Vision Solar Systems - 100 L Gravity Fed Type
9
Copyright © 2013 - 2016 Enervision. All Rights Reserved.
Issue 2.0 Mar 2013
Maintaining the energy in the thermal storage tank
The tank is constructed of double shell much like the tubes. The inner shell is stainless steel and
contains all the connections and seals for the pipes and tubes. This tank is open to the atmosphere and
is not under any pressure. The outer tank or shell is a thin sheet that protects the system and between
the two layers is a thick layer of thermal insulation that restricts the loss of heat from the water in the
inner tank. Using a low pressure tank to contain the thermal mass of water has its advantages. Firstly,
it limits the maximum temperature of the water in the tank as the water cannot rise above 100ºC – the
point at which water boils at sea level. Secondly, by not having a tank of hot water under pressure,
there is no need for management to ensure the safety of people, animals and possessions near the
tank.
Supplying the Hot Water
Water is supplied to the plumbing by gravity feed. This requires that the tank is mounted above the
water outlet to ensure a supply of water. The higher the tank is mounted above the water outlet, the
greater the pressure of the water at the outlet. Note that the tank is designed for zero pressure, to
extend the height of the cistern would place the tank under pressure, and this will affect the safe
operation of the system, and can be harmful to persons and property.
The Backup Heating
The need for backup heating is twofold – firstly, to compensate when there is little or no sunshine,
and, secondly, when water usage exceeds the daily solar heating capacity. In these situations the
backup element acts as a substitute heat source to the sun. It is a good practice to fit the timer to the
supply feed for the element. With electricity conservation being the objective, a timer with a
reasonable reserve (time it will continue to maintain time without the mains available) and the ability
to set up exclusion zones should be installed. This will prevent the use of the electrical element during
peak electricity usage period and also during the sun’s normal heating time.
Considerations
There are some considerations to take into account in a system like this as the performance of the
system differs to that of a normal hot water system.
1)
The supply of water is via a gravity feed. Do not expect high pressures from the system. Flow
rates will depend on the height of the tank above the water outlet, and the restriction of the
pipe work to the water outlet.
2)
The temperature of the water is dependent on the solar energy collected, and the amount of
water used. On sunny days with little water usage, the temperature can very high, and on
cloudy days with lots of water used, you can run out of hot water.
3)
As the hot water is drawn from the system the water in the tank cools. This means that the
water temperature tapers off as the water is used. In other words, initially the water
temperature may be 65ºC, after half the water is used it may drop to 55ºC and at the end it
may be as low as 40ºC. This means you will initially use a low amount of hot water and mix
with cold water to get the water to the desired temperature and at the end you may be using
mostly hot water with very little cold water mixed in.
Safety and Operation
There are certain great safety features in this type of system;
1)
The tank of hot water is at low pressure so the water cannot be superheated under pressure
and will never exceed boiling point. If left unused for an extended period of time the excess
solar energy collected each day will boil out into the cistern tank and, at worst, some water
can be lost through the overflow. This provides inherent safety of overheating and possible
bursting of the tank.
