Manual 01 - Section 07








Residential Solar Information Manual
Section 7 - Solar Product Performance

SOLAR THERMOSIPHON " HOW IT WORKS"

All the Solahart domestic solar hot water systems are dependant on the natural thermosiphon principle for circulation of the heated collector fluid to the storage tank or vessel jacket. As the solar radiation heats the fluid within the collector it becomes less dense and therefore lighter than the cold water stored above it in the storage tank or vessel jacket. This causes the cold (heavier) fluid to flow down the collector and push the heated (lighter) fluid up into the storage tank or vessel jacket.

The flow rate through the collector circuit is dependant upon the temperature difference between the tank or jacket and the collector. The greater the temperature difference, the higher the flow rate.

After the sun leaves the collectors, the fluid quickly cools and becomes heavier than the fluid in the tank or jacket. This causes the thermosiphon circulation to stop. There is no heat loss from the storage tank or jacket to the collectors.

The diagram indicates the flow path of the hot fluid from the collector to the vessel in an L system, and to the jacket in a J or K System.



OPEN CIRCUIT, L SYSTEM THERMOSIPHON FLOW PATTERN

The thermosiphon flow path of the open circuit, L system is as shown below. The potable water in the bottom of the tank flows down the cold down pipe at the left hand side of the collector and enters the bottom collector header to replace the heated water within the collector. The heated collector water flows from the collector top header via the hot pipe into the centre of the tank.

The location of the cold water inlet on the elbow of the collector cold down pipe enables the cold water to enter the tank without forcing cold water into the collector.
The use of open circuit systems is restricted to frost-free and good, potable water quality areas. If used in areas where the potable water has a high mineral content calcification of the collector riser tubes can reduce system performance.

Closed circuit, jacketed systems should be used if either of these conditions apply.

CLOSED CIRCUIT J OR K-SYSTEM THERMOSIPHON FLOW PATTERN

The typical thermosiphon flow of the closed circuit J or K system is as shown below. The collector fluid hot return pipe to the jacket is located on the left side and the cold down pipe on the right.

The short cold pipe protrudes into the collector top header and partitions of the header tube to form an internal down pipe. (In some models the cold pipe may extend to the bottom right hand corner of the collectors.)

A slot in the inserted section of the cold down pipe facing the riser tubes of the collector creates the flow path for the cooler jacket fluid to the bottom of the collector.

The number of risers used to transport the fluid to the bottom of the collector is dependant on the collector configuration. Dedicated cold down pipes are manufactured for one, two and three collector combinations.

As the hot collector fluid is pushed into the jacket it rises along the outside of the main vessel wall whilst exchanging heat energy through the cylinder wall into the potable water. In this process the collector fluid gradually looses heat (becoming heavier) and falls toward the cold down pipe connection for return to the bottom of the collector. This process pushes the hotter (lighter) fluid back via the hot pipe to jacket to continue the process.


WORLD SOLAR RADIATION GUIDE MAP

MARCH

Average W/m2 per day for the month

JUNE

Average W/m2 per day for the month

SEPTEMBER

Average W/m2 per day for the month

DECEMBER

Average W/m2 per day for the month


RATED ELECTRICAL OR GAS HOT WATER DELIVERY

The rated electrical hot water delivery of a Solahart storage tank or heat exchanger is determined by a draw-off test performed in accordance with Australian Standard 1056.1-1991. The following paragraph explains the Australian Standard test procedure used for electric storage hot water systems and is applicable to solar hot water systems fitted with an electrical or gas booster.

The vessel is filled with cold water and the electric or gas booster element energised. After the first thermostat cycle the tank temperature is allowed to stabilise for 24 hours with power available at the thermostat.

The hot water draw off is commenced between 20 and 30 minutes after the thermostat has opened at the set temperature. During the draw off the electric booster is switched off and the water flow rate is maintained at a constant rate of 12-13 litres/minute.

The temperature of the hot water flow is measured at intervals of 5 litres and stopped when the water temperature has fallen to 12°C below the temperature of the first 5 litres.

The quantity of water drawn before the 12°C temperature drop is the rated electrical or gas hot water delivery.

The rated electrical hot water delivery of Solahart systems is based on sickle type elements with the sickle in the horizontal plane. The rated electrical or gas hot water delivery does not apply for off-peak electric systems when the sickle element is fitted in the down position. Fitting the sickle element in the sickle down position will increase the rated electric hot water delivery by approximately 25%. Refer to Section 3 for the rated electric or gas hot water delivery.


COLLECTOR INSTANTANEOUS EFFICIENCY

The collector instantaneous efficiency should not be confused with the system efficiency or system Solar Contribution Factor (SCF). The system SCF is based on a 12 month period in which the total system (vessel, collectors & pipe work) is characterised.

A collector instantaneous efficiency test is undertaken over a thirty minute period when the solar radiation and water flow through the collector is steady. The procedure is described in the Australian Standard AS2535 (test method for collector instantaneous efficiency) and summarised below.

The collector to be tested is fitted to a sun tracking device to ensure that the collector is facing directly at the sun for the duration of the test.

When the collector inlet temperature is at 80°C, six measurements are taken of collector output temperature, water flow rate, solar radiation and wind speed over a one minute period. This information is used to calculate the efficiency each instance. The process is then repeated 4 minutes later, if the discrepancy between the first and the second batch of measurements is greater than 2% the test is rejected and conducted again.

Once the 80°C plot point has been determined, the collector inlet temperature is lowered to preset points and the tests repeated to provide collector instantaneous efficiency ratings at the various inlet temperatures. Using this information the collector instantaneous efficiency curve is drawn.

The collector instantaneous efficiency curves on the following pages have efficiency on the Y axis and the collector load factor [calculated by the formula (Tw - Ta - 3) / G] on the X axis.

Tw = mean collector temperature = 0.5 x (Ti + To) Ta = ambient temperature G = solar radiation Ti = collector inlet temperature To = collector outlet temperature


COLLECTOR EFFICIENCY CURVES: L AND I COLLECTORS

L COLLECTOR

(Tw - Ta - 3) / G kW/m2

I COLLECTOR

(Tw - Ta - 3) / G kW/m2



COLLECTOR EFFICIENCY CURVES: J AND K COLLECTORS

J COLLECTOR

(Tw - Ta - 3) / G, kW/m2

K COLLECTOR

(Tw - Ta - 3) / G, kW/m2