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NVH SRIRAM
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ABSTRACT
solar air conditioning refers to any air conditioning (cooling) system that uses solar power.
This can be done through passive solar, solar thermal energy conversion and photovoltaic conversion (sun to electricity). The U.S. Energy Independence and Security Act of 2007[1] created 2008 through 2012 funding for a new solar air conditioning research and development program, which should develop and demonstrate multiple new technology innovations and mass production economies of scale. Solar air conditioning will play an increasing role in zero energy and energy design.
In particular, in hot and humid summers, an important proportion of the overall electric power is dedicated to satisfy air conditioner loads. Outdoor weather conditions are crucial in determining residential energy consumption for heating, ventilation and air-conditioning (HVAC) household appliances. In this paper we address the modeling of outdoor weather conditions impact on predominantly air conditioner residential load. The main emphasis is on the temperature and humidity segregated load influence where the socioeconomic and life style of the consumer is isolated from the load model. Important field data has been collected for several hot and humid consecutive months covering a wide range of outdoor temperature and humidity. After recognizing that humidity can be divided into three different comfort levels, three-dimension analysis of the data have been conducted and mathematical relations have been extracted to represent the dependencies of the real power with both humidity and temperature. The investigations have shown the sensitivity of the load to temperature and humidity to be in good compliance with the expected natural load behavior.
Every air-conditioning system needs some fresh air to provide adequate ventilation air required to remove moisture, gases like ammonia and hydrogen sulphide, disease organisms, and heat from occupied spaces. However, natural ventilation is difficult to control because urban areas outside air is often polluted and cannot be supplied to inner spaces before being filtered. Besides the high electrical demand of refrigerant compression units used by most air-conditioning systems, and fans used to transport the cool air through the thermal distribution system draw a significant amount of electrical energy in comparison with electrical energy used by the building thermal conditioning systems. Part of this electricity heats the cooled air; thereby add to the internal thermal cooling peak load.
In addition, refrigerant compression has both direct and indirect negative effects on the environment on both local and global scales. In seeking for innovative air-conditioning systems that maintain and improve indoor air quality under potentially more demanding performance criteria without increasing environmental impact, this paper presents radiant air-conditioning system which uses a solar-driven liquid desiccant evaporative cooler. The paper describes the proposed solar-driven liquid desiccant evaporative cooling system and the method used for investigating its performance in providing coldwater for a radiant air-conditioning system in Khartoum (Central Sudan). The results of the investigation show that the system can operate in humid as well as dry climates and that employing such a system reduces air-conditioning peak electrical demands as compared to vapour compression systems.
Introduction
Air-conditioning has been achieved reliably and efficiently over the last few decades due to the popularity gained by vapour compression machines as a result of halogenated hydrocarbons discovery. The need to conserve high grade energy and reducing the harm effects of halogenated hydrocarbons, such as; the contribution to the Earth’s ozone layer depletion and global warming due to emissions of halogenated hydrocarbons during production and use, necessitate exploring alternative techniques. Evaporative cooling, a very simple, robust and low cost cooling technology basically achieved by evaporation of water in air is one proposition. Evaporative water coolers (cooling towers) are devices utilizing the direct contact between water and atmospheric air to cool water by evaporating part of the sprayed water in the air. Despite its potential to reduce cooling energy and peak energy demand, cooling towers are not widely used in many areas because of their declining cooling capacity
with increasing outdoor humidity.
In liquid desiccant evaporative cooling (LDEC) process air is used, dehumidified by a desiccant solution, to cool water by direct evaporative cooling (both require no refrigerant). LDEC is considered to be a modification of direct evaporative cooling that can cater for different climates. Unlike vapor compression cooling which rely on high energy technology, desiccant evaporative cooling relies on desiccant dehumidification (low energy technology) to provide dry air required for ventilation and evaporative cooling. Solar energy or any other type of energy that might otherwise be wasted provides the heat energy required for regenerating the desiccant used by the desiccant dehumidifier during the cooling season (summer) and heating the water circulated through the radiant system during the heating season (winter). This provides dry ventilation air and cold water for a radiant system, and thereby gives a solution to thermal environment control that significantly reduces electrical energy demands, greenhouse gas emissions and dependence on harmful refrigerants.
As an open heat driven cycle affording the opportunity to utilize heat that might otherwise be wasted, a liquid desiccant evaporative cooling cycle can be coupled with solar heating to produce dry ventilation air and cold water for a radiant system. This can significantly reduce cooling electrical energy demands in comparison with conventional vapour compression refrigeration, and should in theory be extremely environment friendly as it eliminates greenhouse gas emissions and dependence on harmful refrigerants. As it delivers cold water and dry air at relatively high COP, solar-operated liquid desiccant evaporative water cooling would be cost effective. The objective of this paper is to study the performance of a solar-driven desiccant evaporative cooling system in providing cold water for a radiant air-conditioning system in Khartoum Sudan. In doing so, a computer program was used to simulate the solar-driven liquid desiccant evaporative cooler. The computer program was developed based on unit subroutines constituting the solar-operated liquid desiccant evaporative cooling system components governing equations.
System Description
The liquid desiccant evaporative water cooler, which is designed to serve as an open cycle absorption system operating with solar energy is shown schematically in. The cooler consists of nine major components: continuous fin tube type process air pre-cooler, air-to water air cooler, an isothermal vertical tube type falling film absorber, adiabatic packed bed tower regenerator, solution-to-solution strong solution pre-cooler and weak solution pre-heater, water-to-solution solution cooler, solution-to-thermal fluid solution heater, solar collector thermal fluid heater, counter-flow packed bed type evaporative water cooler and appropriate instruments for various measurements. Arabic numerals indicate working fluids states at specific locations; thick solid lines represent air flow, thin solid and dashed lines represent solution and water flow respectively.
The liquid desiccant system is connected in a flow arrangement that allows thermal fluid storage and is capable to work in two automatic modes as may be selected by the user. One automatic mode is for full system operation in which all components including the thermal fluid storage circuit operate, while the second is for solar heating only. In the full automatic mode, pump 1 pumps absorbent solution from regenerator sump (state 13) through the solution-to-solution heat exchanger where it is pre-cooled by exchanging heat with cold solution leaving the absorber sump. The solution then flows through the solution-to-water heat exchanger where it is cooled to state 9 by water from the evaporative water cooler and supplied to the absorber distribution system.
The cold solution to trickle down in counter flow to air stream and collects in the absorber sump. A fan draws ambient air through the air-to-air heat exchanger where it is pre-cooled to state 2 and through the air-to-water heat exchanger where it is cooled to state 3 to the absorber chamber. In the absorber, water vapour is removed from the sensibly cooled process air entering the bottom of the absorber (state 3) by being absorbed into the absorbent solution. Part of the dehumidified air leaving the absorber (state 4) is taken to facilitate ventilation purposes while the remainder is brought into direct contact with sprayed water in the evaporative cooler. The temperature of the absorbent solution in the absorber is maintained constant using a water-to-solution heat exchanger enclosed within the absorber chamber through which cold water from the evaporative water cooler is circulated. To maintain the liquid desiccant at the proper concentration for moisture removal, pump 2 pumps weak solution from the absorber sump (state 10), through the solution-to solution heat exchanger where it is pre-heated to state 11 by recovering heat from the hot solution leaving the regenerator. The pre-heated solution is then pumped through the solution-to-thermal fluid heat exchanger where it is heated to the required regeneration temperature (state 12). The hot solution then trickles down the regenerator distribution system in counter flow to atmospheric air entering at the bottom of the regenerator. The vapour-pressure difference between the ambient air and the hot solution causes ambient air to absorb water vapour from the solution (i.e. re-concentrate the absorbent to state 13).
The hot air is discharged to the atmosphere while the re-concentrated solution (state 13) is pumped through the solution-to-solution pre-cooler and the solution-to-water cooler to the absorber distribution system. During solar heating, pump 4 supplies the thermal fluid-solution heat exchanger with the required amount of the hot thermal fluid from the hot fluid storage tank. After it exchanges its heat with weak solution, the leaving warm solution is mixed with another amount of warm thermal fluid from the warm fluid storage tank and pumped through the solar collector heater to the hot thermal fluid storage tank. During night, pump 5 supplies the thermal fluid-solution heat exchanger with the required amount of hot thermal fluid from the hot thermal fluid storage and store the warm fluid in the warm fluid storage tank. The regenerator and the associated flow system and components are all similar towhat was shown at the absorber side. The system regeneration side is shut down if the thermal fluid storage tank cannot supply thermal fluid at sufficiently high temperature or if the absorbent solution concentration in the absorber pool rises above a set limit. Psychometric cycle of process air flowing through the solar driven liquid desiccant evaporative water cooler employed solely to provide cold water for a radiant system. Lines 1-2, 2-3 represent the path of the process air (ambient air) through the air-to-air and air-to-water heat exchangers. Line 3-4 represents the path through the absorber and line 4-5 the pass through the evaporative water cooler.
System’s Simulation
The simulation process constitutes description of the procedure used to model the system components and a main program that integrates these components. The main program calls the unit subroutines to page link the components and form a complete cycle. Mass and energy governing equations are written by taking each system component as a control volume and divide the domain of interest into a finite number of computational cells using finite difference technique. A mathematical solver solves simultaneously the system components governing equations.
Active Solar Space Cooling
Solar space cooling is quite costly to implement. If the solar system is used for space cooling only, installed costs can run $4,000-$8,000 per ton. It is best to use a solar system that serves more than just the cooling needs of a house to maximize the return on investment and not leave the system idle when cooling is not required. Significant space heating and/or water heating can be accomplished with the same equipment used for the solar cooling system.
Solar-powered refrigerators
Solar-powered refrigerators are most commonly used in the developing world to help mitigate poverty and climate change. By harnessing solar energy, these refrigerators are able to keep perishable goods such as meat and dairy cool in hot climates, and are used to keep much needed vaccines at their appropriate temperature to avoid spoilage. The portable devices can be constructed with simple components and are perfect for areas of the developing world where electricity is unreliable or non-existent. [1] Other solar-powered refrigerators were already being employed in areas of Africa which vary in size and technology, as well as their impacts on the environment. The biggest design challenge is the intermittency of sunshine (only several hours per day) and the unreliability (sometimes cloudy for days). Either batteries (electric refrigerators) or phase-change material is added to provide constant refrigeration.
History of solar refrigeration
"In developed countries, plug-in refrigerators with backup generators store vaccines safely, but in developing countries, where electricity supplies can be unreliable, alternative refrigeration technologies are required”.[3] Solar fridges were introduced in the developing world to cut down on the use of kerosene or gas-powered absorption refrigerated coolers which are the most common alternatives. They are used for both vaccine storage and household applications in areas without reliable electrical supply because they have poor or no grid electricity at all.[4] They burn a liter of kerosene per day therefore requiring a constant supply of fuel which is costly and smelly, and are responsible for the production of large amounts of carbon dioxide.[5] They can also be difficult to adjust which can result in the freezing of medicine.[6] There are two main types of solar fridges that have been and are currently being used, one that uses a battery and more recently, one that does not
