Tom Veatch

Solar Heating of Potable Water

Here are the design elements for such a system.

• Placement, Angle, Orientation: Place and orient the collectors to maximize collected heat value.
  it to maximum perpendicular to the sun at the time the sun produces the most valuable radiation.
  • Placement should be where maximum sun is available. An instrument including a hemi-spherical mirror and solar-angle markings can be used to see how trees occlude the sky from a given place; this lets you find a place where daytime sunlight can be maximized.
  • Direction or Azimuth: Since the sun moves East to West but is highest in the sky in the South (if you are located in the Northern hemisphere), the collector should generally aim South. But anywhere roughly south is approximately equivalent. Some say within 45 degrees of South, orthers say within 15 degrees of South.
  • Elevation: Collector angle from the horizontal should be close to the local latitude, which is close to the average elevation of the noonday sun at that location. So:
    • At the equator, collectors should be roughly horizontal.
    • At the north pole, roughly vertical.
    • In Seattle at 48 degrees latitude, the collectors should be sloped at roughly 48 degrees from the horizontal.
    • However where winter heating is more precious than summer heating, the collectors should orient toward the winter sun, which is 23.5 degrees less than the latitude, and 47 degrees lower in the sky than the summer sun. (Earth-Sun geometry is explained here. ) Then there will be some loss of efficiency in the summer (when there is probably too much heat being captured) but maximum efficiency in the winter (when the smaller amount of heat that does get captured is more precious and useful).
    • So at 48 degrees latitude in Seattle, the collectors should be sloped at roughly 48+23.5=71.5 degrees from the horizontal, or 18.5 degrees from the vertical plane.
    • But in your particular application, if the solar collectors are undersized even in the summer, and assuming that every BTU of backup heat costs the same whether summer or winter, then the slope should be equal to the latitude, which will optimize the heat capture over more of the year.

• Collector Type: Evacuated tubes heat pipes, or flat plate collectors?

  Flat plate collectors lose a lot more heat because they are not vacuum-tube insulated. They can be cheaper to buy and install. They are somewhat less efficient, but also somewhat tougher. Hail will not break them as easily; snow will melt away from them more quickly (because there is more heat loss!).

  Evacuated tube collectors are highly insulated by the vacuum layer. If there would be significant heat loss from a flat-plate collector because the collector temperature is far above ambient temperature, then use evacuated tube collectors.

  So in a very cold environment, or where maximizing efficiency is key, use evacuated tube collectors. In a milder environment, where wringing every bit of efficiency out of the system not as important, use flat plate collectors.

• Closed loop or open loop?
  • Closed loop systems run a heat transfer liquid through the solar collectors to pick up heat, and through a heat exchanger (a spiral of piping inside a water tank) to give the heat off to the water.
    • Freeze protection by using antifreeze (glycol) as the heat transfer liquid. Usually the glycol used is food grade propylene glycol rather than ethylene glycol, so that a leaking heat exchanger won't poison someone who drinks the heated water.
    • Glycol systems must have their pH level checked semi-annually or annually, since high temps can break down the glycol and make the liquid acidic, which will gradually damage the system.
    • Heat exchangers may be required to have double walled piping, with an air space between the walls which is vented to the open atmosphere. This reduces efficiency considerably. This requirement depends on local plumbing codes that regulate what piping and materials are allowed to contact potable water.
    • Piping need not be carefully graded since no draindown is needed. But piping should still be drainable, avoiding air and liquid traps.
    • Automatic air vent at high points of the system, to remove air, protected with a manual shutoff valve.
    • Air separator before the pump, to capture bubbles in the stream and remove them.
    • An expansion tank is needed for a closed loop system, so heated and expanding liquid does not burst something.
    • Fill/drain valves are needed, preferably one of each with a normally open ball valve between them. To fill or flush the system, shut that ball valve, put a hose from a new bucket of glycol to the (pump inlet side) fill valve, and let the pump pull the new fluid around through the system, pushing air or old glycol out the drain valve. When the drain valve runs clean, stop the pump, close the fill/drain valves, and re-open the normally-open ball valve: the system is now charged with fresh fluid.
    • Makeup fill system for glycol, in case of loss of volume?
  • Open loop systems use the potable water supply itself as the heat transfer liquid. No heat exchanger is needed, which eliminates one source of inefficiency.
    • Freeze protection by automatic draindown
    • Exposed piping must be graded (1/4 inch per foot), so that draindown is complete. Any licensed journeyman plumber is perfectly capable of building properly graded piping systems.
    • (A demonstration system with piping, pump, tank, and collector at within, say, 6 feet of elevation, cannot be open loop, since the in an open loop system the draindown air tank must be above the storage tank and below the collectors, which would make the demonstration system very tall.)
    • Size the piping for >2fps liquid flow while pumping.
    • Any air vent at the high point(s) of the system must be protected with a manual shutoff valve, normally closed, so that the system does not become waterlogged.
    • Fill/drain valves are not needed since the water supply itself is used to fill the solar subsystem, and since the tank will have a drain which should be at the bottom of the system.
    • An unpressurized draindown system can use a vacuum breaker to let air in to drain the liquid out. But an open loop system is pressurized, so a drainback air tank is needed (which can be the top of the storage tank, but ideally high in the insulated part of the building to lower the pump head and drain volume required). The air tank should be sized to cover expansion tank needs also, and during operation or maintenance it should be pumped up to the pressure of the city water supply. This tank should have an air-pump fitting, and a sight glass to show the level (to detect waterlogged or too-much-air conditions. Periodic maintenance, or some type of air-admittance valve, may be needed to adjust the air level in the drainback tank.

      Run a separate pipe line from the top of the drainback air tank to the high point of the system, protected with a check valve, so that when the pump is not pushing liquid against the check valve, the air can come up through it and fill the system from its high point(s), draining the liquid down into the heated part of the building.

    • Add a filter between the tank and the pump so that scale or lime etc. doesn't get stuck up inside the collectors.

• One tank or two? A separate storage tank or single, combined storage & backup-heater tank.
  • A separate storage tank lets the solar system perform optimally, without interference from the backup heater.
  • A combined storage/backup water heater tank uses the water heater as solar heat storage. If the backup heater needs to run at all, it may be stealing opportunities from the solar heating subsystem, heating water that could have waited for the solar system to heat it. This reduces the solar energy yield. Also solar efficiency is higher when the input water is coldest, so if the backup heater has partly heated the solar fluid, it has reduced the solar collectors' efficiency.
  • I prefer a separate storage tank plus a tankless water heater as the backup heater. The solar subsystem is optimized, and there is no standby loss from the nonexistent water heater tank. Often the tankless water heater won't have to turn on, and when it does, it won't have to work nearly as hard to heat solar preheated water as it must with the unheated, cold water supply. Especially with the occasional heavier loads, like two people showering at the same time, the tankless heater's capacity is much greater when the water is solar pre-heated. Therefore tankless and solar go well together.
• Sizing
  • Collector size A rule of thumb for domestic hot water: About 40 square feet of collector area for the first two residents; 20sf more for each additional two residents. A resident is an adult or a child over age 12.
  • Storage tank size At least 1 gallon per square foot of collector area. For sunny areas, up to 2.5 gal/sf. Make a generous allowance for storage, e.g., for a 60sf collector, use an 80 gallon storage tank.
  • Pipe sizing Depends on the collector used. Some use 1/2 inch lines, some use 3/4 inch lines, some use 1 inch lines, and of course some are metric.
  • Drainback air tank sizing Calculate the volume of the piping in the freeze-exposed space. Then size the tank at least that size. Also apply the minimum of the expansion tank requirement for the system (approx 1 gal per 60 gal storage). You may need to double this if the air tank is intended to be at most half full of air.
  • Circulator pump sizing Aiming for a flow rate of 2 feet per second in 3/4 inch tube calculates out to about 1/3 gallons per minute or or 20 gph, which is quite small. 12V circ pumps are often used. Gravity circulation exists but is a weak force, used in tank-over-collector systems suitable for mountain cabins; it is not further discussed here.

• Additional design elements.
  • Check valve in the solar loop protects against heat dumping at night, with the pump off. A floating ball check valve can be installed in the downward return piping, which allows downward flow past the floating ball, but stops upward flow when the floating ball plugs up the top.
  • Flexible Tubing.
    • Plastic tubing, such as PEX: Use no PEX tubing in the solar loop; since stagnation temperatures can exceed 350 degrees F.
    • CSST: Caleffi offers systems using corrugated stainless steel tubing, slightly over-sized due to the corrugations which cause turbulent flow and thus pump energy losses.
    Flexible tubing is incompatible with downhill grading of piping, which is:
    • required for drainback or draindown designs
    • recommended by the Florida State publication, irrespective of freeze-protection method (i.e., also for closed-loop glycol systems)
    • required for maintenance on any system; otherwise the necessary maintenance task of simply draining the system is not possible.
  • What is the stagnation temperature limit? If over 400 degrees F, soldered joints can melt, and brazed or mechanical joints may be required. If relief valves fail, systems under pressure could imaginably approach these temperature limits and melt apart.
  • Temperature and Pressure relief valve on the storage tank, just like a water heater tank.
  • Pressure relief valve on the solar subsystem, piped to a drain. just like a water heater tank.
  • Mixing valve to reduce high-temperature solar heated water down to the limited temperature of domestic hot water. If the solar heated water is above the scalding limit, it should be mixed with cold water down to below 140 degrees F (commonly to 120) before being sent to the fixtures.
  • Radiator for stagnation relief. When the system overheats, keep the pump on and route the solar liquid to a radiator to dump the heat out. This requires the right controller, sensors, and a motorized diverting valve, in addition to the radiator and piping.
  • Circulator pump sizing, 2fps in 3/4 inch tube is about 1/3 gpm or 20 gph, which is quite small. 12V circ pumps can be used with a PV panel, for off-grid power and to turn on/off with the sun's intensity. A 110V to 12V transformer may be used instead of the PV panel.
  • Controller, sensors, programming, BTU metering. The system should have sensors and program logic to handle the following test conditions.
    • Normal operating mode. All hot water demand satisfied by the collectors. Backup heated not needed. Balanced load & supply over a day. Temporary imbalances of load > supply, and supply > load.
    • Sub-normal operating mode. Backup heater needed.
    • Normal non-operating mode. Collectors cool/cold. Pump off. Draining, if a drainback system.
    • Stagnation or overheat mode.
    • Setup/charge/drain/disassemble modes.
  • Insulate the piping to/from the collector; also the pump, etc.

• Preferred System Summary: This summary references part numbers in Drawing #TCV2, Dated 1/9/2009, entitled Solar Hot Water System, with a design characterized as open loop, tankless, drainback, responsive, and heat-dumping.
  • Open Loop. Parts include:
    • Solar Storage Tank (SST) (TCV2.B) (No double-wall heat exchanger piping inside)
    • collector assembly (TCV2.B) Mounted on roof with weight-spreading, solid mechanical linkage to underlying rafter/joist structure, roof penetration sealed against water intrusion, elevation angle and orientation fixed by suitable hardware.
    • Air vent protected by manual valve at top of system (TCV2.F)
    • Pressure relief valve at top of system (TCV2.G)
    • circ pump (TCV2.H)
    • pressure gages (TCV2.K)
    • T and P relief valve on tank (TCV2.M)
    • Drain valve on tank (TCV2.O)
    • Controller (TCV2.P)
    • Insulation on piping, valves, tanks, pump.
  • Drainback Freeze Protection. Parts include:
    • Drainback air tank (DAT), sight glass, air-pump fill valve (TCV2.D)
    • Drainback check valve (CV) (TCV2.E)
    • Drainback piping from tank to DAT to CV to top of system
  • Overheat protection. Parts include:
    • Radiator/heat-dump (TCV2.I)
    • Controller-driven, multi-way motorized valve (TCV2.J)
    • Piping from valve to radiator to storage tank.
  • No-storage backup heater. Parts include:
    • Tankless auxiliary heater with included tempering/ mixing valve (AUX) (TCV2.C)
    • Isolation valves for aux heat to work w/o solar preheat (TCV2.L)
    • Piping from DCW and SST-output (preheated) to AUX
  • Responsive storage so that the second shower is also hot and solar heater heats the coldest water rather than mixed water (more efficient). Parts include:
    • SST option 1: horizontal baffle separating upper half from bottom half of Solar Storage Tank (TCV2.N)
    • SST option 2: three side inlet/outlets.
    • SST option 3: dip tube for cold inlet to enter from top inlet to bottom part of SST.
    • Multi-way motorized valve option: add one output path alternative to the middle of SST.
    • Piping from motorized valve to middle of SST.

  Space Heating

  Solar water heating for space heating is not much considered here because they need to be so much bigger. Space heating systems require perhaps triple the collector size of a DHW-only system, plus or minus a factor of three. Closed-loop, glycol systems with efficient, single-wall heat exchangers are code-compatible if space heating is the only application. Note, however, the increased maintenance requirements on glycol systems mentioned above.

  Swimming Pool Water Heating

  Swimming pool heating is also barely mentioned here because pools are relatively few in Western Washington, my location. However the application is important and in fact ideal. In solar heating of swimming pool water, the key fact is that the ambient temperature of the air during the swimming months and the temperature of the pool water are so close to each other mechanical heat losses are relatively negligible, therefore system efficiency is very high and also insulation is unnecessary. Every pool should use a solar pool heater. Indeed in some jurisdictions this may be required by law.

  Copyright © 2009, Thomas C. Veatch. All rights reserved.
  Modified: April 8, 2009