Solar Water Heating Design

Solar water heating system in Northern USA and Canada use a closed loop or active solar water system. This design is based around the need for a glycol-based heating fluid that will not freeze during the winter months. In other southern locations drain back system or compact solar water heaters can be used when the risk of freeze damage is minimized. However, where heavy freezing occurs it is best to be safe and protect the system with a heating fluid that can withstand temperatures below 0 C.

A closed loop solar heating design transfers its energy to potable domestic hot water using a heat exchanger. The heat exchanger transfers heat energy created by the collector to the potable water system to be used by the homeowner. There are two types of heat exchange systems. Internal and external. Both are effective however in test the internal outperforms as 100% of the heat is transferred to the potable water.

Our solar water heating designs use a differential solar controller that manages the heat control. The controller is equipped with sensor that monitor the temperature in the solar collector and the temperature in the potable water. If the heating fluid inside the solar vacuum tube collector is greater than that of the potable water, the controller will activate a solar pump causing the hot fluid to be transported through the heat exchanger thereby transferring the solar energy to the potable water. When the temperature drops in the collector to less than the temperature in the water tank then the solar controller will shut down the pump such as in the evening.

Sizing the system

When designing a solar system you must be sure to properly size it. This means a balance of energy in, storage for the energy and energy out. This can be more difficult when trying to do only winter space heating and the energy out in the summer will be almost NIL. And as such a dissipation technique will need to be used or alternatively a pool can be heated.

Sizing a solar water heating system has some general rules of thumb. One 30 tube collector should have a minimum of 40 gallons of hot water to act as a solar thermal storage. So, 3 collectors should use a 120 Gallon tank. Larger system can combine multiple tanks together to form a larger buffer tank by piping them in parallel. Calculating the demand of the system will be useful as you don't want to over size it such that there is more heat than is being used. This can be done by a rule of thumb with domestic hot water i.e. 20 gallons of hot water per person in a residential application.

When in doubt we have some very powerful software that allows us to size the system correctly. You can take advantage of our Free Simulations by clicking on the calculator.

Stagnation

Our solar water heating designs are built around vacuum tube solar collectors and as such need to handle the overheat protection of stagnation which happens when the system produces more energy than the household consumes. This often happens during a period of inactivity such as holidays. All our systems are built to safely shut down during stagnation. During stagnation the pump is turned off and the solar heating fluid turns to steam inside the solar collector. The extra volume of steam is absorbed in the system by an oversized expansion tank. A properly designed solar water heating system needs to ensure it can safely go into stagnation mode any time there is a problem or imbalance of energy.