A Great Marriage

The process of photosynthesis that we all learned about in science class uses solar energy to convert carbon dioxide from the atmosphere into cells that ultimately grow into plants. The big ones are called trees, and they have provided humankind with carbon-based fuel for millennia.

Although it is not quite as easy to convert wood into heat compared to other fuels, the “carbon neutral”
nature of burnt wood has rekindled interest in using it for fuel. Especially as the availability, price and environmental consequences of fossil fuels continue to be unsettling issues.

STORED SUNSHINE

In my area of upstate New York, firewood is plentiful and relatively inexpensive. You can buy it for $60/face cord, delivered. At a nominal 80 per cent conversion efficiency in a wood gasification boiler, that works out to about $11.18 per million Btus delivered. In rural areas, the primary fossil-based heating fuels are #2 fuel oil and propane. The current price for #2 oil is about $3.80 per gallon. At 85 per cent conversion efficiency that works out to $31.92 per million Btus delivered, almost three times as much as heat provided by a wood gasification boiler.

State-of-the-art wood-gasification boilers are a great way to convert firewood into heated water at relatively high (80-85 per cent) efficiency. After that, modern hydronics technology offers many ways to store and distribute that water for both space heating and domestic hot water.

Still, most people who use such a boiler for heating in cold weather, do not want to maintain its operation
in summer when the only load is likely to be domestic water heating. In these situations, solar energy can provide an ideal “summer substitute” for firewood, and modern hydronics technology provides many elegant ways to merge these two energy sources together.

Figure 1 shows one way to combine a wood gasification boiler and solar thermal array into a common system.  The two heat sources are linked at the storage tank. Either one can operate independently of the other. It is likely that the boiler will not get fired if the owner is expecting a period of significant solar gain. However, if the boiler is operating and the sun comes out, the collector array can still operate, provided the collector temperature rises several degrees above the storage tank temperature.

The solar subsystem is a closed loop filled with an antifreeze solution. Heat is transferred to the system water using a generously-sized brazed plate heat exchanger. The circulator between the storage tank and lower temperature side of the heat exchanger includes a check valve. This prevents heat migration into the solar subsystem when it is not operating.

A thermostatically controlled “loading unit” is seen near the inlet to the wood gasification boiler. This device consists of a circulator, thermostatic mixing valve and flapper-type check valve. Its purpose is to prevent inter–nal flue gas condensation, which creates creosote, within the combustion chamber.

When the circulator in the loading unit is operating, the pressure differential it creates forces the flapper-type check valve to remain closed. The internal thermostatic element regulates the proportions of hot water from the boiler outlet and cooler water from the storage tank that are blended together to keep the boiler inlet temperature suitably high. This is typically about 130F for 20 per cent moisture content firewood.

During a power failure, the flapper type check mechanism in the loading unit allows natural convection flow between the boiler and thermal storage tank. In consideration of this, the path between the boiler and storage tank should have low flow resistance. The check valve seen on the supply pipe from the boiler to the storage tank must be a flapper type check valve.

SIDEARM DOMESTIC

Domestic water is preheated by an external stainless steel heat exchanger as shown in Figure 2. The domestic water flow switch closes whenever it detects a demand of 0.6 gpm or higher. This turns on a small circulator, that immediately moves water from the storage tank through the primary side of the heat exchanger. Domestic cold water passes through the other side of the heat exchanger. Its temperature rise depends on the water temperature within the storage tank.

If the tank is 130F or higher and the target domestic water delivery temperature is 120F or lower, a suitably-sized heat exchanger could provide the full temperature rise. If the storage tank is cooler, the heat exchanger provides preheating. A thermostatically controlled electric tankless water heater adjusts the input wattage to its element to “top off” the temperature rise as required.

Although Figure 1 shows a zoned radiant panel distribution system, there are plenty of other possibilities. To make the most of stored energy, configure the distribution system to handle the design heating load with a supply water temperature no higher than 120F. A mixing device, such as the three-way motorized valve shown in Figure 1, should always be installed to protect a low temperature distribution system from potentially high water temperature in the storage tank.

COMPLEMENTARY RENEWABLES

Each of the subsystems has its strengths and limitations: the solar subsystem is ideal for summer, while the wood fired boiler covers the colder/cloudier winter. When good hydronic design is used to merge them into a common storage and distribution system, these two renewable heat sources become complementary. Perhaps one of your future projects can benefit from this synergy. <>

John Siegenthaler, P.E., is a mechanical engineering graduate of Renssellaer Polytechnic Institute and a licensed professional engineer. He has over 34 years experience in designing modern hydronic heating systems. He is also an associate professor emeritus of engineering technology at Mohawk Valley Community College in Utica, NY.