SOLAR DOMESTIC HOT WATER Todd Cory Heating water with the sun is very practical and cost effective. While photovoltaics range from 10-15% efficiency, thermal water panels range from 50-80% efficiency. In combination with a wood stove coil/loop, virtually year round domestic hot water can be obtained without the use of fossil fuels. DESIGN Solar water heating panels consist of parallel vertical pipes attached to an absorber plate with horizontal header pipes at the top and bottom. All of this is enclosed in an insulated box with a tempered glass cover. A heat transfer fluid circulates through these pipes to the DHW (domestic hot water) storage tank. I have made several panels myself with little difficulty. While not as efficient as commercial units, the cost savings make home construction worthwhile. Most commercial units come in 4x8' or 4x10' copper with black paint or chrome finish. A high quality 4x10' black chrome absorber panel can produce = 45,000 BTU's per day. One BTU is the amount of energy required to raise 1 pound of water 1 degree F. Current cost for such a panel is around $600. As a large part of these panels is made of copper, prices will vary with the market prices of this metal. SIZING As with electrical production the system must be sized to the user's needs. Comparing the parallels with electrical production may make sizing easier to understand. ¥ BTUs gained per dayÅwatt hours of charging ¥ Storage in gallons of waterÅamp hours of battery storage ¥ Usage in gallons of waterÅamp hours of load Here are some things to consider when sizing (gallons of usage per day) in your DHW system. ¥ Numbers, length and temperature of showers/baths per day ¥ Future or present hot tub heating requirements ¥ Future or present hydronic heating requirements An "average" (HOME POWER readers would most likely consider this excessive) household of 2-3 would need a minimum of 50 gallons of storage and 1 4x8' (32 sq. ft.) of absorber panel. This is strictly ball park. Usage needs will determine system size. STOVE LOOPS Unlike solar electric systems, sun/heat energy can also be transferred by a coil/loop inside the wood stove. You can make your own out of steel or copper pipe or purchase a much superior unit made of stainless steel from the folks at Hydrocoil in Grass Valley, Ca. By incorporating solar hydronic panels on the roof for production in the summer months and a wood stove loop for the less sunny winter months, domestic hot water production can be maintained year round, and storage tank capacity reduced. PLUMBING CONFIGURATIONS Basically there are 5 variations of plumbing arrangements in solar DHW systems. 1) DRAIN DOWN (active pumped) 2) DRAIN BACK (active pumped) 3) CLOSED LOOP (active pumped) 4) PASSIVE CONVECTION (tank on roof type) 5) PASSIVE PUMPED ("geyser" type) The main concerns in choosing a system type are: 1) availability to operate active pumps and electronic controls 2) amount of below or near freezing weather in your area 3) efficiency and maintanence requirements of design Below follows a brief description of each plumbing arrangement with advantages and disadvantages for each. 1) DRAIN DOWN When the control senses heat available to be gained, an "HG-SPOOL" (drain down valve) opens, filling the panel(s) and operating a pump, which circulates potable water directly from the DHW storage tank through the panel(s). When temperatures approach freezing, the "HG-SPOOL" closes, draining the remaining water from the panel(s) to the ground. On the Plus Side ¥ Most efficient as the system uses potable water in the panel(s) directly, without the need for heat exchangers ¥ Existing/standard DHW tank can be used On the Minus Side ¥ Needs HG-SPOOL, vacuum breaker, air vent and control all subject to eventual failure, resulting in destroyed panel(s) in freezing weather and costly repairs. ¥ Needs power from the electrical system to hold the HG-SPOOL open and to operate the pump and controls. 2) DRAIN BACK When the control senses heat available to be gained, the pump is activated, circulating distilled/treated water through the panel(s) and a special heat exchanger within or external to the DHW storage tank. A drain back tank holds this water when it is not being pumped through the panel(s). On the Plus Side ¥ High efficiency as a non-toxic heat transfer fluid is used ¥ Low freezing potential as the panel(s) are drained by gravity when there is no heat to be gained On the Minus Side ¥ Needs power from the electrical system to operate the pump and controls. ¥ Needs special drain back tank ¥ Needs special heat exchanger DHW storage tank or external heat exchanger. ¥ Some noise when the pump is moving water through the system. 3) CLOSED LOOP The panel(s) are filled with a non freezing heat transfer fluid. Common fluids are propylene/ethylene glycol, or silicon oil. When the controls sense heat available to be gained, the pump is activated, circulating the fluid through the panel(s) to the heat exchanger, where it heats the potable DHW. On the Plus Side ¥ Virtually eliminates ANY freezing potential. On the Minus Side ¥ Heat transfer fluid (glycols) must be changed every 4-5 years to maintain PH. ¥ Needs special heat exchanger DHW storage tank or external heat exchanger. ¥ Needs power from the electrical system to operate the pump and controls. Only medium efficiency as a non potable heat transfer fluid is used. 4) PASSIVE CONVECTION (tank on the roof type) The DHW storage tank sits on the roof above the panel(s). The panel(s) have a non freezing heat transfer fluid in them which passively convects to heat the potable water in the DHW storage tank. On the Plus Side ¥ The wood stove coil/loop is easily installed in a passive fashion as the DHW storage tank is on the roof. ¥ System is totally passive. ¥ Medium high efficiency as the panel(s) are so near the DHW storage tank and a non-toxic heat transfer fluid is used. On the Minus Side ¥ Inlet and outlet pipes must be WELL insulated to reduce freezing potential. ¥ Some units use the unevironmental CFC freon in the convection loop. ¥ If gylcol is used in the convection loop, it will need to be changed every 5 years to maintain Ph. 5) PASSIVE PUMPED (geyser type) The DHW storage tank sits on a pad which convects heat from pumped fluid in the panel(s) to the potable water. The heat from the sun does the pumping to this heat exchanger which can be located as much as 36 feet below the panel(s). (see HP #8 Page 20) On the Plus Side ¥ No possibility of freezing. ¥ System is totally passive. On the Minus Side ¥ For use of this system with a passive (convection) stove loop, the DHW storage tank must be located a minimum of 15" above the stove. ¥ Medium efficiency as the system utilizes an alcohol heat transfer fluid. CONCLUSION All systems are adaptable to inclusion of wood stove coil/loops. In areas where freezing weather is common, the drain down type is not advisable. For people using alternative electrical energy, any active type system (drain down, drain back,closed loop) is also not advisable. This leaves us with two types of passive systems. The geyser pumped type and the convection tank on the roof type. My personal favorite is the passive convection type, particularly, Solahart's 300 L-J. It incorporates an 80 gallon storage tank and 2 (3 optional) 4x6' black chrome absorber panels. It produces the maximum for any such passive roof mount unit of 35,800 BTUs per day. With installation (without wood stove loop) one could expect to pay around $2,100. It's nearest competitor, the Suncatcher II uses the ozone destroying CFC, freon, (Solahart uses non-toxic propylene glycol) as a heat transfer fluid. It has a 40 gallon storage tank, produces14,500 BTUs per day and with installation (without wood stove loop) one could expect to pay around $1600. The Solahart is easily adaptable for use with a passive wood stove loop, which also greatly reduces and freezing potential. I've worked with Solahart on several of their units in Mt. Shasta and they really stand behind their products. They've been in the business for over 30 years and when in Australia in the early 80's, I was amazed to see them on nearly every roof. As to the geyser pumped type, I have little experience with them. It is a relatively new design and while the manufacturer rates them highly, field testing has shown some sensitivities to exact charging techniques. Also, the system utilizes an alcohol heat transfer fluid which, from what I understand is less efficient than glycol based heat transfer fluids, which in themselves are inherently inefficient. I would welcome further field based reports on this system. For two years I installed/maintained solar hot water systems in Mt. Shasta with Chitwood Energy Management. We did mostly closed loop ethylene glycol systems to eliminate any freezing potential and as grid power was available. Regardless of the system you chose, Solar DHW, like solar photovoltaic electricity, is the way of the future. As with PV's for folks at remote locations, it is NOW a cost effective means for abundant year round running hot water. I would be happy to answer further questions/inquiries regarding DHW solar systems. Please include a S.A.S.E. ACCESS SOLAHART 3560 Dunhill St. San Diego, CA. 92121 (916) 452-5805 SUNSHINE SYSTEMS (Suncatcher) 14340 Dandee Hill Rd. Grass Valley, CA. 95945 (916) 272- 4904 HELIOTROPE GENERAL (HG-SPOOL) 3733 Kenora Dr. Spring Valley, CA. (800) 522-8838 in CA. (800) 854-2674 out CA. HYDRO COIL (stainless steel wood stove coils) 14340 Dandee Hill Rd. Grass Valley, CA. 95945 (916) 272-4904 SAGE ADVANCE CORP. (Copper Cricket) 4209 W 6th Ave Suite A Eugene, OR 97402 (503) 485-1947 CHITWOOD ENERGY MANAGEMENT 508 Sarah Bell Mt Shasta, CA 96067 (916) 926-3539 Todd Cory PO Box 689 Mt. Shasta, CA 96067