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Solar Air-Conditioning Systems Impact on the Built Environment – a Thermodynamic Approach

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Solar Air-Conditioning Systems Impact on the Built Environment – a Thermodynamic Approach C. Koroneos, E. Nanaki, G. Xydis, Solar Air-conditioning Systems Impact on the Built Environment - A Thermodynamic Approach. 45th ISOCARP Congress 2009 Solar Air-conditioning Systems Impact on the Built Environment – A Thermodynamic Approach 1. Introduction Energy consumption in European domestic and tertiary sectors represents about 40% of the annual EU-15 final energy use and about a third of greenhouse gas emissions. Among these, about two-thirds are concentrated in residential sector, the remaining part in commercial building. The household sector represents about 70% of total energy consumption in buildings sector [1]. During the last few decades energy consumption for cooling has increased dramatically in most European countries. The main reasons for the increasing energy demand for summer air- conditioning are the increased thermal loads, increased living standards and comfort demands in conjunction with architectural characteristics and trends. During the summer the demand for electricity in Greece increases due to the extensive use of heating ventilation and air conditioning systems, which increase the peak electric load, causing major problems in the electric supply. In the current practice, air conditioning is exclusively based in the use of electric energy, while the use of solar energy is limited at heating of domestic hot water and in limited applications for space heating and fewer for cooling. It has been estimated that the total energy consumption in Greece, in 2003, for central air conditioning systems was 2909 GWh/y [2], whereas the per capita consumption was estimated at 371 KWh/ y [3]. The extensive use of electrically driven compression cooling equipment is responsible for an increase of greenhouse gases emissions, due to the energy production or to the leakage of the cooling fluids; intensifying the cycle of climate change. The latter is of great significance, especially in the case of public buildings. The energy behavior of the public buildings in Greece varies according to the building’s age and its structural components [4]. Public buildings that have been constructed until 1940 are characterized by heavy structural components, lack of central heating, high energy consumption rates and satisfactory conditions of thermal comfort especially during the summer months. The majority of public buildings in Greece have been constructed during the period 1940-1980 (before 1973); these constructions are characterized by reinforced concrete, lack of thermal insulation, they usually have a central heating system, high energy consumption rates and medium thermal comfort. The energy behavior of the public buildings in Greece has significantly been improved, during the past 20 years, mainly due to the use of thermal insulation. In 1980 the buildings adsorbed 22% of total energy consumption; while in 1994 the absorbed energy came up to 30.9% [5].This increase can be attributed to the increase in the use of electrical energy. Taken into consideration the fact that the energy consumption in public buildings will be greater than 50 kWh/ m 2, coming up to a total of 250 kWh/ m 2; it is obvious the need for rational energy use so as to avoid the economic as well as environmental impacts. For this reason the development of trendsetting technologies for reliable, affordable and environmentally friendly energy is vital. The use of solar energy to drive cooling cycles for space conditioning of most buildings constitutes an attractive concept, since the cooling load coincides generally with solar energy availability and therefore cooling requirements of a building are roughly in phase with solar incidence. Solar cooling systems have the advantage of using harmless working fluids such as water, or solutions of certain salts. They are energy efficient and environmentally safe. They can be used, either as stand-alone systems or with conventional air conditioning, to improve the indoor air –quality of all types of buildings. The main goal is to utilize “zero emission” technologies to reduce energy consumption and CO 2 emissions. 1 C. Koroneos, E. Nanaki, G. Xydis, Solar Air-conditioning Systems Impact on the Built Environment - A Thermodynamic Approach. 45th ISOCARP Congress 2009 The continuous increase in the price of energies, associated with support systems set up by the majority of governments has made it possible for a significant solar thermal sector growth. In particular, the solar energy market in the EU presented significant growth (+12%); still at 1.089 MW th of newly installed capacity it remains below what would be needed to reach EU’s target for 2 2010: 70,000 MW th of solar thermal capacity (100 million m ). The solar thermal capacity in operation reached 9,525 MW th at the end of 2004, which provides 8,164 MW th of clean energy. As far as Greece is concerned, 151 MW th of new solar thermal capacity were installed in 2004 – an increase of 34% compared to 2003. For 2005 a continuation of the pre-2004 trend is expected, with sales in the area of 119 MW th [6]. Today approximately 120 solar thermal assisted cooling systems are presently installed worldwide, with a total cooling capacity estimated at 10MW [7,8].Their specific collector is ~ 3m 2/ Kw for water chillers or 10 m 2 per 1000 m 3/h of air volume flow in desiccant systems. Most of the systems have been installed in Germany, Spain as well as Greece [9].About 69% of theses systems use absorption chillers whereas about 10% of them adsorption chillers. Furthermore, in about 19% of the installations a solid desiccant cooling system is installed. It should be noted that their primary energy savings potential is between 30-60%; however these potentials are often unrealizable with the current systems, due to sub-optimal design, installation, and operation. Although a precise statement on the economic situation of a solar assisted air conditioning system depends on the specific system, in general the annual cost ( the complete cost including capital cost, operation and maintenance costs etc) of a solar assisted air conditioning system are currently above the annual cost of a conventional system. In the context of this paper, the potential for solar applications, mainly for solar cooling are investigated for a medical center in Northern-Western Greece. To be more specific, an exergy analysis is applied, to a solar air conditioning system, so as to analyze the performance of a cycle driven by solar energy. Additionally, an environmental and an economic evaluation of the solar cooling installation in the medical center are presented in this study. The environmental benefits of such a system outweigh the conventional ones. The results are of great significance, as they are indicative of the application prospects in the Greek market. 2. Solar Cooling Solar air conditioning refers to any air conditioning system that uses solar power. This can be done through passive solar, photovoltaic conversion or solar thermal energy conversion. Specifically, solar cooling technologies use solar thermal energy provided through solar collectors to power thermally driven cooling machines. A solar cooling installation consists of a typical solar thermal system made up of solar collectors, storage tank, control unit, pipes and pumps and a thermally driven cooling machine. A conventional energy source functions as auxiliary, mainly for the periods when the cooling load is not covered by the produced energy by the collectors. The schematic installation for the provision of solar air conditioning is presented in Figure 1 [10]. 2 C. Koroneos, E. Nanaki, G. Xydis, Solar Air-conditioning Systems Impact on the Built Environment - A Thermodynamic Approach. 45th ISOCARP Congress 2009 Cooling Refrigerant tower Refri gerant steams liquid Generator Condenser Solution heat exchanger Expansion valve LiBr LiBr/H 2O Absorption Evaporator Refrigerant Refrigerant liquid steams Figure 1: Schematic installation for the provision of solar air conditioning using as refrigerant water [ 10 ] The basic principle for production of cold by heat is based on the evaporation technique. The basic advantage of solar cooling is, that the greater heat exists the more cooling is produced, advantage that is excellently synchronized with the requirements for summer cooling in buildings. In Table 1, the basic operations characteristics of the main solar cooling techniques are summarized. 3 C. Koroneos, E. Nanaki, G. Xydis, Solar Air-conditioning Systems Impact on the Built Environment - A Thermodynamic Approach. 45th ISOCARP Congress 2009 Method Closed Open Cooling cycle Closed circulation of cooling Cooling means in contact with the means atmosphere Principle of method Cold water production Air humidification +evaporative cooling Absorption mean Solid Solid Liquid Liquid Cooling/absorption water/silica gel H2O/ LiBr, H2O/ silica gel, H2O/CaCl, mean NH3/ salt NH3/H2O H2O/ LiCl, H2O/ LiCl cellulose Available techniques Cooling Cooling Air-conditioning adsorption absorption based on ---- machines machines absorption (ABA) Available power 70-1050 kW 50 kW-5MW 30-350 kW (per (KW) (A) unit) ---- 250 Kw-5MW (B) Coefficient of 0.3 – 0.7 0.6 -0.75 (A) 0.5-1.0 >1.0 performance COP 1.0- 1.20 (B) Driving Temperature 60-90 0C 80-110 0C (A) 45-95 0C 45-95 0C 140-160 0C (B) Solar Collector Vacuum tube, Vacuum tube, Flat plate Flat plate flat plate flat plate collector, air collector, air collector selective collector collector collector Table 1 : Characteristics of solar cooling techniques [11] A classification is presented based on the type of thermal cycle –either open or closed- and absorption medium. The types of thermal cycles are distinguished to: • closed systems: thermally driven chillers that provide chilled water, which is either used in air handling units to supply conditioned air or that is distributed via a chilled water network to the designated rooms to operate decentralized room installations. These systems usually use cooling towers or similar equipment to reject heat. The market available machines for this purpose are the absorption chillers.
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