In Canada, our available solar profile can be 3-4x greater in the high gain months (summer) than in the low gain months (winter). For a relatively constant load such as domestic hot water, depending on load/capacity, a solar thermal domestic hot water system sized to provide the majority of the domestic hot water in summer, will fall short in winter. If it is sized to provide the majority of domestic hot water in winter, there will be an excess in summer.
Wood & Solar Thermal Annual Profile.pdf
A great solution to this imbalance is to use complementary seasonal technologies such as a wood boiler and solar thermal. Excess heat from the wood boiler space heat is utilized for domestic hot water preheat during the winter (low solar) months. In the summer months when the wood boiler is not in use, the solar provides the domestic hot water preheat. During the shoulder seasons, both will contribute.
This type of system typically requires control of two separate loops, one for the boiler and one for the solar. In this case, this was accomplished with separate circulation pumps and sensors. The solar array and control is no different to a standard solar domestic hot water system.
With a near perfectly unshaded solar roof such as this one and efficient wood boiler, an estimated 70-80% of the domestic hot water needs are met with this combined thermal system for a family of 4. Best of all, its renewable!