William S. Duff Klaus Vanoli

Colorado State University Institut fur Solarenergieforschungs

Department of Mechanical Engineering Hameln Germany

Fort Collins CO 80524 USA k. vanoli@isfh. de

bill@enar. colostate. edu

Research on Integrated Compound Parabolic Concentrator (ICPC) evacuated solar collectors has been going on for more than twenty years [1, 2]. University of Chicago and Colorado State University researchers developed a novel ICPC design in the early 1990s that can be easily manufactured and solves many inherent problems of previous ICPC designs [3, 4]. This ICPC evacuated collector operates nearly as efficiently at double effect (2E) absorption chiller temperatures (150C) as do more conventional collectors at much lower temperatures. With this collector, a 2E chiller can cool a building using a collector field that is about half the size of that required for a lower temperature (90C) ordinary single effect (1E) absorption chiller. In 1998 this novel solar collector and a solar driven 2E absorption chiller were demonstrated for the first time in an office building in Sacramento, California. This project has now been operating for over six years [5, 6, 7, 8, 9, 10, 11, 12, 13, 14].

In Germany in 1978 two different evacuated tubular collectors began providing 50C domestic hot water to the Solarhaus Freiburg apartments. One of these, the Corning evacuated collector, had been providing 90C hot water to a 1E absorption chiller at Colorado State University’s Solar House I since 1975. This collector was uninstalled in 1978 and shipped to Freiburg. In 1982 the Philips VTR261/Stiebel — Eltron heat pipe evacuated collector array replaced the Philips Mark IV evacuated collector that had been installed on the Solarhaus Freiburg in 1978.

This paper presents an evaluation of the quality, reliability and performance of the ICPC, Corning and Philips VTR261 solar collectors over their respective operating periods.

Collectors

Novel ICPC

The novel ICPC evacuated solar collector tubes are 125 mm (5 inches) in diameter and 2.7 meters (9 feet) long, each having an effective aperture area of 0.317 m2. A cross­section of the collector tube illustrating the two orientations is shown in Figure 1. The aperture area is defined as W x L, where W is the outside diameter of the tube and L is
the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of soda-lime glass. Each evacuated tube contains a thin wedge shaped absorber, positioned horizontally in half the evacuated tubes produced and vertically in the other half. The bottom half of the glass tube is silvered to form the matching CPC reflector running the length of the tube. A small feeder pipe is placed inside the 12 mm pipe that has been bonded to the absorber to allow fluid to flow into and out of the evacuated tube. The module manifolds are a concentric pipe-inside-pipe design as well.

The 336 tube 106.5 m2 aperture area collector array at the Sacramento demonstration is made up of three banks with 112 evacuated tubes each bank. The evacuated tubes in the banks are plumbed in parallel in a reverse — return arrangement. The tubes are oriented with their long axis north-south at an angle of 10o from the horizontal.

The north bank consists of all horizontal fin tubes and the middle bank consists of all vertical fin evacuated tubes. The south bank includes an even mixture of the two types. The three banks are in­turn plumbed in parallel in a reverse — return arrangement.

Corning

The Corning evacuated solar collector tubes shown in figure 2 are 103 mm (4 inches) in diameter, have a center-to — center distance (pitch) when set into the array of 111 mm and are 2.44 meters (8 feet) in length. Each tube has an effective aperture area of 0.232 m2. The aperture area is defined according to the Figure 2: Corning Evacuated Tube IEA SHAC Program Task VI definition

[17] as W x L, where W is the pitch between tubes in the array and L is the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of borosilicate (Pyrex) glass and each evacuated tube contains a flat fin shaped absorber running the length of the tube. A pipe that has been bent into a U-shape is bonded to the absorber to allow fluid to flow into and out of the evacuated tube.

The 144 tube 33.3 m2 aperture area collector array at the Solarhaus Freiburg is made up of two banks with 12 evacuated tube modules each bank. The tubes are oriented with their long axis east-west at an angle of 55o from the horizontal. The six evacuated tubes in each module are plumbed in series with the modules in the banks plumbed in parallel in a reverse-return arrangement. The two banks are in-turn plumbed in parallel in a reverse — return configuration.

Philips VTR261

The Philips VTR261 evacuated solar collector tubes shown in figure 3 are 67 mm (2.64 inches) in diameter, have a center-to-center distance (pitch) when set into the array of 104 mm and are 1.75 meters (5.75 feet) in length. Each tube has an effective aperture area of 0.163 m2. The aperture area is defined according to the IEA SHAC Program Task VI definition [17] as W x L, where W is the pitch between tubes in the array and L is the exposed transparent part of the collector tube when placed in the array (excluding covering portions of supports, non-transparent end caps and covering portions of headers). The tubes are made of soda-lime glass and each evacuated tube contains a flat fin shaped absorber running the length of the tube. An anodized aluminium ripple reflector is positioned behind the evacuated tubes. A heat-pipe is bonded to the absorber to deliver thermal energy to a condenser located external to the evacuated space.

_ 2 , Figure 3: Philips VTR261 Evacuated Tube

The 180 tube 29.5 m2 aperture area

collector array at the Solarhaus

Freiburg is made up of three rows with five 12 evacuated tube modules each row. The evacuated tube condensers in each row are plumbed in series with the rows plumbed in a reverse-return arrangement. The tubes are oriented with their long axis north-south at an angle of 55o from the horizontal.