Stranded heat recovery using software controls

By Bristol Stickney, technical director,
Cedar Mountain Solar Systems, Santa Fe, N.M.

?In a well-designed solar heating system, the solar collectors provide most of the heat throughout the year, and the conventional boiler or other familiar heat source becomes the “backup system.” Even in the sunniest climates, there are days when the heat provided by the sun is not enough to satisfy the entire heating load. Whether this is due to extreme weather or extreme heat demand, even if the situation is rare or intermittent, the need for heat must be satisfied to keep the users satisfied. This is why no contemporary solar heating system is complete without a backup system that is reliable, fuel-efficient and properly controlled.

In a solar water heater, the backup can be as simple as an electric heating element that only turns on when the water drops below “shower temperature.” In solar combisystems installed in recent years, condensing boilers have become very popular backup burners because of their small size and high fuel efficiency. A typical backup boiler might burn propane, natural gas, electricity or fuel oil. The controls must prevent the burner from firing whenever solar heat is available at a useful temperature, either directly or from the solar heat storage.

Figure 31-1 shows the typical piping configuration that I have installed many times to provide these functions in a solar combisystem. Using a primary/secondary “flow center,” the solar collectors, heat storage, backup boiler, domestic hot water (DHW) and space heat zones can all be connected in a standard plumbing pattern. Notice that there are three heat sources and three heat loads. Solar combisystems always include multiple heat sources and loads. Even in this simple home heating design, the solar collectors, the boiler and the heat storage can provide heat (sources), while the heat storage, DHW and living spaces may demand heat (loads). The heat storage system can act as a source or as a load.

Unconventional backup heat

One major benefit of using a flow center design is that any source (and any load) can be “plugged-in” with two pipes. In some installations this has worked out nicely using alternative heat sources; some of them have been unconventional. When designing for energy efficiency, “waste heat” and convenient local fuel sources are hard to ignore, especially when they are expedient, plentiful and inexpensive. If you have multiple local resources, you might want to connect three or four heat sources, or even more, if the connection is relatively easy and if the potential for savings is real.

These resources generally come in two flavors: On demand and Intermittent. Figure 31-2 shows how other sources of heat can be easily added to a primary loop using two pipe connections at closely spaced tees. In a primary loop, the heat sources are best placed in order of lowest temperatures first and higher temperatures last, around the loop.

On demand

Alternative heat sources that can be started and stopped on demand (only when heat is needed) can be installed, similar to a “boiler” and controlled similar to multiple boilers. This is done most often to take advantage of alternate fuels, as the price of fuel changes over time or the performance of the equipment changes seasonally. So, for example, a ground-source heat-pump or an electric boiler might be placed just before a propane boiler in Figure 31-2 so that these fuels can be used exclusively or combined to achieve the best backup fuel costs from month to month. Solar heat, when placed upstream in the loop, is used for fuel offset by preheating any of these units and shutting them down during good, sunny weather.

Intermittent

Heat is sometimes generated out of sync with the need for heat and not controlled by the need for heat. Some typical examples would be Solar, a wood stove with heat coil, fireplace with heat coil, wood fired hydronic boiler, other multi fuel hydronic boilers (coal, pellet fuel, small biogas digester) and waste heat from other mechanical equipment such as generators or large refrigeration compressor systems). These sources can be used, stored and controlled in much the same way as solar heat is treated and can be a nice complement to the solar heat. When installed upstream of the solar heat storage system (as seen in Figure 31-2), non-solar intermittent heat can be stored in the solar storage as well.

Example: Wood-fired hydronic backup

In 2006, a homebuilder near Santa Fe found that the building site provided an ample supply of firewood that was essentially free. The only other backup fuel in that location was propane. The construction of the home was planned in two stages; solar collectors were to be added in the second stage. This meant that the house would be occupied for several years before the solar collectors were installed. The house was well insulated with radiant floor heat, using mostly concrete floors.

The homeowner chose to install a large wood-fired hydronic boiler (outdoors) that came with a built in 600-gallon tank. This was used as both a heat storage tank and as an intermittent alternative heat source. A primary loop was installed in the mechanical room with the wood-fired hydronic heat connected with two pipes upstream of a condensing propane boiler. The piping connections were done in the same order as labeled in Figure 31-2.

The homeowner could make one large wood fire on a cold day and run the heating system (using standard automatic home heating solar controls) on the heat stored in the water tank. When that was depleted, the condensing boiler would kick in. When the solar collectors were eventually installed both wood and propane usage was reduced significantly, and more heat was stored in the mass floors by the control system. The two-pipe, primary-loop connections allowed each heating device to be easily installed and controlled in a modular way over time.

Example: “Waste” heat from a generator

An off-grid home built around 2007 near Cerrillos, N.M. gets most of its electricity, which is stored in batteries, from photovoltaic panels. During extreme weather, an Onan generator fires up (on propane) to recharge the batteries when the solar cannot supply all the electrical needs in the home. This house was built from the ground up with solar heated warm mass floors, solar hot water and condensing boilers. When we realized (in 2009) that most of the propane heat from the generator was heating the great outdoors in winter, we decided to capture it. The top rubber hose on the automotive-style radiator was diverted to a stacked-plate heat exchanger. This allowed the automotive “water pump” in the engine to pump hot antifreeze from the engine through the heat exchanger before returning to the engine. The heat exchanger was treated like a solar collector, with controls that pumped heat away from the engine into the primary loop whenever the engine was hot enough. In this way, the “waste heat” from the propane burned by the generator is allowed to contribute to heating the floors, the DHW and even the swimming pool. The primary loop in this home was connected in a similar configuration to the labels shown in Figure 31-2.

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems, since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.

Bristol Stickney, partner and technical director at Cedar Mountain Solar Systems in Santa Fe, N.M., has been designing, manufacturing, engineering, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC and is involved in training programs for solar heating professionals (visit www.cedarmountainsolar.com or www.solarlogicllc.com for more information.)