ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 37-2012 SOLAR ENERGY EQUIPMENT.pdf

1、37.1CHAPTER 37SOLAR ENERGY EQUIPMENTSOLAR HEATING SYSTEMS. 37.1Air-Heating Systems. 37.2Liquid-Heating Systems . 37.2Solar Thermal Energy Collectors 37.3Row Design 37.6Array Design 37.7Thermal Energy Storage 37.11Heat Exchangers 37.15Controls 37.17PHOTOVOLTAIC SYSTEMS 37.19OLAR energy use is becomin

2、g more econo mical as the costSof energy continues to climb, especially with increasing govern-ment and utility incentives as well as growing interest in green and/or sustainable construction. In addition, many countries considersolar and renewable energy as a security measure to ensure theavailabil

3、ity of power under adverse conditions. While the UnitedStates continues to grow its solar industry, China, Europe, Asia, andthe Mediterranean basin are leading development of advanced man-ufacturing techniques and applications. However, equipment andsystems are still very similar in all markets; the

4、refore, this chapterprimarily discusses the basic equipment used, with particular atten-tion to collectors. More detailed descriptions of systems and designscan be found in Chapter 35 of the 2011 ASHRAE HandbookHVAC Applications.Commercial and industrial solar energy systems are generallyclassified

5、according to the heat transfer medium used in the collec-tor loop (i.e., air or liquid). Although both systems share basic fun-damentals of conversion of solar radiant energy, the equipment usedin each is entirely different. Air systems are primarily limited toforced-air space heating and industrial

6、 and agricultural drying pro-cesses. Liquid systems are suitable for a broader range of applica-tions, such as hydronic space heating, service water heating,industrial process water heating, energizing heat-driven air condi-tioning, and pool heating, and as a heat source for series-coupledheat pumps

7、. Because of this wide range in capability, liquid systemsare more common than air systems in commercial and industrialapplications.As shown in Table 1, global installed thermal capacity of solarcollectors by type at the end of 2009 reached 172.4 gigawatts(GWth) (Weiss and Mauthner 2011). Unglazed p

8、lastic collectors areused mainly for low-temperature (e.g., swimming pool) water heat-ing; their main markets are in North America (i.e., the United Statesand Canada) with an installed capacity of 12.9 GWth, followed byAustralia (3.3 GWth). Glazed flat-plate and evacuated-tube collec-tors, which are

9、 mainly used to generate domestic hot water andspace heating, dominate the market in China (101.5 GWth), Turkey(8.4 GWth), Germany (8.3 GWth), Japan (4.0 GWth), and Greece(2.9 GWth).According to the European Solar Thermal Industry (ESTIF2010), at the end of 2009 the total thermal capacity in operati

10、on inthe EU exceeded 22 GWth, corresponding to 31.7 million m2ofglazed collector area. Germany is the leader in terms of market vol-ume, with 38% of the European market, whereas Austria, France,Greece, Italy, and Spain together account for 39%. In terms of solarthermal capacity in operation per capi

11、ta, the European average is at43.6 kWth/1000 capita. Cyprus, where more than 90% of all build-ings are equipped with solar collectors, leads Europe with 646 kWth/1000 capita, followed by Austria at 301 kWth/1000 capita, andGreece at about 253 kWth/1000 capita. Solar hot-water systems arenow mandator

12、y in new buildings according to solar ordinances inSpain, Portugal, Italy, Greece, and elsewhere in Europe. China dominates the world market with 70% of the existingglobal capacity, producing 77% of the worlds solar hot-water col-lectors in 2009, followed by 12% in Europe (REN21 2010). Practi-cally

13、all installations in China are for domestic hot water only. Thetrend in Europe is towards larger solar-combi systems that provideboth domestic hot water and space heating, accounting for half ofthe annual market (Balaras et al. 2010), as well as multipurposeheat-pump-assisted solar systems (Todorovi

14、c et al. 2010). The U.S.market for solar hot-water collectors (excluding unglazed swim-ming pool heating) is still relatively small but is gaining ground(especially in California), and in 2009 total capacity increased 10%to some 2.1 GWth(REN21 2010).Photovoltaic (PV) systems, an entirely different c

15、lass of solarenergy equipment, convert light from the sun directly into electricityfor a wide variety of applications. In 2009, the global PV marketreached 7.2 GW and the cumulative PV power installed totaled over22 GW worldwide (producing about 25 TWh on an annual basis),compared to 15 GW in 2008 (

16、EPIA 2010). Europe is leading theway with almost 16 GW of installed capacity in 2009 (about 70% ofglobal PV installations), followed by Japan (2.6 GW) and theUnited States (1.6 GW). Germany maintains the largest marketshare with around 3.8 GW newly installed PV power in 2009 (morethan 52% of global

17、PV market), followed by Italy (711 MW), Japan(484 MW), and the United States (477 MW). Common PV produc-tion is dominated by Chinese and Taiwanese manufacturers (59% ofPV cells and 55% of PV modules), followed by Europe (17% and28%), Japan (9% and 4%) and the United States (5% in both cases).SOLAR H

18、EATING SYSTEMSSolar energy system design requires careful attention to detailbecause solar radiation is a low-intensity form of energy, and theequipment to collect and use it can be expensive. A brief overviewof air and liquid systems is presented here to show how the equip-ment fits into each type

19、of system. Chapter 35 of the 2011 ASHRAEHandbookHVAC Applications covers solar energy use, and bookson design, installation, operation, and maintenance are also avail-able (ASHRAE 1988, 1990, 1991).Solar energy and HVAC systems often use the same componentsand equipment. This chapter covers only the

20、 following elements,The preparation of this chapter is assigned to TC 6.7, Solar Energy Utili-zation.Table 1 Worldwide Solar Capacity by TypeCapacity, GWthEquiv. Glazed Collector Area, 1 000 000 m2Glazed flat-plate collectors 55.1 78.6Evacuated-tube collectors 96.4 137.7Unglazed plastic collectors 1

21、9.7 28.2Glazed and unglazed air collectors 1.2 1.7Total 172.437.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)which are either exclusive to or have specific uses in solar energyapplications:Collectors and collector arraysThermal energy storageHeat exchangersControlsThermal energy storage is a

22、lso covered in Chapter 51, heatexchangers are also covered in Chapter 48, as well as pumps inChapter 44, and fans in Chapter 21.AIR-HEATING SYSTEMSAir-heating systems circulate air through ducts to and from an airheating collector (Figure 1). Air systems are effective for spaceheating because a heat

23、 exchanger is not required and the collectorinlet temperature is low throughout the day (approximately roomtemperature). Air systems do not need protection from freezing,overheat, or corrosion. Furthermore, air costs nothing and does notcause disposal problems or structural damage. However, air duct

24、sand air-handling equipment require more space than pipes andpumps, ductwork is hard to seal, and leaks are difficult to detect.Fans consume more power than the pumps of a liquid system, but ifthe unit is installed in a facility that uses air distribution, only aslight power cost is chargeable again

25、st the solar space-heating sys-tem. Thermal storage for hot-air systems has been problematic aswell because of the difficulty in controlling humidity and moldgrowth in pebble beds and other such devices, particularly in humidclimates.Most air space-heating systems also preheat domestic hot waterthro

26、ugh an air-to-liquid heat exchanger. In this case, tightly fittingdampers are required to prevent reverse thermosiphoning at night,which could freeze water in the heat exchanger coil. If this systemheats only water in the summer, the parasitic power consumptionmust be charged against the solar energ

27、y system because no spaceheating is involved and there are no comparable energy costs asso-ciated with conventional water heating. In some situations, solarwater-heating systems could be more expensive than conventionalwater heaters, particularly if electrical energy costs are high. Toreduce parasit

28、ic power consumption, some systems use the lowspeed of a two-speed fan.LIQUID-HEATING SYSTEMSLiquid-heating systems circulate a liquid, often a water-basedfluid, through a solar collector (Figures 2 and 3). The liquid in solarcollectors must be protected against freezing, which could damagethe syste

29、m.Freezing is the principal cause of liquid system failure. For thisreason, freeze tolerance is an important factor in selecting the heattransfer fluid and equipment in the collector loop. A solar collectorradiates heat to the cold sky and freezes at air temperatures wellabove 0C. Where freezing con

30、ditions are rare, small solar heatingsystems are often equipped with low-cost protection devices thatdepend on simple manual, electrical, and/or mechanical compo-nents (e.g., electronic controllers and automatic valves) for freezeprotection. Because of the large investment associated with mostcommer

31、cial and industrial installations, solar designers and install-ers must consider designs providing reliable freeze protection, evenin the warmest climates.Thermosiphon solar domestic hot-water heating systems oper-ate at low, self-regulated collector flow rates. Their performance isgenerally better

32、in temperate climates than active (using a pump tocirculate the fluid) systems operating at conventional flow ratesand is comparable to pump systems operating at low collector flowrates. Thermosiphon systems dominate the market in residentialand low-rise building applications in southern Europe and

33、China,and Australia. The collector fluid is circulated by natural convec-tion, eliminating the need for the pump and controller of an activesystem. The hot-water storage tank is placed above the collectorarea. The flow rate varies depending on the absorbed solar radia-tion, fluid temperatures, syste

34、m geometry, etc.Direct and Indirect SystemsIn a direct liquid system, city water circulates through the collec-tor. In an indirect system, the collector loop is separated from thehigh-pressure city water supply by a heat exchanger. In areas of poorwater quality, isolation protects the collectors fro

35、m fouling by min-erals in the water. Indirect systems also offer greater freeze protec-tion, so they are used almost exclusively in commercial andindustrial applications.Freeze ProtectionDirect systems, used where freezing is infrequent and not severe,can avoid freeze damage by (1) recirculating war

36、m storage waterFig. 1 Air-Heating Space and Domestic Water Heater SystemFig. 2 Simplified Schematic of Indirect Nonfreezing SystemFig. 3 Simplified Schematic of Indirect Drainback Freeze Protection SystemSolar Energy Equipment 37.3through the collectors, (2) continually flushing the collectors withc

37、old water, or (3) isolating collectors from the water and drainingthem. Systems that can be drained to avoid freeze damage are calleddraindown systems. Although all of these methods can be effec-tive, none of them are generally approved by sanctioning bodies orrecommended by manufacturers. Use cauti

38、on when designing directsystems in freezing climates with any of these freeze protectionschemes.Indirect systems use two methods of freeze protection: (1) non-freezing fluids and (2) drainback.Nonfreezing Fluid Freeze Protection. The most popular solarenergy system for commercial use is the indirect

39、 system with a non-freezing heat transfer fluid to transmit heat from the solar collectorsto storage (Figure 2). The most common heat transfer fluid is water/propylene glycol, although other heat transfer fluids such as siliconeoils, hydrocarbon oils, or refrigerants can be used. Because the col-lec

40、tor loop is closed and sealed, the only contribution to pump pres-sure is friction loss; therefore, the location of solar collectorsrelative to the heat exchanger and storage tank is not critical. Tradi-tional hydronic sizing methods can be used for selecting pumps,expansion tanks, heat exchangers,

41、and air removal devices, as longas the heat transfer liquids thermal properties are considered.When the control system senses an increase in solar panel tem-perature, the pump circulates the heat transfer liquid, and energy iscollected. The same control also activates a pump on the domesticwater sid

42、e that circulates water through the heat exchanger, where itis heated by the heat transfer fluid. This mode continues until thetemperature differential between the collector and the tank is tooslight for meaningful energy to be collected. At this point, the con-trol system shuts the pumps off. At lo

43、w temperatures, the nonfreez-ing fluid protects the solar collectors and related piping frombursting. Because the heat transfer fluid can affect system perfor-mance, reliability, and maintenance requirements, fluid selectionshould be carefully considered.Because the collector loop of the nonfreezing

44、 system remainsfilled with fluid, it allows flexibility in routing pipes and locatingcomponents. However, a double-separation (double-wall) heatexchanger is generally required (by local building codes) to preventcontamination of domestic water in the event of a leak. The double-wall heat exchanger a

45、lso protects the collectors from freeze dam-age if water leaks into the collector loop. However, the double-wallheat exchanger reduces efficiency by forcing the collector to oper-ate at a higher temperature. The heat exchanger can be placedinside the tank, or an external heat exchanger can be used,

46、as shownin Figure 2. The collector loop is closed and, therefore, requires anexpansion tank and pressure-relief valve. Air purge is also neces-sary to expel air during filling and to remove air that has beenabsorbed into the heat transfer fluid.Overtemperature protection is necessary to ensure that

47、the sys-tem operates within safe limits and to prevent collector fluid fromcorroding the absorber or heat exchanger. For maximum reliability,glycol should be replaced every few years. In some cases, systems have failed because the collector fluid inthe loop thermosiphoned and froze the water in the

48、heat exchanger.This disastrous situation must be avoided by design if the water sideis exposed to the city water system because the collector loop even-tually fills with water, and all freeze protection is lost.Drainback Freeze Protection. A drainback solar water-heatingsystem (see Figure 3) uses or

49、dinary water as the heat transportmedium between the collectors and thermal energy storage.Reverse-draining (or back-siphoning) the water into a drainbacktank located in a nonfreezing environment protects the system fromfreezing whenever the controls turn off the circulator pump or apower outage occurs.For drainback systems with a large amount of working fluid in thecollector loop, heat loss can be significantly decreased and overallefficiency increased by including a tank for storing the heat transferfluid at night. Using a night storage tank in large systems is anappropriate strat