Solar Water Heaters Bastián Acevedo Bustos Electronic Engineering Student, Tarapacá University Environmental Studies Student, York University
Systems
Active System Active systems use one or more pumps to circulate water and/or heating fluid in the system.
Passive System Passive systems rely on heat-driven convection or heat pipes to circulate water or heating fluid in the system.
Direct System Water circulates from the geyser/solar tank through the solar panels (which are absorbing infra-red radiation from the sun), and heats up as it passes through
Indirect System Indirect or closed loop systems use a heat exchanger that separates the potable water from the fluid, known as the "heattransfer fluid" (HTF), that circulates through the collector.
Passive Direct Systems ICS or Batch Heater: An integrated collector storage (ICS or Batch Heater) system uses a tank that acts as both storage and solar collector.
CHS: Passive Direct Systems A convection heat storage unit (CHS) system is similar to an ICS system, except the storage tank and collector are physically separated and transfer between the two is driven by convection.
Active Indirect Systems Pressurized antifreeze or pressurized glycol systems: Pressurized antifreeze or pressurized glycol systems use a mix of antifreeze (almost always non-toxic propylene glycol) and water mix for HTF(Heat Transfer Fluid) in order to prevent freeze damage. Drainback System: A drainback system is an indirect active system where the HTF circulates through the collector, being driven by a pump. When the pump is off, the HTF drains out of the properly sloped collectors and pipe, leaving them empty and protected from freezing.
Collectors
Flat-Plate Collector Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers.
Integral Collector-Storage Also known as ICS or batch systems, they feature one or more black tanks or tubes in an insulated, glazed box.
Unglazed or Formed Collectors Unglazed or formed collectors are similar to flat-plate collectors, except they are not thermally insulated nor physically protected by a glass panel.
Evacuated-Tube Solar Collectors An evacuated-tube collector contains several rows of glass tubes connected to a header pipe. Each tube has the air removed from it (evacuated) to eliminate heat loss through convection and radiation.
U Pipe Evacuated Tubes A direct-flow evacuated tube collector has two pipes that run down and back, inside the tube. One pipe is for inlet fluid and the other for outlet fluid.
Heat pipe evacuated-tube collectors Heat pipe evacuated tube collectors contain a copper heat pipe, which is attached to an absorber plate, inside a vacuum sealed solar tube. The heat pipe is hollow and the space inside is also evacuated.
Integrated Tank Solar Collectors- Thermosyphon Evacuated Tubes: Where temperatures are not likely to drop into the freezing zone, many evacuated tube solar collectors are made with an integrated storage tank at the top of the collector.
Considerations
Efficiency First, Then Loads Insulate pipes and storage tanks, install high-quality, restricted- or low-flow faucets and showerheads, and lower your water heater s thermostat setting if possible. Minimize losses, reduce your hot water demand, and make your solar hot water system both smaller and less expensive.
SIZING https://secure.solar-rating.org/certification/ratings/ratingssummarypage.aspx http://pvwatts.nrel.gov/
Storage Sizing Your climate and the total area of the collectors will determine the storage tank capacity. The storage tank should be large enough to satisfy the demands of a household for one day, at a minimum.
Backup Source For natural gas or propane, traditional tank-type and tankless are the most common. For electricity, you can choose from tank-type, tankless, or heat pump/hybrid water heaters.
Solar Loop Type Specify the solar loop type that will work best for your application and climate.
HARSH CLIMATE SYSTEMS Drainback Antifreeze MILD CLIMATE SYSTEMS Integral Collector Storage Thermosyphon Direct Pump
Choosing the Right Collector Whether you choose flat-plate or evacuated tube collectors, focus on how much energy a collector will produce on an average day at your site. Next, choose your collector make and model, and determine how many you ll need to offset your water heating needs.
Space Available Calculate how much space is available for the solar thermal collectors. Collectors can be mounted on the ground, the side of the building (like awnings), or even pole-mounted.
Budget RETScreen Clean Energy Project Analysis software RETScreen can provide performance estimates for a number of renewable energy and energy-efficiency technologies. http://www.retscreen.net/
Mechanical Room Measure the space available for mechanical equipment. For solar water-heating systems, you will need room for the existing water heater and the new solar tank.
Installation Basics
Collector Tilt, Orientation, & Access We recommend that solar collectors used for year round domestic hot water face true south(northern Hemisphere) or true north(southern Hemisphere) and be tilted up from the horizontal at an angle equal to the latitude of the site plus 15 degrees.
Mounting Solar Collectors Mounting hardware can be supplied by the collector manufacturer or you can build it yourself. Flush mounts (also called stand-off mounts) are used to mount the collectors at the same pitch as the roof.
Whether the rack is homemade or manufactured, painted angle iron can be used for mounts in areas of low humidity. An important consideration to keep in mind regarding all types of roof mounting is that the mount hardware must be fastened directly to the structural members of the roof the joists, rafters, or trusses.
Roof Penetrations The wires needed for the collector sensor can be run alongside one or both of the insulated pipes to the collectors. Roof penetrations for piping need to be slightly larger than the diameter of the piping and its insulation. Plastic pipe insulators are handy devices for running a pipe through any roof sheathing or structural member.
Sealing the mount screws or bolts and the part of the mounts that are directly in contact with the roof surface can be done with roof sealant.
Ground Installation If the piping is underground, it and its insulation should be encased in a larger PVC pipe. The classic ground installation uses a very simple pier of concrete to secure the collectors to the ground.
Wall Mount Installation Factory or site-built mounts can be easily adapted for mounting collectors to the side of a home or other building. The mounts should be lag screwed or bolted directly to the center of the wall studs rather than just the wall sheathing.
Collector Piping Copper is the favored material for collector piping. Chlorinated polyvinyl chloride (CPVC) piping is sometimes used for passive water heaters. The collector supply pipe is always connected from the pump to the cold inlet at one end of the bottom header pipe of the collector. The return pipe runs from the hot outlet of the collector(s) and runs to the heat exchanger next to the storage tank.
Exposed Pipe Insulation All pipe insulation exposed to ultraviolet (UV) rays of the sun needs protection for long-term durability. A good UV-resistant paint will last from five to ten years, and manufacturers of high-temperature, closedcell insulation have recommended products. If you want a maintenance-free covering for the insulation that will last a lifetime, flat, architecturalgrade aluminum used for camper shells and gutters is a good solution
The Control System Almost all solar water heaters use the same type of electronic control, a differential control (aka differential thermostat). The differential control is used to control the system if AC pumps are used. It is important that the sensors have a good mechanical and thermally conductive connection to the pipe.
Closed Loop and Anti-freeze Freeze protection is accomplished by circulating a nontoxic antifreeze fluid to remove heat from the solar collectors and transfer it to the domestic water via a heat exchanger. The antifreeze remains contained within a closed loop, so it never mixes with the domestic water.
Rules Rule #1 The pumps should be placed so that they are pumping the coldest fluid in the system. The coldest collector loop fluid is found after it has been circulated through the heat exchanger where it has lost most of its heat. The coldest DHW is at the bottom of the storage tank. Rule #2 Pumps should always be mounted so that the impeller shaft is horizontal. Mounting a pump with the shaft vertical will put too much pressure on the shaft bearings and cause premature failure. If possible, the pump(s) should pump upwards; this prevents trapped air from collecting in the pump chamber, which is possible with some pump models. Rule #3 The check valve should be placed between the two boiler drain valves that are used for purging and filling the system. These boiler drains are also used for any future maintenance of the collector loop fluid.
Rules Rule #4 The expansion tank, pressure gauge, and the boiler drain and check valve assembly are usually placed downstream from the heat exchanger, near the collector fluid pump. Therefore, they are on the cold side of the loop as well. Rule #5 If you place the expansion tank with the pipe fitting down (tank upside down), the tank will continue to function if the internal bladder fails. An expansion tank placed with the fitting horizontal will still hold air with a bladder failure, and may continue to function. A tank placed with the fitting up, upon failure, will introduce all the air in the tank to the collector loop piping. This is a common cause of failure in older systems. Rule #6 A coin vent may be located anywhere that an air pocket is likely to form within the piping. Air pockets are most likely to form where the fluid is at its hottest, or where the piping makes a downward turn. A coin vent is usually placed at the collector outlet where the piping turns downward. Another may be located at a similar location in the collector loop at the closed loop module assembly.
SDHW Closed Loop Parts List Solar Collector(s) Pump(s) Differential Control Heat Exchanger Storage Tank Expansion Tank Pressure Relief Valve Pressure Gauge Coin Vent or Automatic Air Vent Boiler Drain Valves Check Valve Miscellaneous Plumbing Parts
Putting It All Together Connecting to the Heat Exchanger Connecting the preassembled module to a tank with an integrated heat exchanger is rather straightforward. The heat exchanger outlet (cold) feeds the pump inlet, and the collector outlet connects to the heat exchanger inlet (hot). One and Two-Tank Storage Systems Most SDHW systems use a separate tank to store solar heated water. This is in addition to the backup auxiliary water heater. Cold water supply from the house will feed the cold inlet of the solar tank. The hot outlet of the solar tank will feed the cold inlet of the auxiliary gas or electric water heater. Collector Loop Fluids When the collectors are securely mounted and all components are assembled and wired, you are ready to test the whole system for leaks, purge it, and fill it with an antifreeze solution or synthetic oil.
Charging the System with Antifreeze You will almost always need about 2 gallons (7.6 l) of solution per collector (1 gallon of glycol) but the quantity will vary with the collector manufacturer, size of the collector, and size of piping in the system. Make sure to have a little extra if in doubt.
Follow-up & Maintenance Installation follow-up starts with casual observation during the first couple of weeks after starting the system up. The system should turn on shortly after the sun comes up, but exact times are hard to gauge. The turn-on time changes with the seasons and the temperature of the cold water. The system should also turn off before sundown.