Traditional and Innovative Cooling Systems Energy efficient, alternative strategies for thermal comfort Matthew Montry and Randy Maddox picture cover page minimum resolution 300 dpi width of the picture not wider than the text blocks height of http://upload.wikimedia.org/wikipedia/commons/6/69/ IceCreamSandwich.jpg the picture minimum 0.6” from last text block csd Center for Sustainable Development UTSoA - Seminar in Sustainable Architecture Traditional and Innovative Cooling Systems Matthew Montry Randy Maddox Fig. 1 Ancient wind towers, or badgir, in Dubai Introduction energy, while making more efficient use of the electricity they do require. The cooling of spaces to enhance human comfort is not a modern idea Conventional AC systems (Figure 1). However, the exploitation of physical and chemical properties The conventional choice for a of refrigerant fluids to transfer ther- building AC is a vapor compression mal energy is a fairly recent human (VC) system. Compared to other accomplishment, traceable to several system types they are much more experiments conducted in the 19th widely marketed, and they run solely century.1 off a standard source of electric- ity. They are relatively inexpensive The first modern, electric air con- to install because they require little ditioner (AC) was the invention of equipment, and they are available Willis Haviland Carrier in 1902.2 in cooling capacities that will serve Since that time we have come to the entire spectrum of need, from take for granted that a building will a single room to a large building. be equipped with an air conditioning Furthermore, they are a very mature system, and few have an under- technology that has seen significant standing of AC paradigms and tech- improvements in efficiency over the nologies beyond that of a conven- past few decades. tional, electrically operated unit. One such measure of efficiency In this section we first describe some is the coefficient of performance fundamental features of conventional (COP), defined as the ratio of ther- AC technology. Then we explore mal energy transferred by an AC innovative strategies and AC system system to the amount of input energy types that use alternative forms of required. It is not uncommon for a 1 UTSoA - Seminar in Sustainable Architecture residential AC system to have a COP of 4 or greater.3 In a VC system, a refrigerant fluid is first induced to evaporate by being injected from a tube of high pres- sure to another of very low pressure. Lower pressure on a liquid means a lower boiling temperature, so that the molecules evaporate readily. Also, evaporation means that the fluid mol- ecules take on energy, thereby cool- ing their surroundings. Traveling to the compressor, the gaseous fluid is now forcefully driven into an adjacent high pressure tube. This compres- sion keeps the pressure in the first tube low, maintaining the process of evaporation at the other end. The refrigerant is now cooled with air or water, and at this high pressure it is more inclined to condense, sloughing off the thermal energy it gained by evaporation. The liquid then returns to the starting point to begin the cycle again. Commonly used refrigerants for VC systems have changed over the years, primarily because of dis- Fig. 2 Water is cooled in an open system through evaporative cooling. covered dangers to human health and the environment. Even today the most common refrigerant in a conventional VC system is R-22, consider new ways of thinking about that might be appropriate in certain a hydrofluorocarbon with a global AC systems, and we see the value contexts. warming potential 1800 times that of of exploring alternative AC technolo- 4 carbon dioxide. In the US, R-22 will gies that use much less electricity, District Cooling be phased out of use in new equip- because they are driven by other ment in 2010, and out of use alto- energy types. District cooling is an efficient method gether by 2020. of transferring energy, in this case by In what follows, we first consider an chilling water, at a central location to As we realize that we must reduce innovative way of thinking about how be distributed to a network of cus- our carbon footprint and address we cool air—a district cooling sys- tomers. A central water chilling plant other environmental concerns, we tem. Then we describe several AC “eliminates the need for separate begin to question the implications of technologies that are driven by ther- systems in individual buildings.”5 the fact that much of our electricity mal energy, and which might serve is generated by inefficient processes as alternatives to a conventional District cooling is most effective that have dramatic environmen- VC system. Finally, we describe where there is a concentration of tal impacts. We see that we must several geothermal cooling systems cooling loads, such as universities, 2 Traditional and Innovative Cooling Strategies cooling developments.9 The system tally friendly alternatives to the use of typically runs at night, when energy generated electricity. load and demand are lowest, to meet peak demand with “enough [en- Absorption and Adsorption Sys- ergy] to equal the demand of 4,500 tems homes.”10 Two alternative AC technologies Other Austin District Energy locations resemble the VC system in that they include the Domain (with the “na- induce evaporation of a contained tion’s first packaged modular CHP liquid refrigerant . The fundamental [Combined Heat and Power] plant to difference is that either absorption exhaust heat directly into an absorp- (the uptake of a gaseous coolant into tion chiller”), and the Mueller Energy a liquid medium) or adsorption (col- Fig. 3 District cooling tower in downtown Austin, 3rd & San Center, considered among “the most lection of molecules onto the surface Antonio environmentally friendly energy of a collecting medium) replaces systems in the world.” Beyond dis- condensation to keep pressure low trict cooling, the Mueller CHP also in the chamber. These systems then “industrial complexes, densely popu- provides steam heating and onsite require an additional step to recover lated urban areas and high density electricity production.11 the refrigerant in a liquid state, and building clusters.”6 When planning this is where thermal energy plays a area development to take advantage District cooling eliminates the need role. of district cooling, it is also possible for multiple, less efficient systems to retrofit current buildings to use in individual buildings in favor of a Absorption systems chilled water. single, larger system in a central location. With the infrastructure thus Among solar heat driven AC technol- In a typical district cooling plant, wa- located off site, noise level in a build- ogies distributed in Europe, absorp- ter is chilled through evaporative pro- ing is lessened, and no space needs tion systems are by far the most cesses within a cooling tower. Heat to be allocated for a cooling unit. The widely used, with 59 percent of the is emitted when water is pumped to only disadvantage to such a system European market of this type.12 Two the top of the tower then allowed to is the possibility of system failure, in chemical pairs are most common: flow down through plastic or wood which case a single incident prevents ammonia absorbed into water, or shells (Figure 2).7 In open/direct many buildings from operating. water absorbed into a strong lithium cooling towers such as those found bromide (LiBr) and water solution. on the UT campus, water is allowed Thermally Driven Cooling Systems to come into contact with the outside The water/LiBr solution illustrates the air. While pollutants may enter an Recently, AC systems that are driven process well. First, liquid water in a open-loop system, a closed-loop sys- by thermal energy have gained at- chamber devoid of air boils readily tem is available, which cools the air tention, not because they are neces- and fills the void because of the low by more indirect means.8 sarily newer than VC technology, or pressure. This extracts heat from even because they have a higher the container walls and cools the Austin has two downtown chilling COP. The advantage of a thermally space outside it. At the other end plants, which can chill water to 44 driven system is that much of the of the low pressure tube, a strong deg-F (Figure 3). This water is then needed energy may be collected and LiBr/water solution absorbs the piped to various buildings and into stored from readily available sourc- vapor, weakening in the process, but their refrigeration coils, where fans es. Though natural gas is a common maintaining evaporation in the first cool the building by blowing air over source of the required heat, solar chamber. the cooled coils. Austin’s downtown thermal energy or even waste heat cooling plants are a 33,000 ton from co-generation may be used, The weakened solution then moves capable system, with ample capac- both of which represent environmen- to another chamber, where it is heat- ity to service “future mission critical” 3 UTSoA - Seminar in Sustainable Architecture ed to increase pressure and cause much of the water to evaporate. This vapor then passes into another chamber, where it is cooled by water under high pressure. This induces condensation and a return to a lower temperature, so that the water may begin the cycle again. Absorption chillers have until recently only been available for large scale applications 50kW and above (Figure 4). However, systems appropriate for residential scale buildings are becoming available (down to 5kW).13 Wang notes that the main problems with the LiBr technology on a small scale are cost and reliability.14 Hen- ning notes that “the main obstacles for large scale application, beside the high first cost, are the lack of practi- cal knowledge on design, control 15 and operation of these systems.” Fig.