Some more ways to merge conventional hydronics and active solar energy subsystems.

Figure 1
Last month's column discussed the hybrid solar/boiler heating system in my home. The solar drainback portion of the system uses six flat-plate collectors and dates back to 1981. The oil-fired boiler was added in 2001. Together, these subsystems provide a portion of our space heating and much of the domestic hot water.

Given the evolution of our system, there are piping details that are not optimal, but still allowed new equipment to be used with minimal disruption of existing piping. Designing such a system from scratch would allow better details. This month we'll look at some of those details as well as general design principals for matching up an active solar energy subsystem with a boiler-based hydronic distribution system.

Figure 2

Obeying The Law

We've discussed the first law of thermodynamics on many occasions. Simply put, the first law states that all the energy that goes into a device must come out of that device. For example, the heat flowing into a mixing assembly must come out of that mixing assembly. It might come out at a different temperature or be carried along by a fluid stream at a higher flow rate than when it went in, but every Btu must be accounted for. Another way to interpret the first law is that you can't "lose" energy. To predict temperatures in different parts of a system, designers must account for every Btu the way a good accountant keeps track of every penny in business cash flow.

Moving energy around would be simple if all thermal processes only had to obey the first law of thermodynamics. If this were true, you could convert solar energy directly back into fuel without losing one Btu in the process. You could also move heat from cool soil into your warm house without the need for a heat pump.

Too bad nature doesn't work that simply. Instead, it imposes another constraint on any process involving energy flow. That constraint is the second law of thermodynamics. Like the first law, it can be stated mathematically in terms of something called entropy, but it's easier to understand qualitatively rather than quantitatively.

The second law of thermo deals with the "quality" of energy. In essence, it says that "high grade" forms of energy such as the chemical energy in fuels or electrical energy are more versatile and easier to control than "low grade" forms of energy such as heat. It also implies that high-grade forms of energy will naturally try to convert themselves into lower grades of energy.

A good example of the second law's validity is the ability to maintain the chemical energy in fuel oil for years, even centuries, compared to the situation of converting that energy in heat and trying to store it without losses for more than a few hours. Like the first law, there's no way around the constraints of the second law, and this imposes some important concepts for solar heating system designers.

To maximize the collection of low-grade heat from solar collectors, fuel should not be consumed to maintain a storage tank at an elevated temperature. Doing so decreases the amount of collectable solar energy because collector efficiency is related to the water temperature at which the collector operates. The cooler the storage tank, the cooler the collector and the higher its efficiency.

Recognizing this constraint means you will want to pipe the boiler so it can supply heat to the distribution system without having to warm the storage tank in the process. A simple way to do this is to use individually controlled circulators to supply a common piping segment that in turn supplies the loads, as shown in Figure 1.

When the temperature of the solar storage tank is too low to supply heat to the distribution system, the boiler is fired and its associated circulator is turned on. The circulator for the solar storage tank is turned off. In this mode, the distribution system functions identically to a nonsolar system. The remaining low-grade heat in the solar storage tank slowly dissipates to the surrounding space (which should certainly be within the thermal envelope of the building). When the sun returns, the cool storage tank allows the collectors to come on at a relatively low temperature and, thus, gather heat at higher efficiency.

The boiler and solar storage tank could also be piped as independently controlled secondary circuits serving a common primary loop, as shown in Figure 2. This configuration does require an additional circulator for the primary loop.

Figure 3

Solar DHW Only

One of the most economically feasible solar options is supplying domestic water heating. It's a year-round load and, thus, takes advantage of high solar yield in warmer months. The size of the solar collector array for an average residential DHW load is much smaller than that required to achieve a high percentage of solar space heating.

Since many residential hydronic systems have an indirect domestic water heater, it's natural to look at ways of using that tank for solar storage as well as a boiler-supplied water heater. Unfortunately, maintaining such a tank at normal DHW supply temperature using the boiler violates the second law principle discussed earlier.

Another issue is tank volume. A typical indirect water heater in a residential system has a volume of 30 to perhaps 60 gallons. Given their heat transfer capability, especially when connected as priority loads, these relatively small tanks can provide good performance. However, for good performance, the solar storage tank should contain 2 gallons of water for each square foot of collector area. Solar energy systems supplying DHW for an average home can have 60 to 90 square feet of collector area and need 120 to 180 gallons of water storage for optimal performance. Buying a large commercial-size indirect water heater to gain the extra storage capacity will be expensive.

One way around this is shown in Figure 3. This system uses a smaller 30- or 40-gallon indirect water heater in combination with a larger insulated storage tank. The indirect heater contains potable water in copper coils suspended in the insulated steel shell. The water in the tank is "system water" rather than potable water. This allows the solar storage tank to be part of a closed loop system constructed of standard carbon steel. A small cast-iron circulator moves water between the two tanks whenever the solar storage tank is a few degrees warmer than the indirect tank. This function is managed by a differential temperature controller like the one used to turn the collector pump(s) on when the collectors are a few degrees warmer than the storage tank. This circulator remains off at other times, allowing the solar storage tank to be taken "offline" when the boiler is heating the indirect tank. Be sure to include an anti-scald-rated tempering valve on the outlet of the indirect to keep DHW delivery temperatures under control on hot, sunny summer days.