تجزیه و تحلیل اقتصادی کاربرد گسترش در مقیاس متوسط سیستم های تهویه مطبوع با سرد کننده معمولی، R1234yf و CO2
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|29095||2013||11 صفحه PDF||سفارش دهید||7530 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : International Journal of Refrigeration, Volume 36, Issue 5, August 2013, Pages 1472–1482
Expanders have been shown to improve the energy efficiencies of refrigeration systems. The current technology is also adequate to manufacture and integrate expanders to practical air-conditioners. In this paper, an economic analysis of the installation of expanders on to existing vapor compression cooling systems, particularly medium scale air-conditioners, is presented. Various refrigerants, including the established and the newly proposed varieties, are considered. From the investigations, it was found that when the expander efficiency is 50%, the payback periods of most conventional systems are below 3 years in high temperature countries with high electricity tariffs and are above 5 years in other countries. Expanders are especially attractive for the transcritical CO2 and the R404A systems. The payback periods are shorter for systems with highly efficient expanders, high cooling loads, high ambient temperatures and for low refrigerating temperature applications.
At the 15th session of the Conference of Parties (COP 15) to the United Nations Framework Convention on Climate Change (UNFCC) in Copenhagen, Denmark, in 2009, climate change was underlined as one of the greatest challenges. Deep cuts in global emissions and low-emission development strategy were recognized as crucial to combat the issue (United Nations Framework Convention on Climate Change, 2009). These calls to curb greenhouse gases emissions are continuations of the 1997 Kyoto Protocol (United Nations Framework Convention on Climate Change, 1997) which calls for the reduction of the emissions of, among others, carbon dioxide and two groups of refrigerants, i.e. hydrofluorocarbons and perfluorocarbons. The production and emission of the more environmentally damaging substance, chlorofluorocarbons (CFCs), have been regulated even earlier, in 1987 under the Montreal Protocol (United Nations Environment Program, 1987). In response, more environment friendly refrigeration systems have been investigated in recent years. Two aspects are of particular concerns, namely the use of environmentally friendly refrigerants and the energy consumption issue. With the phasing out of the use of CFCs, chemical substances like the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs), were proposed and have been used as temporary alternatives. However, these compounds are considered to be greenhouse gases (Chen, 2008; Tsai, 2005a, b). As a response to these concerns, even more environmentally friendly refrigerants, mainly R1234yf (Honeywell,) and natural refrigerants (Hwang et al., 1998; Lorentzen, 1988, 1994, 1995; Lorentzen and Pettersen, 1993), particularly CO2 and ammonia (NH3) have been proposed as replacements. As for the energy consumption issue, while it is actually a natural process, high energy consumption is damaging to the environment mainly because of our current over-dependence on energy produced by conventional fossil fuel burning processes which produce greenhouse gases (especially CO2). The world energy consumption had grown steadily in the past decades. In 2010, the overall consumption grew by 5.5% (Enerdata,) while electricity consumption, which accounted for about 40% of the total energy consumption (U.S. Energy Information Administration, 2011), increased by 6.4% (Enerdata,). This growth was driven mainly by the growing demand of the developing countries, particularly China (the world's largest energy consumer in 2010) and India (the third largest, below the US). In Singapore, the electricity consumption increased by 9% in 2010 (AsiaOne,) with about 20% of the electricity consumed by households (Ministry of the Environment and Water Resources of the Republic of Singapore, 2008). It is also reported that 40% of the world annual energy consumption was used for the provision of building-related facilities such as lighting, heating and air conditioning (Omer, 2008). In the UK, for example (Nicholls et al., 2009), in 2009, buildings were responsible for around 50% of all carbon emissions in the country. About half of the household electricity consumption in Singapore in 2009 was for air conditioning and refrigerators (AC&R) (Ministry of Trade and Industry of the Republic of Singapore (2007); Tay. In Guangzhou, China, AC&R contributed 30% of the total household electricity consumption in 2003 (Kondou et al., 2011). In the US, cooling and refrigeration contributed about 24% of the total electricity consumption of commercial buildings, while air conditioning consumed more than US$25 billion or about 20% of the total household electricity consumption in 2005 (U.S. Energy Information Administration, 2010). In the UK, the electricity consumption of AC&R applications contributed about 8% of the total greenhouse gases emissions in 2008 (Cowan et al., 2009). Globally, AC&R applications accounted for about 15% of the world electricity use (Coulomb, 2006). Most of the electrical energy used to operate AC&R systems is, and will still be for the foreseeable future, produced from fossil fuels which produce greenhouse gases, mainly CO2. This accounts for about 80% of the total greenhouse gases emissions of AC&R while the rest is from the release of refrigerants into the atmosphere (International Institute of Refrigeration, 2004). It is therefore very important not only to have a refrigeration system using an environmentally friendly refrigerant, but also to have one with good energy efficiency. Various methods have been proposed to improve the energy efficiency of refrigeration systems. One of the ways is to recover the power loss during expansion by replacing the conventional expansion valve with an expander (shown in Fig. 1), the idea that was initially proposed to increase the efficiency of the CO2 refrigeration system (Lorentzen, 1994). When applied to conventional R22 and R134a systems, the coefficient of performance (COP) has been reported to increase by up to 15% (Robinson and Groll, 1998) and 12% (Goncalves and Parise, 2008), respectively. When applied to a transcritical CO2 system, where the pressure difference between the suction and discharge lines is very high (in the range of 70 bar), the COP can increase by up to 50% (Fukuta et al., 2006).An expander can be seen as a compressor operating in reverse. Therefore, any compressor mechanism can theoretically be used as an expander. These include the rolling piston (Hua et al., 2010; Li et al., 2009; Matsui et al., 2009, 2008), rotary vane (Fukuta et al., 2009, 2006; Jia et al., 2011), scroll (Kim et al., 2008; Kohsokabe et al., 2006), piston (Baek et al., 2002, Baek et al., 2005a and Baek et al., 2005b; Nickl et al., 2005), screw (Kovacevic et al., 2006), swing piston (Guan et al., 2006), revolving vane (Subiantoro and Ooi, 2009, Subiantoro and Ooi, 2012a and Subiantoro and Ooi, 2012b), etc. With the current state of technology, expander prototypes with overall efficiency of more than 60% have been reported (Nickl et al., 2005). However, some important issues must be addressed when designing an expander. These include the two-phase expansion process (Fukuta et al., 2008, 2009) and the suction and discharge control. Many compressors utilize pressure difference to control the opening and closing of the suction and discharge valves. Unfortunately these mechanisms cannot work for expanders since the pressure in the working chamber must be controlled by the suction and discharge valves, not the other way around. The simplest method may be to use externally controlled valves such as the solenoid or the pneumatically activated valves. However, the time delays inherent to these types of valves may cause problems (Baek et al., 2005a and Baek et al., 2005b; Goncalves and Parise, 2008). Therefore, it is common to find different and novel mechanisms to control the suction and discharge processes. Another challenge is to select the method of integration of the expander to the refrigeration system. There are at least 3 available methods: 1) to use a generator and supply the generated power to motor, 2) to connect the expander to the motor and the compressor using mechanical systems, 3) to design and manufacture the expander and the compressor as one component. Method 1 is less desirable due to the need of a generator. Method 2 is simple and common (Kakuda et al., 2009; Matsui et al., 2009), but the main technical challenge is to deal with the matching of the frequencies between the compressor and the expander. Method 3 is ideal but is challenging to design. Nevertheless, some integrated expander/compressor mechanism designs have been reported recently too (Kovacevic et al., 2006; Peng et al., 2006). Fortunately, our current technology is adequate to address all the major issues related to expander and its integration to commercial refrigeration systems. The reservation is mostly due to the additional cost involved. Hwang (2009) discussed briefly about its worthiness in an editorial, noting that the expander efficiency is a crucial factor. Henderson et al. (2000) has carried out a study about the economical consideration of using expanders in heat pumps. To the authors' knowledge, no study of the application for air-conditioning and refrigeration purposes has been reported. It is the purpose of this paper to present a detailed study on the economical consideration of the installation of expanders for cooling purposes, with particular emphasis on medium scale systems (cooling capacity of 5.27 kW). Various refrigerants, including the established and the newly proposed ones, namely R22, CO2, R134a, NH3, R32, R404A, R1234yf, R410A, R407C and R438A are considered.
نتیجه گیری انگلیسی
In this paper, theoretical analysis has been carried out to analyze the benefits of using an expander instead of an expansion valve in the vapor compressor cooling systems when using various refrigerants. The analysis shows that expanders have been shown to be able to improve the energy efficiencies of refrigeration systems. The current technology is also capable of manufacturing commercially practical expanders and integrating them into existing vapor compression refrigeration and air-conditioning systems. However, the additional cost involved has delayed the introduction of expanders to the market. An economic analysis of the installation of expanders to refrigeration systems has been carried out to address the financial issue. The emphasis is on medium scale air conditioning systems where the cooling load is 5270 W, the ambient temperature is 35 °C, the evaporating temperature is 7.2 °C and the condensing temperature is 54.4 °C. Various refrigerants, namely R22, CO2, R134a, NH3, R32, R404A, R1234yf, R410A, R407C and R438A are considered. The payback period is the main parameter in this study. Accounting for various factors, the expander retail price has been assumed to be US$100. The system is assumed to operate 2000 h per year for the base case. This is a typical annual operating usage in high temperature regions. From the investigations, the followings are found: 1. In general, expanders are financially feasible to be installed into refrigeration systems, particularly medium scale air conditioners. Assuming that the compressor and the expander efficiencies are 75% and 50%, respectively, the payback periods are less than 5 years for all the systems studied, except for that with ammonia as the refrigerant. Expanders are especially attractive for CO2 and R404A systems with payback periods of less than 1 and 3 years, respectively. 2. It is important to use a highly energy efficient expander. For illustration, an increase of the expander efficiency from 30% to 60% in an R1234yf system cuts the payback period from 5.3 to 3.4 years. 3. Payback period is not significantly affected by the degradation of the efficiencies of the compressor and the expander, even when the degradation is 5% per year. 4. Expanders are more attractive to be employed for systems with high cooling loads such as room and automotive air conditioners. It is less attractive for smaller systems such as a household refrigerator. 5. Expanders are most attractive for places with high ambient temperature such as in the tropics and the desert regions. 6. Expanders are very suitable to be employed for systems with low refrigerating temperatures. When the evaporator temperature is −30 °C, the payback periods of most of the systems are less than 1 year. 7. The variations of the payback period with electricity tariff for various refrigerants have also been studied. In hot temperature countries with high electricity tariffs like Singapore and Brazil, the payback periods are short, less than 3 years for most conventional systems and less than 6 months for the CO2 system. For hot temperature countries with relatively low tariffs like India, Indonesia, Vietnam and South China, the payback periods of most of the conventional systems are currently between 5 and 10 years. It is noted, however, that the electricity tariffs in these countries are expected to increase following rising local energy demands and global trends of increasing energy prices. 8. Expanders are not economically attractive to be installed in air conditioners in temperate regions, except in CO2 systems. The payback periods of conventional systems are more than 10 years while that of CO2 systems are about 5 years. 9. The installation of expanders can save about 1–4 MWh of energy over the course of 8 years in conventional systems, depending on the usage, operating conditions and type of refrigerant used. When used in a transcritical CO2 system, the energy saving is even higher, at almost 16 MWh. To conclude, our study shows that in most practical cases, the use of expanders in commercial refrigeration systems not only reduces the environmental impact of the systems, but also has practical payback periods for both the conventional and especially, the transcritical CO2 systems. The study concludes that the use of expanders in commercial refrigeration systems seems to be financially justifiable. Therefore, we recommend for refrigerators and air-conditioner manufacturers to seriously consider employing expanders in their future refrigeration units.