One of the most cost-effective ways to include renewable technologies into a building is by incorporating solar water heating. A typical solar water-heating system reduces the need for conventional water heating by about two-thirds, minimizes the expense of electricity or fossil fuel and reduces environmental impact.
Most solar water-heating systems have two main parts: a solar collector and a storage tank. Solar water heaters use the sun to heat either water or a heat-transfer fluid in the collector. Heated water is then held in the storage tank ready for use, with a conventional system providing additional heating as necessary. The tank can be a modified standard water heater, but it is recomended that a solar hot water tank is used.
Solar water heating systems can be either active or passive.
Active solar water heaters
Active solar water heaters rely on electric pumps, valves, and controllers to circulate water, or other heat-transfer fluids through the collectors. These are the three types of active solar water-heating systems:
- Direct-circulation systems use pumps to circulate water through the collectors. These systems are appropriate in areas that do not freeze for long periods and do not have hard or acidic water.
- Indirect-circulation systems pump heat-transfer fluids, such as a mixture of glycol and water antifreeze, through collectors. Heat exchangers transfer the heat from the fluid to the potable water stored in the tanks. Some indirect systems have "overheat protection," which is a means to protect the collector and the glycol fluid from becoming super-heated when the load is low and the intensity of incoming solar radiation is high.
- Drainback systems, a type of indirect system, use pumps to circulate water through the collectors. The water in the collector loop drains into a reservoir tank when the pumps stop. This makes drainback systems a good choice in colder climates. Drainback systems must be carefully installed to assure that the piping always slopes downward, so that the water will completely drain from the piping. This can be difficult to achieve in some circumstances.
Passive solar water heaters
Passive solar water heaters rely on gravity and the tendency for water to naturally circulate as it is heated. Because they contain no electrical components, passive systems are generally more reliable, easier to maintain, and possibly have a longer work life than active systems.
- Integral-collector storage systems consist of one or more storage tanks placed in an insulated box with a glazed side facing the sun. During the winter, they must be drained or protected from freezing. These solar collectors may be best suited for areas where temperatures rarely go below freezing. They are also good in households with significant daytime and evening hot-water needs; but they do not work well in households with predominantly morning draws because they lose most of the collected energy overnight.
- Thermosyphon systems are an economical and reliable choice, especially in new homes. These systems rely on the natural convection of warm water rising to circulate water through the collectors and to the tank (located above the collector). As water in the solar collector heats, it becomes lighter and rises naturally into the tank above. Meanwhile, the cooler water flows down the pipes to the bottom of the collector, enhancing the circulation. Some manufacturers place the storage tank in the house's attic, concealing it from view. Indirect thermosyphons (that use a glycol fluid in the collector loop) can be installed in freeze-prone climates if the piping in the unconditioned space is adequately protected
Close-Coupled systems:
These are the most common systems on the market. It consists of roof-mounted solar collectors, combined with a horizontally-mounted storage tank located immediately above these collectors. In most close coupled systems a pump is not required. Heated water rises naturally through the solar collectors and enters the storage tank. When this happens, cooler water at the base of the storage tank is forced out and flows down to the bottom of the collectors. This cycle is continuously repeated while the sun is shining. Most commercially available solar hot water systems employ this cycle, commonly referred to as ‘thermosyphon flow'.
Split Systems:
There are two types:
Thermosyphon system:
This system works on the same principle as the close-coupled thermosyphon system, except that the storage tank is located within the roof space . The base of the tank must be situated at least 300 mm above the collectors.
Pumped system:
The tank is located below the level of the collectors, usually at ground level. Water must therefore be pumped from the tank to the collectors and back by a thermostatically controlled pump. Pumps are not expensive to run, costing between one and two cents per hour. This system can be used when the roof structure isn't strong enough to support a close-coupled system. The system is traditionally boosted by off-peak electricity. Pumps can be mains or photovoltaically driven.
What temperatures can be expected?
The full capacity of a solar storage cylinder takes a full day of sun in order to heat to its full temperature on the whole capacity. To increase the temperatures, a larger collector area could be used.
Expected ranges for the Cape area would be:
* figures are given to be an average and could vary higher or lower.
Winter: 35-45ºC
in between seasons: 40-50ºC
Summer 45-55ºC
The solar radiation absorbed is roughly a third of the quantity in Winter compared to that absorbed in Summer. Heating of the top half of the stored water will take approximately 1½ hours. The electrical element will heat the water up to the point where the thermostat is set at, and then switch off. This set point can be adjusted between seasons for optimum use (summer 45ºC, winter 55ºC) but would usually be set on installation only. Heated water will not loose its temperature over night as the cylinder is well insulated.
Basic Principles:
The water will only start heating if the sun falls on the collector and if the stored water has a lower tempertature than the inside of the collector. It will not back-circulate at night as this is against the laws of nature.
A solar cylinder usually only has one element that heats the top half of the storage cylinder, leaving the bottom half to heat by the sun. This means that even if the element is left on, solar energy still gets absorbed into the lower unheated part (half of the capacity). When the bottom temperature reaches the temperature of the electrically heated water, then only will the temperature be reaised to a higher point.If it is a rainy or very overcast day, little heat will be gained from the sun, resulting in very low water temperature if the element is kept off.
When to switch on the element:
Most people switch their element on for winter only, and keep the element switched off in the summer time. It is advisable to install a timer to do the switching automatically. A timer can govern the element in order to trigger connection 1½ hours before use, and switch off before water is drawn. Timers can be purchased at Solardome SA, please enquire. To optimise your solar system savings, adapt the use of the water to be within 2 hour periods. This means that people should try to use all of the water either at night or all in the morning. This prevents hot water from mixing with cold water if only a part of the capacity is drawn.
An example:
When a 100 liter storage cylinder has 50 liters used, this results in 50 liters of cold water entering the cylinder and 50 liters of hot water left available.The different temperature water will not mix noticeably for up to a 2 hour period, where after mixture will result in a drop in the higher temperature water (although the lower temperature water has been raised).When mixture occurs, the temperature usually drops to a lower temperature than what can be used, and the element needed to be switched on.
If however all the 100 liters of hot water was taken out within a 2 hour period, then no electrical work would have been needed.
TRY TO ALL USE THE BULK OF THE WATER EITHER ALL AT NIGHT OR ALL IN THE MORNING.
Problem finding:
- If water leaks from a valve with more than a drip every now and then = faulty valve = phone plumber.
- If your element does not heat although the circuit breaker is switched on = it could be a faulty element or thermostat = phone electrician.
- If water continuously runs out of either the cylinders' casing or the collector panels' casing = leaking copper = phone installer.
*Generally a customer has no maintenance on the solar system, but it could be advised to hose off the collector panel if it becomes very dirty.
