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I Drain Down □ Glycol I Drain Back any additional flow into the collector. At the same time the pump turns off and the valve allows the water existing in the collectors and piping to drain out. The ac valve requires a special controller (DTT-74 or DTT-794) and two 10 KQ thermistors, which are available from Heliotrope General. The DC PV controlled Solar Sidebar uses a 10 Watt PV module. All the major components needed are included and assembled. The system only needs to be attached to the collector and storage tank. No controller is required. It is a nice unit, and is quick and easy to install. Heliotrope has recently improved the drain-down valve and offers a limited ten year warranty on this product.
The indirect systems have both a collector loop and potable water loop. The loops flow in opposite directions and the fluids in these loops never mix. The potable water loop picks up the heat from the collector loop through a heat exchanger, which is nothing more than a copper pipe or pipes inside another pipe or container. The indirect systems we have explored are the drain-back and the propylene-glycol anti-freeze.
Drain Back System
Our home built drain back tank is a 16 x 16 x 16 inch steel box with a 10 foot coil of 1/2 inch copper pipe inside (Figure 2). The potable water is pumped with a small (< 1/40th HP) stainless steel or bronze pump through the copper coil, picking up heat from the collector water around it. Inside the steel box and around the heat exchanger is the collector water, treated with a rust inhibiting solution of sodium-hydroxide, trisodium phosphate, morpholine, and sodium dichromate (from Hicks Water Stoves & Solar Systems), which gets pumped up to the collectors by a second larger and usually cast iron (1/12 HP) pump whenever they are hotter than the water in the bottom of the tank. The two pumps are controlled by a Heliotrope General (DTT-84) differential controller and two 10 KQ thermistors. This type of system is very common in North Carolina, especially in larger 500 or
750 gallon versions providing space heating to the house as well as hot potable water. These systems can perform very well. However, our system did not (Chart 1). The weaker performance is probably related to the increased quantity of fluid this design has to heat and inadequate insulation of the drain-back tank.
Glycol Systems
Recently we have focused our attention on indirect propylene glycol systems. These systems are simple, reliable, and inexpensive. We have been using external heat exchangers which connect directly to either an electric water heating tank (see photos) or a separate solar storage tank that would be plumbed in series with a gas fired water heating tank. These external exchangers cost a lot less than a storage tank with a built in exchanger, can eliminate the necessity of purchasing a new solar storage tank for someone who wants to add solar to an existing electric hot water heating system, and provide more flexibility in choice of storage tank size. An external heat exchanger can be
Right: A complete system minus the collector.
purchased for about $100 (see photo). Special solar storage tanks with built in heat exchangers can be purchased, but they are only available in a small number of sizes and are quite expensive. AET offers only one tank with a built in wrap-around heat exchanger. It is an 80 gallon tank with copper coil wrapped around the outside of the bottom half of the tank. Insulation is wrapped around the tank and exchanger. It costs $576. Shipping could add another $100 to the cost. One can purchase a regular 80 gallon electric water heating tank for about $230 at a local building supply center and other sizes are available for less. Adding an external exchanger brings the cost up to $330. So for $330 one can get essentially the same equipment as the 80 gallon tank with a wrap-around exchanger, with a delivered cost over $600.
Figure 3 shows a basic schematic for an indirect system with an external heat exchanger. The photos
Right: A complete system minus the collector.
Figure 3 shows a basic schematic for an indirect system with an external heat exchanger. The photos
also show a similar indirect system, configured slightly differently than Figure 3. The system depicted has a ball valve where a check valve would normally be placed. I did not feel a check valve was needed because the collector was mounted on the ground below the elevation of the storage tank and therefore should not reverse thermosiphon at night. The system in the photo also has an extra air vent installed in the collector supply line, several analog thermometers installed, and a slightly different expansion tank position. The storage tank, exchanger, and the pipes to the tank and collectors should be well insulated. Low flow shower heads should also be installed.
A 50/50 mix of water and propylene glycol (boiler antifreeze from Camco Manufacturing) is pumped into the system. Most full size systems hold 6 gallons or less. Some glycols come already diluted with water. Make sure you read the label. A 4 x 10 foot panel holds about 1.5 gallons. The boiler drain valves on either side of the check valve enable filling, pressurizing, and draining the system (Figure 4). We fill and pressurize to about 15 psi with a Teel drill driven pump (model 1P866). The pressure should be a little more than the pressure or static head that the fluid will exert from the elevation difference between the tank or exchanger bottom and the collector top, 1 psi for every 2.25 feet or .44 psi per foot difference. 15 psi equals 33.75 feet of static head, and is more than our modules have.
Left: Some serious problem solving going on here.
When the system is operating (Figure 3) the glycol mixture is pumped with a small (< 1/25th HP) cast iron, bronze, or stainless steel pump from the bottom of the exchanger through a check valve and fill/drain assembly and into the bottom of the collector. The check valve prevents reverse thermosiphoning at night or on cloudy days and needs to be pointed the correct way. An expansion (pressure) tank and pressure gauge are also depicted in this side of the collector supply loop, but could be installed anywhere in the loop. The expansion tank (normally 2 gallon) has an air filled bladder which gets compressed by the expanding hot fluid in the collector loop. This protects the system components from excessive pressure. The pressure in the expansion tank should be measured before filling the system and if needed, adjusted so that it is close to the static head pressure. They normally come precharged with 12 psi, which would be good for most situations. If a flow meter is desired it would normally be positioned vertically in this side of the supply loop. One should also install a ball valve below or above the flow meter so that the flow rate can be controlled. Some flow meters have valves built in.
The glycol fluid exits the top of the collector in the corner diagonally opposed to the supply corner in order to maintain a balanced flow through the collector. The heated glycol then passes by a 150 psi air vent installed vertically at the high point of the system and an adjustable pressure relief valve set at about 90 psi. This is a little less than the expansion tank bladder's maximum psi rating and should protect all components if for some reason the pressure would rise that high. We have had some "pop off" problems with the pressure-temperature valves commonly used on water heating tanks and like to use the adjustable pressure relief valves from AET.
Potable water is normally taken from the bottom of the tank at the drain opening. The drain is re-installed in a tee fitting at the opening. This water from the bottom of the tank can be either pumped or naturally convected through the exchanger. It flows in the opposite direction of the glycol. The hot water from the top of the exchanger is returned either at the side of the tank or in the top of the tank. This could be where the temperature/pressure relief valve is installed or in the cold in port at the top of the tank. The P/T relief valve could be removed and installed with the return water in a tee fitting or installed somewhere else in the potable loop. If the cold in port is being used for returning the solar heated water to the tank, then the cold water can be delivered to the tank in the drain port at the bottom of the tank and the cold water dip tube can be taken out and cut to reduce it's length so that water is delivered about 10 inches below the electrical element. AET recommends perforating the dip tube all up and down it's length. An air vent should be installed at the highest point of the hot water return line. This is especially important for systems that naturally convect the potable water through the exchanger. If the system has only one tank with electric elements then the bottom element should be disconnected. In this kind of system the flow rates of the potable water should be slow (less than .5 gpm) to avoid excessive mixing of the water and the temperature of the return water should be close to the thermostat setting of the element. The flow rates will be slow if the potable loop naturally convects.
Chart 2 shows temperatures from a sunny June day for each to the heat exchanger. The temperatures are for a system similar to the one depicted in Figure 3 which naturally convects the potable water through the exchanger. The average temperature of the water in the tank at the end of the day was 130° F.
Chart 2: Pump Indirect Glycol external heat exchanger temperatures in ° F
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