Solar air conditioning represents a potentially huge market for solar energy in which the resource availability and load are well matched on both daily and seasonal bases. Analysis has shown that flat plate collectors cannot be economically viable for solar-driven absorption cooling under foreseeable conditions. Clearly, some kind of high-temperature collector is required even for single-effect absorption systems. For several decades, we have been promoting the use of evacuated CPCs, in particular, the
FIGURE 7.7: Closeup view of the fin absorbers and the end caps of some of the “easily manufacturable ICPC tubes used in the Sacramento project.
ICPC, as the ideally suited solar collector for this application. In particular, there is a marked improvement in going to a double-effect system that should justify the further development of such systems.
A project to demonstrate the operational viability of the combination of the two concepts was initiated in 1995. This system was installed in 1998 and is still operating at the time of this writing (2008). The project and system are described more fully in several regular progress reports (see, for example, Duff, Winston, O’Gallagher, Bergquam, and Henkel, 2004; Duff, Daosukho, Winston, O’Gallagher, Bergquam, and Henkel, 2005; and references therein).
The ICPC design has been described in some detail in Chapter 3, and the collector design and cross-section were shown in Figure 3.7. A photo of a portion of the array under test is shown in Figure 7.7, and the thermal efficiency curve measured at the Sandia National Laboratory is shown in Figure 7.8.
A close-up of a portion of the ICPC array used in the Sacramento project is shown in Figure 7.9. The whole array was composed of 336 ICPC tubes of the design in Figure 3.7. They were arranged in three banks of 8 each for the 14-tube modules on the roof of an 8000-ft2 one-story office building. Note that the CPC design had a very large acceptance angle effectively collecting from
FIGURE 7.8: The thermal efficiency of a panel of nontracking (completely stationary) ICPC collectors of the design used in the Sacramento project as measured at the Sandia National Laboratory. Note that the efficiency remains above 50% at a temperature of 175 °C above ambient or above 200 °C for air conditioning applications.
FIGURE 7.9: Closeup view of a portion of the ICPC array used to drive a double-effect chiller cooling project in Sacramento, California, beginning in 1998.
the full sky. The array was aligned with the individual tubes parallel with the north-south direction. The total net area of the array is 1141 ft2 (106 m2). Thermal storage was provided in a 3785-L (1000 gallons) tank. The whole array was used to drive a 20-ton, double-effect Li-Br/water absorption chiller. This chiller was commercially available but was modified to be hot water fired. Backup in case of cloudy days was provided by an auxiliary gas backup boiler (70 kW or 240,000 BTU/hour), which turned out not to be used very much.
The collector array was monitored instantaneously, and all-day efficiencies of each of the three banks were monitored separately. The collectors were typically operating at 150 °C, which corresponded to 110 °C above ambient on warmer days and 130 °C above ambient on cooler days, and the array performance was measured. A typical daylong performance curve is shown in Figure 7.10.
While operating in the range of 120-160 °C, daily collection efficiencies of nearly 50% and instantaneous collection efficiencies of about 60% were achieved in 1998 and 1999. Daily values of the Coefficient of Performance (C. O.P.) of 1.1 were achieved for the double-effect chiller. Two differently oriented fins (vertical or horizontal) gave essentially identical performances. The ICPC collector performance remained unchanged in nearly 2 years of operation. Eventual problems with the chiller pump resulted in reverting to a single-effect chiller (COPs of about 0.5-0.7) after 2002.
FIGURE 7.10: Typical clear day performance curve for the prototype ICPC collectors used to drive a double-effect chiller in a demonstration project in Sacramento, California.
The system easily met the full building load. The actual load varied between 12 and 16 tons as opposed to the rated capacity of 20 tons. This meant that the chiller was operating de-rated and under less-than-ideal conditions in that often the daylong operational performance was not optimized (collectors turned on late by hand on most days). Despite this, the system achieved typical daylong COPs of 0.9-1.1 even under a 12- to 16-ton load. It would be expected that the system would perform even better (COP ~1.2) under the projected full load.
In summary, this project has shown that
1. The high temperatures required for 2E chiller operation can be readily achieved with a nontracking evacuated concentrating collectors.
2. Reliable system (combined array-chiller) operation can be maintained over several years.
3. The overall system performance is four times that of a flat plate array operating a single-effect chiller. This corresponds to one-fourth the collector area for the same load!
4. The collector design is simple and readily manufacturable.
5. The collector has demonstrated the potential to be reliable over time scales of decades.
6. Solar air conditioning on a large scale for residential applications is a very realistic nearterm possibility.