20 Questions And Answer On Domestic Hot Water Solar Systems - part 2 Of 4
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20 QUESTIONS AND ANSWERS ON THE FUNCTION OF SOLAR SYSTEMS FOR DOMESTIC HOT WATER (part 1 of 4)
6. How is the hot water reservoirs protected against corrosion?
The reservoirs for domestic hot water have to be designed in a way to stand the pressure of the public water and the corrosion effect of the hot water.
The mild steel is the most common used material for constructing hot water reservoirs. This is because of its solidness. Its answers the pressure requirements (6 or more bars) through reservoir wall thickness of 2-3 mm. The price is also reasonable.
The corrosion protection is achieved with the use of enamel coating of the inside reservoir walls only if:
- The steel is eligible for glass enameling.
- The welds on the side of the water have to be smooth
- The production of the reservoirs is adjusted to the enamel process.
- The use of glass enameling has to answer strictly all the necessary procedures.
- A cathode defense has to be provided.
The enameling is in two layers. The main layer gets the conglomeration with the steel. The surface layer stands the corrosion effect of the hot water
On the base of years of experience we may sat that the enamel hot water reservoirs with magnesium anode for cathode defense (defense the places which may not be enamel covered) are long-life product (decades). It is necessary though the anode work to be checked regularly especially in the beginning
The use of stainless steel
The use of stainless steel requires good quality of the steel and complicate welding process. Otherwise there is a risk of leaks, result from the loading. The reservoirs leaks are due to two factors:
- Pressure inside the reservoir result from public water network. It creates stress on the stainless steel.
- The presence of different corrosion elements in the water, which activate only when the reservoir is under pressure
So when some say that the stainless steel reservoirs do not broke same like steel kitchen pots is wrong. The difference is that the pots are not put under the influence of pressure.
The use of mild steel and inner reservoir, made from corrosion resistant material
The protection of the mild steel, hot water reservoirs from corrosion can be achieved through using inner reservoir. The latter has to be made from corrosion resistant materials. Thin copper sheets (Northern Europe) and polymer materials (Europe and USA) are used for constructing the inner reservoir.
The outside shape of the inner reservoir is similar to the inside form of the mild steel reservoir. The pressure from the hot water in the inner reservoir transmitted to the outside steel reservoir. The latter do not make an immediate contact with the hot water and in this way it is defended against corrosion.
The material for the inner reservoir has to stand the expected high tempered water. It has to be physiologically inert and approved for contact with sustenance.
Special steps have to be made in order to avoid the formation of vacuum in the inner reservoir, when the hot water is being used. One of them is the installing of vacuum switch breaker on the cold water circle.
The use of copper
The use of copper is an expensive alternative for a reservoir, made to stand the pressure from the public water network. Copper with 1 mm thickness is used in Cyprus when the solar systems fro domestic hot water is supplied with water from an open reservoir. The latter is situated higher than the domestic hot water solar system, which causes aesthetically problems.
7. Is it possible the solar domestic hot water system to be used in the lower floors of a block of flats?
Let’s imagine that we have a five floors block of flats and a consumer that lives on the second floor. When the consumer turn on the hot water, on dependence of the tubes’ length there will be 3-6 liters cold water at firs before the hot water. This is little amount of water in comparison with the total volume of water that we use for laundry or dish washing
Of course for needs which require less that 3-6 litters that will be inconvenient. A solution can be the use of well insulated tubes for hot water.
So in conclusion solar domestic hot water systems can be use in block of flats even in lower flats, but the seller has to inform the buyer for this primitive water consumption before the hot water reaches the flat.
8. What is re-circulation and is it recommended?
There is solar domestic hot water systems (thermosyphon), which are installed (both the collectors and the hot water reservoirs) on roofs (flat). In this case the hot water comes directly from the reservoir.
There are also domestic hot water solar systems, which collectors are installed on roofs, the hot water reservoir in on lower level (like basement or a ground floor). In this case a pump is required to transfer the energy from the collector to the reservoir while the differential thermostat (control system) controls the pump function. These domestic hot water system have larger capacity (larger collective area, bigger reservoir) than simple thermosyphon system.
In these cases the reservoir is situated in the basement in order the triple energy reservoirs (boilers) to be used. They can be connected to:
- Solar collectors
- Heating system (additional heater, when the heating system works)
- Electricity (additional heater when the heating system works)
With these triple energy reservoirs there is a possibility of using another pump, which supply the hot water to the consumption exits in the house and then turns it back into the reservoir. Suitable tubes’ system is required for that. And in this way the consumer has hot water on the moment without waiting. This system for hot water distribution is known as re-circulation system is used in hotels.
The constant circulation of domestic hot water in the water distribution network leads to wastes. From the made calculations in small and large hotels we can conclude that approximately 20%-30% from the total energy, which is consumed for water heating is waste into the distribution network. In addition to that in many cases, when a concrete hotel is not full, the wastes can reach 50% from the total energy.
When we are going to install system for hot water distribution with re-circulation pump we need to have:
- Good insulation of the water distribution tubes.
- Limit the use of the re-circulation pump (install of clock switch or some other method) as far as the conditions allow that.
It is duty of the seller/spider man to inform the consumer for the large wastes into the hot water distribution network so the one can decide pro and con of using re-circulation system.
- It is well known that solar collectors for domestic hot water have to be orientated south (northern hemisphere). What happens when this is not possible? What is the difference when it is orientated north without low effectiveness levels?
Difference to 20° from the south orientation causes insignificant decrease in the amount of solar radiation on the collector surface. For bigger diversion to 45°, the decrease is from 15% during winter and 5% during summer.
- Most of the solar collectors for domestic hot water are situated on 45°towards the horizontal plane. If we want to situate the collector on bigger or smaller angle (systems with pump) can we expect problems with the functioning or increased effectiveness?
When the gradient is from 25° to 50° there is difference in the annual energy profit, which do not go beyond 5%. So he answer has to be “No
- When we are on vacations, do we need to cover the solar collector during this time?
When the water in the solar domestic hot water system is not being used during summer or other vacations, the temperature of the water in the reservoir reaches high levels (90°-95°). The solar systems for domestic hot water are designed to stand these kinds of temperatures. There is no need to cover the collector during vacations. Covering it will prevent though the water to reaches such temperatures unnecessary.
- When the forecast is for freezing hot to protect the solar system fro domestic hot water?
When the consumer has a system with closed loop the one do not need to do anything. The spider mad uses water and antifreeze in the appropriate proportion (different for each region) in the closed loop during installation or during regular maintenance. In regions where there are rare freezing solar systems for domestic hot water with open loop are popular. If a drain hole is made, the consumer has to drain the solar system in keeping with the spider man’s instruction. If you do not have such drain hole, contact your seller for help.
- What are problems with closed loop solar systems for domestic hot water? What happens when there is leak (from water plus antifreeze) into the hot water? Is this dangerous for our health?
The percentage on antifreeze depends on the lowest temperatures during winter in the given region. The pressure into the closed loop of the collectors is lower than the one into the hot water reservoir. If the closed loop is broken there won’t be leak into the domestic water, the opposite will happen. Even if the pressure in the hot water reservoir increases and there is leak of water and antifreeze into the tubes’ network this will not be a problem because the using antifreeze is not generous for our health.
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