Solar Hot Water Heaters during Winter Conditions

Solar water heating (SWH) or solar hot water (SHW) system is a good alternative to conventional electric geysers and fuel-fed boilers. It is an attractive and sustainable option, with its pollution free nature and near zero operational cost. Be it summer or winter, you need hot water for showers, laundry, washing dishes etc. In fact, you need more hot water in the winter. SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. 

You have to adjust your solar water heater to accommodate for winter conditions. Many of the regions rarely experience freezing temperature while others have to deal with them much of the year. Many of the regions have good amount of solar energy available even during winter conditions and SWH systems need protection from freezing for its operation.

Closed-loop indirect systems that use a heat transfer fluid (HTF) or drain back systems are two types of solar hot water heater used in cold regions.

Fig. Closed Loop System and Drain Back System

(Source: http://en.wikipedia.org/wiki/Solar_water_heating)

Closed Loop System: 

The basic components of a closed loop system are: 

- A storage tank

- Solar collector

-A differential controller that senses temperature difference between water leaving the solar collector and the water in the storage tank. 

- A heat exchanger to transfer heat between the heat transfer fluid (HTF) and the end-use water. 

Using an antifreeze (a chemical additive which lowers the freezing point of a water-based liquid) solution as the HTF is a common freeze-protection strategy. A closed loop solar hot water system uses a heat transfer fluid such as glycol or a mixture of water and glycol that circulates through the collector. 

When the Sun is out and HTF in the collector is about 8–10 °C warmer than the HTF in the tank, the controller turns the pump on. HTF is pumped from the bottom of the tank through the collectors, picking up heat as it goes. After being heated in the collector, the HTF travels to the heat exchanger, where its heat is transferred to the end-use water. This process goes on as long as the collectors are at least 5 °C hotter than the tank, heating the tank continually. Whenever the difference falls below 5 °C, the controller turns the pump off. The collector loop is always full of HTF irrespective of whether the pump is on or off. 

Propylene glycol is usually used as antifreeze as it’s less toxic than ethylene glycol and can be considered as "non-toxic antifreeze”. [1]

Drain Back System: 

Drain back systems are closed-loop, active (use one or more pumps to circulate water or heating fluid) systems. In addition to a storage tank, solar collector, a differential controller and a heat exchanger as in the closed loop system, drain back system has a reservoir–a tank-which holds the heat transfer fluid (HTF) in the drain back/collector loop. 

A heat-transfer fluid (usually water) contained in an unpressurized, closed loop is pumped through the collectors. These systems use a heat exchanger that separates the end-use water from the fluid. If the pump is off, the HTF drains into the drain back reservoir and none remains in the collector, leaving them empty and protected from freezing. All piping above the drain back tank, including the collectors, must slope downward in the direction of the drain back tank so that system is able to drain properly. 

When the controller detects that the collector temperature exceeds the stored water temperature by a preset amount, the pump is activated. When the temperature difference decreases to a set amount, the pump shuts off and all the water drains from the collectors back into the tank. Because the water automatically drains out of the collector and the piping when there is no sunlight (temperature difference falls in the absence of the Sun) and no further water is pumped into it, drain back systems have a built-in freeze protection. 

Although some drain back systems use a blend of antifreeze and water for enhanced freeze protection, many use only distilled water as the HTF which has the highest heat transfer characteristics. The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance thus more effective at gathering heat. [2] However, water freezes at a high temperature (0 °C), offering little or no freeze protection and expands as it freezes which increases the possibility of damaging the solar collectors or the piping. 

References:

[1] Antifreeze: http://en.wikipedia.org/wiki/Antifreeze [accessed (26th Nov. 2013)]

[2] Specific Heat: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html

[3] Closed Loop Solar Hot Water: http://www.homepower.com/articles/solar-water-heating/domestic-hot-water/closed-loop-solar-hot-water

[4] Drain Back Solar Hot Water Systems: http://www.homepower.com/articles/solar-water-heating/domestic-hot-water/drainback-solar-hot-water-systems

[5] Understanding Solar Hot Water Systems – The Drain Back Design: http://www.solarhotwater-systems.com/understanding-solar-hot-water-systems-the-drain-back-design/

I wrote this article originally for Experimentation Online - the premier student science newspaper based in Ireland and the UK. Reproduced here because the original website is no longer active.