A high-speed cylindrical solar-water heater

Solar Energy, 197 I, Vol. 13, pp. 339-344.

Pergamon Press.

Printed in Great Britain

TECHNICAL NOTE A High-Speed Cylindrical Solar-Water Heater STEPHEN A. VINCZE* (Received 17June 1970)

A HIGH-SPEED solar water heatert based on a novel principle is described, illustrated and test results are given. INTRODUCTION Solar heaters powered by conventional flat-plate heat collectors have various drawbacks, the greatest of which is perhaps the flat surface of the heat collector itself. Consider a flat plate of unit width and length [Fig. I(a)]. If the incidence of the sunrays is at fight angles to the plate, the effective width B of the solar beam equals the plate width W. whereas the incident solar flux F is proportional to the effective beam width, viz. (I)

F~B=W.

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W.

(2)

Between a = 0 and 90 ° the beam width will increase from 0 to W. between 90 and 180 ° decrease to 0 again. The useful heat supplied by a conventional flat-plate heat collector and transmitted into the storage tank associated with it equals the incident solar radiation less all losses, namely: (l) Reflection and absorption losses in the glass coverings; (2) Conduction losses in the flat plate, in the water tubes and in the joints between the flat plate and the water tubes; (3) Radiation and convection losses at the back and at the edges of the flat plate; (4) Radiation and convection losses in the connecting pipes between the heat collector and the water tank. Thus, it is not surprising that the average yearly efficiency of such solar-water heater systems is of the order of barely 30 per cent. Contrast the flat plate with a cylindrical surface. A circular cylindrical heat collector, Fig. I b. having the same projected area as the flat plate considered above, will exhibit the same beam width and the same incident solar flux for any angle of incidence of the sunrays, B = Ba = W = constant.

(3)

Installed in front of a heat reflecting surface, e.g. a roof. a wall or similar, the reflected solar beam of the same order of magnitude as the direct solar beam will be utilised as well and virtually the whole cylindrical surface, Fig. l (c), irradiated. Also, it is possible to utilise the cylindrical heat collector as a storage vessel, providing internal ducts for thermosyphonic circulation of the heated fluid. The circular cylindrical "glass house" used instead of the flat glazing of the flat-plate heat collector improves the performance *Consulting engineer, P. O. Box 485, Wellington, New Zealand. tN.Z. Patent No. 1447981145072 (Australian and foreign patents pending). 339

340

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Fig. 1(a). Flat-plate heat collector. Fig. l(b). Circular cylindrical heat collector. Fig. 1(c). Cylindrical heat collector installed in front of a heat reflector. considerably, since there will be less reflection losses. The heat is appfied where it is needed, viz., direct on the vessel containing the water; the heat lost by conduction and by radiation and convection at the back and at the edges of the flat-plate heat collector are eliminated. There being no external connecting pipes between the cylindrical heat collector and its incorporated water tank, these losses are also avoided. All this will result in simple, economic construction and in exceptionally high efficiency of the heater. By suitable choice of the surface/volume ratio, the heating speed can be varied at will. OPERATING PRINCIPLE Figure 2 illustrates the operating principle of the heater. The notations used are: (1) Cylindrical heat collector and storage vessel; (2) Annular space between vessel 1 and guide 6; (3) Inner "glasshouse"; (4) Outer "glasshouse"; (5) Insulating air space; (6) Cylindrical guide. The water in the narrow annular channel heats up rapidly. The warm water rises and the cooler water inside the vessel descends, establishing a vigorous thermosyphonic circulation which, in turn, promotes the heat transfer to the fluid. ACTUAL REALISATION Figure 3 shows an actual solar-heater installation based on the above principles. In this instance, two heaters are connected between the cold water tank and the hot water tank of an existing electric water heating system, augmenting its capacity by 10-20 gallons per day, depending on solar conditions. TESTS The tests referred to in the following have been carried out with only one heater unit similar to those shown in Fig. 3. The main dimensions of the heater were: Capacity 22.2 I, projected collector area 0.186 mg. total cooling surface 0.645 m ~. The tank/heat collector was made of stainless steel, painted matt black. Inside, the cylinder was fitted with a cylindrical circulation guide; made of temperature resistant plastic material.

A high-speed cylindrical solar-water heater

341

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The heater was provided with cylindrical double glazing made of clear plastic, with 1 in. air space between tank and inner glazing and 1 in. air space between inner and outer glazing. The "glasshouse" had solid, non-transparent,'spring-loaded ends, made of approx. 2 in. thick plastic insulating material, The heater was oriented north, tilted 63 ° to the horizontal and propped u p at ground level against a 6 ft high, aluminium-painted, corrugated iron fence, running east to west. F o r test No. 1 only, an approximate 7 ft z area of the ground in front of the heater was covered by newspaper sheets, serving as a "heliostat". A t the beginning of test No. 1 the heater was filled with water of 9°C temperature. In tests Nos. 1 to 5, the water temperatures were measured at the top outlet of the tank, in test No. 6 at the top and also at the bottom outlet of the same, whereas the air temperatures were measured in the shade adjacent to the heater. All tests were carried out at Wainuiomata near Wellington. Since the nearest Meteorological Office taking continuous Eppley pyranometer recordings is located at Kelburn, reasonably close to the test site, meteorological recordings there are also shown in Tables ! and 2 for comparison. The bottom temperatures were taken by draining a small quantity of water into a vessel and measuring the temperature there. The bottom temperature readings could be on the low side because of possible rapid cooling during draining and measuring.

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S.A. VINCZE ANALYSIS AND CONCLUSIONS Analysis of the tests reveals the following:

(I) The total heat collected in the tank of the heater during 5 days (Table 1; tests Nos. 1. 2, 4, 5 and 6, total test duration 38.5 hr) amounted to 4.292 kWh, the average heat collection to 0.858 kWh (2920 B.t.u.) per day or to 4.6 kWh/m 2 per day, corresponding to 598 W/m ~ mean rate of heat collection. (2) The maximum and the minimum heat collections per day amounted to 0.980 kWh (5-27 kWh/m z) and to 0-724 kWh (3.88 kWh/mS), respectively. (3) The total heat loss during the five nights (Table 2; tests Nos. 2a to 6a, total test duration 62.2 hr) amounted to 2.440 kWh, the average heat loss to 0-488 kWh ( 1660 B.t.u.) per night, or to 2-62 kWh/m: cooling surface per night, corresponding to an average heat loss of 39.2 W, or 60-7 W/m s cooling surface. (4) The daily average rate of the water temperature rise (all tests) amounted to 4'34~C/hr, the maximum daily rate to 5.16°C/hr and the minimum to 3-73°C/hr. (5) The average rate of cooling during the night amounted to l'5°C/hr (maximum 1.96°C/hr, minimum 0.62°C/hr), the minimum on the night following a cloudy day with barely 0.3 hr sunshine, high winds and 0. II in. of rain. (6) The wind and its direction have a great influence on the performance. (7) The water circulation inside the heater was excellent. (8) The water temperature never fell below + 9°C, even on frosty night (white frost on the ground, measured ambient air temperatures 0°C in the morning). (9) The exceptional performance and the "absurdly" high average e~ciencies returned in Table I are explained by the fact that: a. The incident solar radiation recorded at Kelburn is measured on a horizontal surface, whereas the heater was tilted at approximately right angles to the Sun. b. The heat collected in the tank is referred to the temperature rise at the top of the tank, whereas the water temperature rise (see Table 1: test No. 6) at the bottom is 7°C lower; the mean temperature rise (at midheight of the tank) is 3.5°C lower than at the top. c. Owing to the heat reflected from the fence and from the ground, not only the projected area but the whole circumference of the cylindrical heat collector is utilised. The outstanding heat insulation of the water tank is another contributing factor. The installation shown in Fig. 3 has been in operation since November 1969 and is saving a very considerable amount of electricity. Measurements are still proceeding and will be reported on later.