phase_Change_materials

USE OF RENEWABLE PHASE CHANGE MATERIALS
TO REDUCE CARBON DIOXIDE EMISSIONS
Galen J. Suppes, Michael J. Goff, and Shailesh Lopes,
Department of Chemical Engineering, W2028 Engineering Bldg. East,
Columbia, MO, 65211
Abstract
Phase change materials (PCM) can reduce heating, ventilation, and air conditioning (HVAC) costs by a number of different mechanisms, including: eliminating air conditioning costs by storing nighttime coolness for use during the day, eliminating heating costs by storing daytime warmness for use during the night, and by load-shifting of electricity through thermal storage. Well-designed strategies and utilization of phase change materials could easily reduce HVAC-related carbon dioxide emissions by more than 25% from current levels. It is anticipated that these reductions in emissions can be achieved while saving consumers money and creating new markets for agricultural commodities. Greenhouse gas reduction potential of PCSs is further enhanced by producing them from fats and oils. This paper summarizes experimental data on the synthesis of PCMs from beef tallow and quantifies the cost and greenhouse gas savings from their use.
Introduction
Figure 1 summarizes carbon dioxide emissions in the U.S. by source i[1]. As evident by Figure 1, electrical power generation is the largest producer of carbon dioxide emissions, and furthermore, emissions in this sector are growing faster than any other sector. When electricity is combined with residential and commercial fossil fuel heating, the sum of 33.9%, 5.4%, and 3.4% total a massive 42.7% of the total greenhouse gas emissions. Due to magnitude of this contribution to greenhouse gas emissions, thermal energy storage can pave the way to reducing carbon dioxide emissions to 1990 levels or less for decades to come. In many if not most instances, reductions in greenhouse gas emissions can be achieved while saving the consumer money.
Thermal energy storage can reduce greenhouse gas emissions through multiple mechanisms, including: 1) Peak load shifting of electricity, 2) Eliminating part of the heating or air conditioning loads, and 3) Enhance the performance of alternatives to fossil fuel combustion heating. The purpose of this paper is to provide an introduction into the topic of phase change materials and to describe how phase change materials can reduce greenhouse gas emissions through each of the three indicated mechanisms.
Phase change material store coolness or warmness, typically through a latent heat of freezing that is about one hundred fold greater than sensible heat. I is the most commonly used phase change material and is used to keep food cool in a cooler or a drink cold in a cup.
While a 0︒C storage in ice can be used to store coolness for a house, it would require a more expensive air conditioner and consume more energy to go to this low temperature as compared to typical evaporator operation at about 10︒C. Alternatively, a phase change material that
freezes at 15︒C freezes at a high enough temperature to be frozen by a conventional air conditioner while freezing at a low enough temperature to provide a temperature driving force to keep the house cool. Particularly good and efficient strategies can be developed to enhance HVAC applications through the proper choice of phase change material with a freezing point typically between 15︒C and 35︒C.
Figure 1. Summary of carbon dioxide emissions by source in the United States.
While some inexpensive salts have been demonstrated for HVAC applications, these salts have two performance problems: 1) they are corrosive and 2) the hydrates typically used in the 15-35︒C temperature range are often not stable for prolonged storage and cycling in HVAC applications.
Narrow freezing point range paraffin waxes distilled from petroleum are often the product of choice. These waxes typically sell for prices greater than $0.35 per pound. When compared to the price of fats and oils ranging in price from $0.10 to $0.30, derivatives of fats and oils can be prepared at lower costs than paraffin waxes. These new fat and oil products are made from renewable resources and have the potential to undercut the paraffin waxes on price. Fatty acid derivatives of fats and oils are able to meet a range of energy storage means in the 15-35︒C temperature range. Current research by the authors of this paper has a focus on low-cost and high conversions of natural fat and oil products for PCM applications.
Efficiency for Energy Storage
When evaluating chemicals for energy storage, thermal storage options have two important performance advantages over battery, water, or other storage options.
The most important performance advantage is the fact that thermal storage and recovery can approach efficiencies of 100% while chemical energy or potential energy storage and retrieval will typically have overall efficiencies of less than 80%. When HVAC is the final application, thermal energy storage can approach 100% efficiency because it is governed by heat transfer that is governed by the first law of thermodynamics. In practical terms this means that in a well-insulated system no energy is lost because it is conserved. In real applications some air handling losses are incurred, but these can be minimized.
Alternatively, battery and pump storage include the transfer of electrical energy to chemical energy (in the case of a battery) or shaft work then potential energy (in the case of pump storage). Fundamentally each of these energy conversions can approach 100% efficiency, but in practice chemical energy and potential energy conversion are each rarely better than 90% efficient and electric motor production of shaft work is rarely better than 95% efficient. Alternatively, conversion of electrical energy to heat and transfer of heat is routinely performed at first law efficiencies greater than 99%.
Thermal energy storage is the most efficient way to store energy when the final application is in HVAC. Matching the PCM freezing points with applications is the most important design criteria to minimize or eliminate any inefficiencies in the energy storage process.
Peak Load Shifting
Peak load shifting is the easiest application of phase change materials and is also the application capable of producing the greatest reductions in carbon dioxide emissions. The concept is quite simple. In the summer time, air conditioners and most industrial equipment tends to naturally operate during the middle of the day when the sun’s heat is the greatest and people are at work running equipment. Without the intervention of electrical power providers, peak demand for electricity would be many times greater than the baseline load at 2:00 in the morning. With intervention through cooperation with customers, peak demand is often reduced to being 50% to 100% greater of baseline load.
To meet peak demand needs, electrical power companies need electrical power generation that can start up and shut down quickly. The preferred generation means are gas turbines typically operating at thermal efficiencies less than 30%. As compared to a modern natural gas combined cycle plant operating at 50% to 53% thermal efficiency, the peak demand unit produces about 40% more carbon dioxide per kilowatt hour of electricity produced.
Also, the use of wind and nuclear energy is limited due to their inflexible nature in meeting peak demand needs. Wind is available on nature’s schedule and nuclear power is capital intensive and best operated a full load at all times. Energy storage can greatly enhance the ability of nuclear and wind power to meet all electrical needs including peak demand needs.
When replacing peak demand gas turbines with wind or nuclear power, nearly 100% of the carbon dioxide emissions are eliminated.
While wind and nuclear energy are unable to meet much of electrical power needs on their own, they can fully meet electrical power needs when combined with energy storage. The sky is the limit when it come to potential reductions in carbon dioxide emissions due to electrical power and HVAC when wind or nuclear energy is combined with energy storage.
Eliminating HVAC Loads
Phase change materials can be used to eliminate HVAC electrical power or fossil fuel loads in some instances. One means of doing this is to store solar energy accumulated during the day and used during the night. Most buildings simply do not have enough built-in heat capacity to substantially moderate temperatures in a 24 hour cycle—PCM devices can be used as a active means to provide this energy storage while tapping into natures sources of heat or coolness available during only part of the 24 hour cycle.
In the winter time, solar energy provides a source of warmth that can be tapped into with varying degrees of success. In the summer time, evaporative cooling that typically achieves much lower temperatures during the night provides a source of coolness. Various means are available to tap into these sources of heat and coolness.
In addition to active storage of solar-originated energy during the winter, passive storage can be achieved by incorporating PCM into wall boards. This passive method simply increases the heat capacity of a building and should be performed on a surface (such as wallboards) that readily allow heat transfer between the room contents and the PCM.
Enhancing the Performance of Alternatives
The previous section described how enhancing the performance of solar heating or evaporative coolers can be used to eliminate heating or cooling costs. In these examples, significant reductions in electrical costs can be achieved. Phase change materials can also be used in combination with vapor-compression cycles to increase the utility of vapor-compression cycles.
Summary
PCM devices can be used to substantially reduce greenhouse gas emissions originating from electrical power generation and residential/commercial heating. While such PCM devices are typically used with HVAC applications, their ability to moderate peak demand impacts most electrical power generation. The ability of electrical power generation means such as wind and nuclear energy to meet consumer needs are greatly enhanced when used with energy storage such as PCM devices.
Acknowledgements
This work was supported by the USDA NRI Competitive Grants Program/USDA award number 2001-355504-11208. Experimental results were presented at the conference and were in the process of being reviewed for publication at the time of the conference.
References
i[1]Emissions by Economic Sector, Excerpt from the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2000, U.S. EPA, Office of Atmospheric Programs, September 2001, see /globalwarming/publications/emissions/index.html.
用于CO_2吸收的离子液体的合成、表征及吸收性能--《精细化工》2007年04期
本文献来源中国知网
以N-甲基咪唑和3-溴丙胺氢溴酸盐为起始原料,合成了一种含氨基的离子液体(ionic liquid,IL)——1-(1-氨基丙基)-3-甲基咪唑溴盐(简写为[NH2p-mim]Br),总产率为90.1%,溴含量为0.980 mol Br/mol IL,胺含量为0.948 mol NH2/mol IL。

探讨了合成工艺,并通过1HNMR、13CNMR、IR和MS对产物结构进行了表征。

吸收实验结果表明,该离子液体能够有效地吸收CO2,在40℃和106 kPa下,质量分数为45%的离子液体水溶液吸收CO2至饱和时,溶液中CO2的摩尔分数可达0.444 mol CO2/mol IL,接近理论吸收量0.5 mol CO2/mol IL;在90℃的真空状态下,吸收的CO2能够完全解吸,重复吸收实验表明,该离子液体吸收CO2的能力无明显下降。

【作者单位】：南京大学化学化工学院江苏南京210093
【关键词】：离子液体;二氧化碳;吸收;解吸
【基金】：江苏省自然科学基金项目(BK2005077)
【分类号】：O621.3
【DOI】：CNKI:ISSN:1003-5214.0.2007-04-003
【正文快照】：
目前,有机胺水溶液吸收法是脱除CO2最主要的方法[1~3],但传统的有机胺具有一定的蒸气压,在脱碳过程中不可避免地产生挥发性有机物(VOCs),导致一定的环境问题,严重影响了该过程的经济性。

离子液体(ionic liquid,IL)是指在室温及相邻温度下完全由离子组成的有机液体物质,其最大的特点是几乎没有蒸气压,热稳定性好[4],因此可作为环境友好的反应和分离介质。

一系列研究表明,CO2在离子液体中的溶解度比其他气体大得多[5~7]。

2002年,文献[8]首次报道了含氨基的离子液体———1-(1-氨基丙基)-3-丁基咪唑氟硼酸盐([NH2p-bim][BF4])的合成,它吸…
An ionic liquid(IL),1-(1-aminopropyl)-3-methylimidazolium bromide,was synthesized using
N-methylimidazole and 3-bromopropylamine hydrobromide as raw materials,the final yield was 90.1%,the bromide content was 0.980 mol Br/mol IL and the amine content was 0.948 molNH2/mol IL.The product was characterized by 1HNMR,(13)CNMR,IR and MS.Experimental result indicated that the IL is an effective absorbent of CO2.The solubility of CO2 in mass fraction 45% aqueous solution of the IL was up to 0.444 mol CO2/mol IL at 40 ℃and 106 kPa,very close to the theoretical maximum loading(0.5 mol CO2/mol IL).The absorption process could be reversed under vacuum at 90 ℃,and the absorption efficiency of the IL has little observable loss after repeated use.。