greater than that of a vapor compression system. The COP of absorption cooling systems is generally lower than that of vapor compression systems, which falls in the range of 0.6-0.8 compared to 1-5.5. The second order efficiency is however, comparable between the two chiller types. Practically this reduced COP is not a problem because the energy used to drive the solar power absorption chiller is ‘free’ and thermodynamically, the quality of the electrical energy used to drive the compression vapor chiller is much higher than the quality of the low grade heat used to drive the absorption chiller. The availability of small capacity absorption chillers is limited. The smallest chiller available on the global market has 15kW cooling capacity. This is produced by EAW in Germany. This critically limits the availability of absorption cooling systems in residential buildings. The operation of absorption chillers for space conditioning can also be water intensive. Li-Br chillers are most suitable for space conditioning applications. This chiller type cannot operate with air-cooling and must have some form of water-based cooling. The most common means of providing water based cooling to chillers is to use evaporative cooling towers. Because water is lost upon evaporation, the use of cooling towers is very water intensive. This problem is however faced by most types of chillers, especially at larger cooling capacities. There are alternatives to cooling tower heat rejection but the viability of these is very site specific and cannot be achieved for all applications. Some of these alternatives include the rejection of heat into the ground or a nearby body of water. Ammonia can be dangerous and care must be taken in design and operation of an ammonia based chiller to ensure that the system is safe. Ammonia, or refrigerant R717, has a safety group classification of B2 according to the Australian Institute of Refrigeration Air-Conditioning and Heating. This means that ammonia is classed as having a high level of toxicity and a moderate level of flammability. It follows that care must be taken in design and maintenance of ammonia systems to avoid the solution reaching concentrations that are explosive. Leak detection systems are also recommended to be installed where ammonia is used as a refrigerant (or absorbent). However, the presence of ammonia is easily detectable as it causes a burning sensation to the eyes. 6 Future research, development and application In the early stages of development of absorption chillers for space conditioning applications, most work was directed towards developing cooling cycles for ammonia/water chillers that use higher concentrations of ammonia. This lowers the required generator temperature and means that the energy required can be more readily supplied by solar thermal collection. A recent study conducted at the Delft University of Technology found that a commercially viable gas fired water/ammonia absorption chiller, designed to operate at 120°C could be easily modified by changing the concentration of ammonia in the charging stage to 42.5% to operate efficiently at a generator temperature of 100°C. This is a significant finding for solar powered refrigeration, but has limited application to solar air conditioning as the evaporator temperature was set at -15°C throughout the study. Such a low evaporator temperature would cause practical difficulties for application to space conditioning as extensive re-heating and re-humidification of the supply air would be required which lowers the efficiency of operation. There are many examples of absorption cooling systems being implemented in commercial buildings worldwide, but very few of residential applications. Locally, the first absorption chiller for residential applications in Australia is to be installed in medium scale multi-residential development to be built in Zetland, Sydney, in early 2008. A lot of research is currently directed towards optimizing absorption chillers for use in combined heat and power application. This has the potential for widespread application throughout the middle-east in the form of district cooling. 7 Conclusion The use of solar thermal energy for space cooling is best achieved by the application of a Water/Lithium Bromide absorption chiller. This system can operate effectively with a generator supply water temperature of 75°C – 95°C which can be reliably supplied by evacuated tube solar collectors. This technology offers significant benefits with respect to the environment and infrastructure. The major restrictions on the widespread implementation of this technology are high capital costs and lack of commercially viable systems on a residential scale. These are market driven forces and there are few technical reasons why this technology cannot be implemented on a large scale. For refrigeration applications Water/Ammonia type chillers are most appropriate. These generally require a higher water input temperature to drive operation which limits the application of solar thermal energy to this application as the cost associated with solar collection is higher because high pressure piping may be required to transport heat in the form of steam. Research indicates however; that the alternation of existing ammonia absorption chillers can significantly improve the operation of solar powered ammonia chillers. Read More