Combining the attributes of solar thermal collectors and a wood-gasification boiler.

Figure 1

One of the benefits of hydronics is the flexibility it offers to combine multiple heat sources within a single system. This applies to systems using both renewable and conventional energy sources.

In past articles we’ve discussed how solar collectors can be combined with an “auxiliary heat source” such as a boiler. The latter automatically supplies heat when the solar storage tank temperature is insufficient to meet the load.

In this issue we will look at ways to combine two renewable heat sources: solar collectors and a solid fuel boiler. There are many possibilities for both of these heat sources. For example, the collectors could be flat plate or evacuated tube. They could be protected from freezing using either antifreeze or a drainback design. Likewise, the solid fuel boiler could use a standard “atmospheric” draft combustion system or a gasification combustion system - the latter being considerably more efficient, and in need of thermal storage for optimal performance.

Figure 2

The Anchor Component

Due to their intermittent heat outputs, solar collectors and wood-gasification boilers require a well-insulated thermal storage tank. This tank is the mediator between the sometimes sporadic generation of heat and the varied loads dissipating that heat.

When combining these two heat sources, it makes sense to use a single storage tank. One method of piping such a combination is shown in Figure 1.

The storage tank is assumed to be pressure-rated and likely to have a volume of several hundred gallons based on the heat output capability of the wood-gasification boiler. A 500-gallon “repurposed” propane storage tank with modified piping connections is a commonly used option. After it is piped and all connections are pressure-tested, the tank shell can be encapsulated with 3-plus inches of spray foam insulation.

Another option is to frame an enclosure around the tank and fill it with blown cellulose insulation. Designers planning to use such a tank should keep in mind that the large water volume will necessitate a larger expansion tank.

The thermostatic mixing valve seen to the right of the boiler is used to boost the boiler’s inlet temperature high enough to prevent creosote formation. When the fire is started, this valve routes flow leaving the boiler directly back to the boiler’s inlet. Once the water entering the boiler has reached a suitable minimum temperature, this valve routes flow to the upper portion of the storage tank.

Stratification keeps the warmest water in a horizontal layer at the top of the tank. This water can move across the upper portion of the tank and flow out along the supply header, making heated water available to the loads without first having to warm the entire tank volume. A float-type air vent ensures no air remains trapped within the tank.

Space heating is straightforward but also state-of-the-art. A motorized three-way mixing valve intervenes between the storage tank and a low-temperature, zoned space-heating distribution system. This valve modulates based on outdoor reset control to maintain smooth heat delivery. It also protects the distribution system against what may be high-temperature water in the storage tank, depending on the operating conditions of the heat sources.

A pressure-regulated circulator varies its speed as necessary to maintain a fixed pressure differential across the space-heating manifold station. This arrangement minimizes the electrical power required by the distribution system.

Domestic water is heated within a stainless-steel, brazed-plate heat exchanger connected to the load side of the storage tank. A low-power circulator controlled by a flow switch moves hot water from the top of the storage tank through the primary side of this heat exchanger whenever domestic water flows through the other side. The high surface-area-to-volume ratio of the heat exchanger provides instantaneous water heating. The thermal mass of the hot water in the storage tank allows for extended DHW draws.

The temperature rise of the domestic water passing through the heat exchanger depends on the storage tank temperature. If the tank is a few degrees warmer than the required DHW delivery temperature, the domestic water may be fully heated when it exits the heat exchanger. If the tank is a bit cooler, the DHW temperature may require “topping off” by the modulating instantaneous electric water heater.

Using an external DHW heat exchanger has two big advantages. First, it’s fully serviceable or replaceable if necessary. Second, it greatly expands the possible storage tank options, especially for larger tanks, compared to designs requiring a tank with a properly sized, removable, internal heat exchanger.

Figure 3

Contingency Plan

The fin-tube elements shown above the boiler are for heat dissipation in case of a power failure. A normally open zone valve opens when power is lost. This allows natural convection to convey heat from the boiler to the fin-tube elements. Many manufacturers of forced-combustion wood-fired boilers suggest this type of subsystem. However, if the boiler is equipped with a “thermostatic loading unit,” such as the one shown in Figure 2, this detail may not be required.

The thermostatic loading unit contains a thermostatic mixing valve as well as a circulator. Its main purpose is to maintain the water flowing into the boiler at a hot-enough temperature to minimize creosote formation. During a power failure, a secondary fluid passage within this unit allows water to thermosyphon between the boiler and storage tank. Given that forced-combustion air flow stops as soon as power is lost, the residual heat within the boiler is simply absorbed by the storage tank.

Solar Contributions

The solar thermal subsystem is a simple closed-loop system with glycol-based freeze protection and external heat exchanger. The collector circulator operates based on a differential temperature controller. Heat can be added to the tank anytime the collectors are a few degrees warmer than the storage tank.

Although the schematic in Figure 1 does not show it, some designers may choose to include a heat dump provision for the solar array. The large storage tank will help minimize the need for such a heat dump under most conditions, but ultimately cannot protect the glycol solution from high temperatures during a power outage on a sunny afternoon.

Solar input will always be greater from spring through fall. Depending on the size of the collector array and the heating load, solar heat gains may negate the need to operate the wood-gasification boiler during milder weather.

Just In Case

Although the wood-gasification boiler and solar array are powerful heat generators under the right conditions, it’s possible some owners would still want to include a fully automatic “auxiliary” heat source for both space heating and domestic hot water. Figure 3 shows one way to do this.

A conventional boiler is shown as the auxiliary heat source. Flow returning from either the space-heating or DHW subsystems is directed to the boiler by a motorized diverter valve. No flow passes through the storage tank when the boiler is operating. This piping also provides hydraulic separation between the boiler and the other circulators in the system.

Although it’s possible for this boiler to provide domestic water heating, there may be a time lag between when the boiler is fired and when it can supply hot water to the flat-plate heat exchanger. This is especially true when the boiler must start from room temperature. The instantaneous electric water heater will provide domestic water heating as the boiler warms up. Eventually the boiler will warm to the point where heat input from the instantaneous heater is not required. Thus, short demands for DHW when the boiler is cool will be met using the instantaneous water heater, whereas longer demands can be sustained by the boiler.

There are several other ways to combine solar thermal systems along with solid-fuel boilers. We will take a look at more options in future Solar Thermal Report issues.