Solar Heat Collection Design
This page discusses the design of the solar heat collection system, which must efficiently transfer the heat from the six solar heat collectors into the storage tank. Click here to see more pictures of the plumbing.
We chose to use 4-foot by 10-foot Heliodyne Gobi flat plate solar collectors because they offer relatively good efficiency, reasonable cost, and they were the largest size we could easily accommodate in the wall space above the garage. These collectors are rated to produce 29,000 BTU per day for each collector in clear sunny weather, and our heat storage design calculations suggest that we would hope to collect about 600,000 BTU over a 4-day sunny period. Simple math says that would take 5 collectors, but ours are oriented vertically in order to reduce the excess summer heat gain compared to orienting them at, say, 60 degrees. The vertical orientation also reduces our winter heat gain, although to a much lesser degree than in summer. And the sad fact is, we never really see a 4-day period with no clouds at all in the winter so 29,000 BTU per day is overly optimistic in our climate. In fact we could have used 8 collectors and it still wouldn't be enough during some parts of winter. We chose the number of collectors - six - using a much simpler calculation. We divided the available space on the wall above the garage by the width of the collectors. We can't fit any more than six, so that's how many we have. We are confident that they will not collect enough heat to meet all our needs during the winter, which is why we have wood stoves for supplemental space heating and electric tankless water heaters for both back-up space heating and domestic hot water. Even though we'll burn some wood and use some electricity, the solar collectors should still meet most of our heating needs.
Design Flow Rate
Heliodyne recommends a design flow rate of 1.25 gallons per minute (GPM) for the GOBI 410 collectors that we use. That's for one collector and the fluid flows through all six collectors in parallel, so we want a total flow rate of 7.5 GPM. We don't need exactly this flow rate but if it's much less than that, the fluid will get too hot as it flows through the collectors. And if it's much more than that, we're wasting energy running the pump. We're using the same model of Grundfos pump for the solar heat collection as for the hydronic heat in the floors, and here is the manufacturer's flow curve for this pump:
The pump has a built-in check valve that keeps water from thermosiphoning back through the collectors at night, which would cause significant heat loss, so we must use the dashed red curves on the graph. The graph indicates that on speed 3, we'll get the desired 7.5 GPM flow rate with a head loss of about 12 feet. This is equivalent to pumping the water 12 feet uphill, and it's the maximum resistance we can tolerate and still get the desired flow rate. The head loss is caused by the friction in all the pipes, collectors and fittings that the fluid must travel through as it circulates in this loop.
Using a simple friction loss calculator, we estimated that the friction loss in the 60 feet of pipe running to and from the collectors would be 11.3 feet using 3/4" pipe, but only 2.8 feet using 1" pipe. Therefore we chose 1" diameter copper tubing for that section and we used soft tubing so it could be bent in gentle curves rather than adding elbows that would cause more friction loss.
For the heat exchanger coils we were limited to 3/4" copper for practical reasons (cost, ease of bending, ability to get them inside the tank) and using only a single coil would produce a huge 18.9 foot head loss at 7.5 GPM! There's just no way we could get sufficient flow through a single 3/4" coil without using a much bigger pump, which would consume more electricity. But by splitting the flow in two, we have only 3.75 GPM going through each coil and the head loss drops to a tolerable 5.2 feet.
Finally we consider the head loss through the collectors. This calculation is easy; because Heliodyne specifies a pressure loss of 2.5 feet at the design flow rate with an array of 6 collectors.
The grand total for these items is 2.8 + 5.2 + 2.5 = 10.5 feet of head loss. There's a bit more friction loss due to the fittings, especially the T fittings where the flow splits and recombines, so we're pretty close to the design goal of 12 feet of head loss and this indicates that we'll probably need to run the pump on its highest speed to get the design flow rate of 7.5 GPM. The friction loss calculator referenced above calculates the friction loss for plain water, so the numbers aren't exactly right for our mix of propylene glycol and water but they're close enough to tell us that we definitely need 1" pipe going to and from the collectors, and our pump is approximately the right size.
The following shows the schematic design of the solar heat plumbing, and a photograph of the components in the mechanical room.
The hot fluid from the top of the collectors enters at the upper right, passes a temperature gauge and then splits at the T into the two heat exchanger coils. It moves downward through the coils and then returns to the lower T, and recombines into a single 1" pipe before passing through the Recirculator Pump. The gauges to the left and right of the pump are 0-30 PSI pressure gauges, and the difference between them tells us the actual head loss that the pump is seeing. That will let us estimate the flow rate through the pump using the pump curve above. One PSI of pressure difference is equivalent to about 2.3 feet of head (with plain water), and when the pump was running on speed 3 we observed about 5.5 PSI difference between the gauges. That gives a head loss of 12.6 feet indicating about 7 GPM flow rate which is pretty close to the desired rate.
In the photo above you can see a black unit on the lower left side. This is a Grudfos Vortex Flow Sensor that connects to the controller and measures the actual flow rate. Thus we have a second way to measure the flow rate, and the controller does indeed indicate a flow of about 7.5 GPM with the pump running on high. Interestingly, the flow rate does not drop much when the pump is reduced to speed 2. As the flow rate drops so does the pressure loss, so by reducing the pump speed we don't actually lose as much flow as we might expect by looking at the graph. Even on speed 2 it stays pretty close to 7 GPM, so we'll run it at that speed for now. Reducing the pump to speed 1 drops the flow rate down to 5 GPM, which is still quite reasonable and it will probably be sufficient to keep the temperature rise through the collectors from getting too high. We'll try that on a bright sunny day, and see if the temperature rise through the collectors stays within reason. If we can reduce the flow rate to 5 GPM, it will reduce the electrical energy consumed by the pump and it will also increase stratification in the tank, giving us hotter water near the top. That reduces the total amount of heat we can store in the tank but having a higher temperature near the top is more useful for heating domestic hot water.
The Air Eliminator is located near the top of the collectors, so that it can automatically vent any air that is present in the system. We placed it high in the attic just after the outlet of the collectors, so it's protected from the weather. It would have been more convenient to locate it down with the rest of the plumbing but it will work better at the highest point in the system. We built a platform in the attic so we have a place to stand in case it ever needs servicing.
It is necessary to keep the fluid under pressure, because otherwise the pump will experience cavitation. See the hydronic heat design page for a discussion of cavitation and why it's bad. Because of the potentially high temperature in this system, the pump needs about 10 PSI of pressure to prevent cavitation. We pressurized it to 15 PSI when cold, and the pressure will go up a bit as the system gets warmer.
The Expansion Tank allows the water to expand as it gets hot, without causing the system pressure to go up too much. Without it, even a moderate temperature increase could raise the pressure a lot. The pressure relief valve is set at 30 PSI, so if the system ever exceeds that pressure it will release fluid (onto the floor and down the drain) to keep the system from bursting. Although it wouldn't damage the system it would make a mess and waste antifreeze so we never want that to happen. We can calculate the required tank volume or we can simply look in the building code book, which tells us that we need about a 2-gallon expansion tank for a system of this volume (roughly 15 gallons of fluid total). Since these tanks come in 2.2-gallon and 4.4-gallon sizes, we chose the larger 4.4 gallon tank to give an extra measure of safety. That will minimize the pressure increase as the system warms up to its maximum temperature and it should keep it well under 30 PSI.