In todays and preceding articles we will share with you the various systems that are being used to heat up water with the sun. We hope to enlighten you and leave you in a position that you are to make a decision on on the system that would best suit your environment and most importantly your needs.
Finding the most efficient and effective solar hot water system for a given situation can be a challenging task. There are a number of key factors that need to be considered when choosing the most appropriate system configuration. These factors include, to a large extent, amount of solar insolation, climate, construction, installation and materials costs, location and accessibility of the system, amount of water needing heating, frequency of hot water use, availability of electricity, availability of materials, and skill level in construction.
1.Drainback Solar water heating system
Drain Back Systems use distilled water as the heat-transfer fluid (HTF) in the collector loop. A pump circulates the water through the collectors. |
| Illustration Of A drainback system |
The water drains by gravity to the storage tank and heat exchanger; there are no valves to fail and the system is completely non-pressurized. When the pumps are off, the collectors are empty, allows the system to turn off if the water in the storage tank becomes too hot.
Since water is used as a heat transfer fluid, it never needs to be changed like pressurized antifreeze systems. Most plumbing codes do not require double wall heat exchangers for drainback systems using distilled water.
- A storage tank, to hold the end-use water being heated. This could be a domestic hot water tank, a several-thousand-gallon tank for a space-heating system, or, in the case of a pool system, the pool itself.
- Solar hot water collectors.
- A differential controller, which monitors the water temperature in the tank and the collector temperature. When the collector temperature exceeds the storage temperature by a set differential (number of degrees), the controller activates the HTF pump.
- A reservoir—a tank in the HTF loop plumbing—which holds the HTF in the drainback/collector loop. The draining of the HTF into the reservoir creates the characteristic gurgling sound of these systems, due to the air space required in the unpressurized reservoir.
- A heat exchanger to transfer heat between the drainback loop and the end-use water (unless it is a pool system). The heat exchanger is often inside the drainback reservoir, and a second pump circulates domestic water through the exchanger, which is immersed in the HTF. Most pool-heating systems circulate the pool water through collectors, eliminating the need for a heat exchange.
The basic components of a drainback system are:
What are the advantages of this system?
- The system has no check valves, no air vents, no pressure gauges, and no expansion tanks.
- Cannot reverse thermosyphon at night.
- Drainback systems may out-heat antifreeze systems by up to 8%.
- The biggest advantage of a drainback system is the high limit feature of differential controllers that turns the pump off, allowing water to drain out of the collectors to the reservoir protecting the collectors from overheating. The system is immune to utility blackouts because the water drains back to the reservoir when there is no electricity to run the pump.
And a few disadvantages
- The drainback system components cost about 15- 20% more than an active open direct loop solar water heating system for residential water heating.
- Larger piping (3/4″ copper pipe) and insulation must be used.
