ASHRAE LV-11-C007-2011 Over Thirty Years of Experience with Solar Thermal Water Heating.pdf

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1、Dr. Stanley. A. Mumma, PE, FASHRAE is a professor emeritus in the Department of Architectural Engineering, Penn State University, University Park, PA. Over Thirty Years of Experience with Solar Thermal Water Heating Stanley A. Mumma, PhD, P.E. ASHRAE Fellow ABSTRACT History is rich with human applic

2、ations of solar energy. The 20thcentury saw two surges in solar energy use, primarily for thermal applications. The first surge was in the 1950s following M. K. Hubberts landmark 1956 paper Nuclear Energy and the Fossil Fuels predicting that oil production would peak by 1969, and the second in the 1

3、970s following the Organization of Petroleum Exporting Countries (OPEC) Oil Embargo. Unfortunately, both surges were followed by nearly full scale public disinterest, particularly when government incentives were discontinued. Now in the 21stCentury with a renewed public interest in good stewardship

4、under the banners of Green, Sustainable, Net Zero Energy Buildings, and/or reduced carbon footprint; solar has reemerged with particular emphasis on photovoltaic (PV) systems. In 1974, as a newly minted Ph.D. and employed as a mechanical engineering faculty member, the authors dean ask him to design

5、 a solar house to be located on the state fair grounds. That started an intensive 10 year solar effort that included a major action oriented effort funded by the US Federal Government entitled the “National Solar Water Heater Workshop” (NSWHW) program1. That work resulted in solar thermal domestic w

6、ater heaters in much of the US built and installed by homeowners. This paper addresses the activities leading up to the NSWHW program, its implementation, and the performance primarily of one of those systems installed and continuously operating in central PA since 1984. The lessons learned may be i

7、mportant since domestic water heating (DWH), which was accomplished in this case via solar thermal, accounts for 13% of the average US residential energy consumption2. The largest average residential energy consumer is space heating at 25%, also a fine application for solar thermal, be it active or

8、passive. To date, space cooling, representing 13% of the average annual energy consumption, is technically possible but economically challenged. Lighting and minor electrical appliances do account for about 36% of the average annual residential energy consumption, justifying societies current PV thr

9、ust. None the less, the relatively steady year around need for DWH makes this second most intensive energy user a prime application for solar. DEVELOPMENT OF A SOLAR DHW SYSTEM FOR THE PHOENIX AZ CLIMATE. When the author moved to Tempe, AZ in 1976 to join a solar pioneer 3on the faculty of a major s

10、outhwestern university, one of his first activities was to build his own solar water heater. It was built very quickly using copper tubes wired to a sheet metal absorber and employing a heat conducting paste. Standard fiberglass insulation was placed behind the absorber housed in a wooden enclosure.

11、 The glazing was standard 34 by 76 inch window glass. This crude system worked amazingly well, and inspired the author to assign the design refinement as a university graduate course semester project. A few of the students researched the collector enclosure material options including fiberglass, she

12、et metal, and extruded aluminum. LV-11-C007 2011 ASHRAE 572011. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in eit

13、her print or digital form is not permitted without ASHRAES prior written permission.Another group explored glazing materials including low iron glass, Plexiglas, other synthetic glazing materials. Another group explored insulation materials including normal and low binder fiberglass, isocyanurate, a

14、nd foam glass. Finally a fourth group explored absorber choices including extruded aluminum fins with copper water ways and a variety of all copper fin-tube assembliesand associated absorbing coatings. As a class we selected what was considered the best design choices, sought a commitment from a loc

15、al member of the solar industry to supply all the system parts, and wrote a proposal for a Federal Government appropriate technology grant to develop a workshop curriculum to assist home owners with the fabrication and installation of a do-it-yourself solar domestic water heater. That proposal was f

16、unded, and the workshop classes began. The response was extremely strong, and the participants well pleased. The freeze protection approach employed was pumped recirculation. When operating properly, warm water is pumped from the storage tank to protect the collectors from freezing. On clear nights,

17、 the absorber temperature can drop 10F or more below the ambient temperature. On one such night the crude collectors the author had installed on his home froze and bust, resulting in a leak. That day, phone calls to his office from workshop participants revealed that his was not the only one with le

18、aks. Every workshop participant was called, and those impacted were assisted with the repair of similarly damaged waterways. We simply swaged in a new U-bend and soldered it in place. With 20-20 hind sight it was obvious that the temperature sensors needed to bound the absorbers to assure that under

19、 any condition the water did not approach freezing, and each participant so notified. There were no more occurrences of freeze ups subsequently reported. MIGRATION TOWARD A CLOSED LOOP SYSTEM: NOVEL DOUBLE WALLED HEAT EXCHANGER As a young faculty member, the author sought ASHRAE mentors. At one of t

20、he winter meetings, an ASHRAE mentor 4shared an idea he had for a double walled heater exchanger. That idea involved transferring energy from one fluid circuit to another fluid circuit along the fins of a finned-tube heat exchanger. That idea was extremely appealing as a fool proof method of prevent

21、ing cross contamination between a toxic heat transfer fluid and potable hot water since a leak in either circuit would end up on the floor, not the other circuit. Still enthused by the idea, when ask at a HTX manufacturers hospitality suite during the same winter meeting if there was any way they co

22、uld be helpful, samples of fin tube heat exchangers without the u-bends installed were requested and secured. Performance testing and analysis of the finned-tube double walled heat exchanger then became the thesis topic for a graduate student. His thesis research demonstrated that the heat exchanger

23、 effectiveness was sufficiently high that its presence did little to reduce the energy collection compared to not using a HTX5. Armed with this very encouraging information, while on ASHRAE business in Washington DC, the author met with his Federal Government program manager and suggested that the H

24、TX made it possible to expand the very successful Phoenix workshops nationwide. The idea was enthusiastically accepted by the Federal Government sponsor, and work commenced on preparation of a proposal to expand the program nationwide. THE NSWHW PROJECT The project consisted of the following main ta

25、sks: 1. Establish staff and maintain a structure to execute a nationwide effort. 2. Work through all the technical aspects of the system including design and testing/rating. 3. Prepare a workshop execution plan, including recruiting and training university sponsors nationwide. 4. Prepare all the doc

26、uments in support of the program such as the tape slide lectures, student handbooks, hardware suppliers handbooks, and university sponsor handbooks. The author was blessed to have 4 graduate students prepared to handle these 4 areas. Briefly, Task 1 was staffed by a retired Air Force Coronal with ex

27、tensive management and engineering experience. Task two was staffed by a graduate Chemical Engineer with many years of experience and tremendous on details. Task three was staffed by the eminently qualified person who had been so successfully running the appropriate technology solar workshops. And f

28、inally Task four was staffed by a fellow with an Architecture background making him invaluable for the huge task of preparing all the visual training materials. Without these key people, the NSWHW would not have succeeded. 58 ASHRAE TransactionsWe also prepared a short video6to explain the NSWHW to

29、potential educational facilities. In the video “the Solar Pioneer” made the following points: a. It was important to make components available to the workshop that met rigid specifications b. A vendor was identified that committed to supplying the necessary components built to the necessary specific

30、ations, such that all the components fit together properly when assembled. c. The instructors were carefully trained, and had previously personally built the collectors themselves. The system schematic is illustrated in Figure 1. A few of the components will be discussed at this point. Collectors: T

31、he figure illustrates the plug in the supply header causing the bottom 6 parallel copper tubes to be in series via the return header with the top 6 parallel copper tubes. The copper absorber fins were plated with a black chrome finish giving it favorable selective radiation properties. Independent l

32、aboratory testing7reported an FR = 0.732, an FRUL= 0.676 Btu/hr-ft2-F, and a b0of -0.21. At the time this performance was unsurpassed by manufactured equipment on the market according to published results8. Double Walled Heat Exchanger: The figure illustrates the two separate HT loops; the collector

33、 glycol loop and the potable storage tank loop. Collector loop pipe fittings: o A group to be discussed later due to problems; expansion tank, air separator, air vent, and pressure relief valve. o The check valve, also subsequently replaced. o The charging point fittings; two hose bibs and a gate va

34、lvealso to be discussed later. Storage tank loop fittings: o Tempering valve used for safety since the storage tank temperature is often in excess of 180F. The valve also greatly extends the storage tank capacity for those fixed volume appliances such as dishwashers and washing machines i.e. if a 20

35、 gallon draw is needed when the tank temperature is 185F and the city water 60F, less than 13 gallons of hot water is used (only 63.6% of the 20 gallon draw) when blended to 140F. If a blended temperature of 120F is needed, less than 10 gallons of hot water is used (only 47.5% of the 20 gallon draw)

36、. o Dip tube used to put water from the HTX into the storage tank. This water generally about 10-20F hotter than the water on the bottom of the tank, or often well below the desired supply temperature, must be introduced below the thermostat used to control the upper electric heating element. Also a

37、 NSWHW option employed PV9to operate the pumps in the system. The module and pump selections are documented in the NSWHW hardware suppliers handbook. OPERATING EXPERIENCE WITH THE CLOSED LOOP NSWHW SYSTEM IN STATE COLLEGE, PA SINCE 1984 The author installed the NSWHW system, illustrated in Figure 2,

38、 at his principle PA residence in December 1984. That system has operated continuously ever since with a high level of success. However the system did need occasional attention. Each item that needed attention will be addressed next as a lessons learned opportunity. a. The following instrumentation,

39、 beyond that included in the hardware package, was installed at the outset to assess performance and assist with trouble shooting: Collector supply and return temperatures. Potable water supply and return temperatures. Supply water temperatures before and after the tempering valve. Running time mete

40、r for pumps, Hot water use flow meter, Energy (kWh) meter for storage tank heating elements. Glycol loop pressure. 2011 ASHRAE 59Figure 1, System SchematicFigure 2, The Authors Installed NSWHW System with Performance Information 60 ASHRAE Transactionsb. It was noted during the winter that some back

41、siphoning was occurring in the glycol loop adversely impacting performance. Back siphoning creates the potential for a freeze up on the potable side of the double walled HTX, but never did at this location. This problem was corrected by removing the check valve on the supply side of the collector lo

42、op, shown in Figure 1, and replacing it with a motor operated valve. c. At that time, so as to prevent a HTX freeze up should a sensor or controller fail, a low limit alarm was placed in parallel with back door buzzer button. In other words, should the temperature of the glycol solution at the HTX d

43、rop close to freezing temperature, the back door bell would sound. That has never occurred during normal operation. d. The air vent valve, associated with the expansion tank air separator in Figure 1, began to leak very slowly after about 5 years, leading to the need to occasionally add glycol solut

44、iona subject to discussed below. The author then realized that the auto industry had been dealing with similar issues for years cooling engine blocks. To overcome the leaky vent valve, the following closed loop system components, costing hundreds of dollars, were replaced with a few dollar radiator

45、cap (ref) and overflow tank: pressure relief valve, expansion tank, air separator, Air vent valve. This modification made addition of anti freeze solution much easier, and took the pressure off the collector loop during the evenings. e. The collector temperature thermistor sensors, used by the contr

46、oller, experience very high temperatures at times and are prone to infrequent failure. As a result the pumps were not operating properlya condition easily detected when adequately instrumented. A supply of very inexpensive commercial matching thermistors were ordered from an electronics house and us

47、ed to replace failed ones several times over the past 26 years. f. The wet rotor pump in the potable loop leaked on two occasions after summer vacation down times. Unfortunately, the manufacturer no longer produced the needed O-ring. However the author did find that the local Caterpillar dealer had

48、in stock the perfect replacement. g. That same pump finally failed after 25 years of service and was replaced. h. The thermal storage tanks take rather severe treatment due to both long periods of over 180F water and highly stratified conditions (cold water at 48F on the bottom of the tank). This le

49、ads to shortened tank lifei.e. only days beyond the warranty period. Fortunately, tank life even under these conditions, can be extended well beyond warranty period by replacing the sacrificial anode semi-annually, as has become the authors practice. i. Vacation overheat protection: If the home main domestic water supply valve is kept open during the vacation, overheat protection can be achieved by operating the pump 24 hrs a daycollect during the day and throw away to night sky at night. If the home main domes