All-glass evacuated tubes are the key component of solar collectors. The evacuated tube is similar to a conventional Dewar flask and consists of two borosilicate glass tubes, a glass with high chemical and thermal shock resistance. The outer side of the inner tube is coated with a sputtered solar selective surface. This coated inner tube is closed at one end and sealed at the other end to the outer tube. The annular space between the outer tube and inner tube is evacuated to virtually eliminate heat loss by conduction and convection.
As the specific gravities of hot water and cold water are different, cold water goes downward and hot water goes upward. When cold water in evacuated tubes is heated, it became lighter and goes to the water tank, while cold water in the tank goes down to the evacuated tubes. Through this circulation, the cold water in the water tank will be gradually heated.
Solar cannot completely replace the need for gas or electricity heating as sometimes there is little sunshine. A correct sized solar water heater can provide 60%-70% of a household need for hot water.
When choosing a solar system for hot water, we must first determine our need for hot water, in quantity as well as in preferred temperature. The typical temperature used for calculation is 45°C, but the required quantity must be calculated according to specific daily needs.
Hot water usage calculation
1) Residences
In family residences, the needs for hot water remain stable during the whole year. An indication for the needs is given by the number of individuals living in the building (or apartment). Usually, the per capita daily consumption of hot water at 45°C is calculated taking into consideration of the following:
Low consumption: 35 liters per capita / day
Medium consumption: 60 liters per capita / day
High consumption: 80 liters per capita / day
In the case where we want to connect the solar installation to the washing machine and the dishwasher, we would have to increase the calculated daily needs of consumption as follows:
Washing Machine: 20 liters / day (1 wash per day)
Dishwasher: 20 liters / day (1 wash per day)
Example:
A family of 4 needs around 240 liters of hot water daily in order to have a medium daily consumption. (60 liters per capita x 4 persons) If we include a washing machine and a dishwasher, then we must calculate a consumption of 280 liters per day.
2) Hotels - Hostels
In buildings such as hotels, hostels, etc..., the needs for hot water are related to the number of guests. In this case, the daily consumption is calculated by the average occupancy of the rooms, from the period of May until August. Using this basis, the size of the proposed installation is determined. Here below we indicate the per capita daily need for hot water at 45°C.
Hostels with shared bath: 35 liters / person / day
Hotels: 40 liters / person / day
2 Star Hotels: 50 liters / person / day
3 Star Hotels: 80 liters / person / day
4 Star Hotels: 100 liters / person / day
Camping: 60 liters / person / day
When comparing peak efficiency levels, it may seem that there is little difference between flat plate and evacuated tubes. In fact, the efficiency of flat plate may be higher, but this is under minimal heat loss conditions. When averaged over a year, the evacuated tube solar collector has a clear advantage. The key points are as follows:
1) Due to the cylindrical shape of the evacuated tube, the solar tubes are able to passively track the sun throughout the day. Flat plate collectors only provide peak energy output at midday when the sun is perpendicular to the collector's surface.
2) Air is evacuated from the tube to form a vacuum. This greatly reduces conductive and convective heat loss from the interior of the tube. As a result, wind and cold temperatures have less effect on the efficiency of the evacuated tube collector.
3) Evacuated tube solar collectors can be used under cold temperatures when flat plate collectors have limited or no heat output.
4) Evacuated tubes are strong, long lasting, and, should one be broken, inexpensive and easy to replace. If a flat plate collector panel is damaged, the whole panel must be replaced.
5) Due to the high efficiency absorption of solar radiation even during overcast conditions, combined with excellent insulative properties of the solar tube, solar tube collectors can heat water all the year around (backup from gas and electricity is still required).
6) Due to the various advantages of evacuated tube collectors over flat plate collectors, a smaller collector can be used to provide the same heating performance. For example, a standard household of 4-5 people would usually require a 250L / 66gallon water storage tank. Depending on your location, only 30 evacuated tubes would be required to provide all summer hot water needs and a large percentage in other seasons.
7) Flat plate solar collectors can produce similar heat output as evacuated tube collectors, but generally only under hot, sunny conditions. When averaged over an entire year, the heat output of evacuated tube collector per net m2 of absorber area, is about 25% to 40%, which is greater than that of flat plate collector.
Due to the enormous amount of energy required to heat a swimming pool, normally the answer is no, as there is not enough space to put a number of collectors required to contribute significantly to the pool heating. That being said, if you have the space and money to devote to a solar thermal heating system for your pool, there is no reason why it cannot be done.
Thus, we designed a patented project collector to realize this. The collector is about 2 m2 in size and can provide 200 liters hot water daily.