Session 6. Sizing a solar water heater
Step 1. (5 minutes)
Step 2. (10 minutes)
Step 3. (10 minutes)
Since most countries in which the PCVs will serve are within 15° of the equator, it is important that the participants use the sun charts or sun angle calculator from Phase III: Session 2, "The Path of the Sun," to describe the sun's path at or near the equator.
Step 4. (10 minutes)
Step 5. (10 minutes)
Step 6. (30 minutes)
Step 7. (30 minutes)
Step 8. (15 minutes)
RULES OF THUMB: SIZING A SOLAR WATER HEATER
Direct Gain Solar Water Heaters
Direct gain solar water heaters are usually sized at 80 liters of water storage for every square meter of collector aperture or area. This ratio of 80:1 will provide hot water (55°C or 130°F) in the afternoon of sunny, warm days if the tank is filled with cold water (15°C or 60°F) in the morning.
If this ratio is changed to 40 liters of water for every square meter of collector (40:1), then the water will heat up faster but will also cool off faster in the afternoon or at night if the tanks are not insulated.
If this ratio is changed to 120:1, the water will barely get warm (38°C or 100°F) but will retain its heat for many hours. This ratio is best used to pre-heat water which will be heated to a higher degree with another heat source. This is the most cost-effective ratio but provides the least hot water.
Flat Plate Collector Solar Water Heaters
The 80:1 ratio also holds true for flat plate collector solar water heaters. Flat plate collector systems, however, have the advantage that the solar heated water is stored in an insulated container so it is less likely to cool down in the afternoon or at night.
Ratios approaching 40:1 tend to be less cost effective. Since it is the collector that determines the total cost of the system, more collector with less storage will cost more and provide you with less hot water.
Ratios approaching 120:1 are more cost effective but will not heat water as hot as an 80:1 ratio system. A system with a ratio of 120:1 will warm the water most of the year, heat it up very well a few months of the year, but will need to be supplemented with another water heater to get hot water (55°C or 130°F) for most sunny and warm months of the year.
Build a solar water heater or an insolation meter when you get to your country and change the storage tank-to-collector aperture ratio to find which ratio will work best for you in your microclimate.
RULES OF THUMB: ORIENTING A SOLAR WATER HEATER
In North America, a common rule of thumb for orienting a solar water heater is to face it within 45° of true south (with true south being the optimal direction) at a tilt of "latitude plus 10 degrees." The Continental United States includes latitudes from 25° to 48°. Therefore, tilt angles can vary from 35° to nearly 60° from the horizontal.
As solar collectors approach the equator, orientation and tilt become more seasonal because the sun moves into both the north and south hemispheres during the course of the year. There Fore, orientation depends on when the solar energy is needed (which months) and the local microclimate (i.e., are there clear mornings or clear afternoons during the months when the solar energy is needed?). If a solar collector is on the equator and facing south with a tilt of 15° from the horizontal, it will work well during the months of September through March. However, from March until September, the sun will be behind the collector. If a solar collector is facing east with a tilt of 15°, it will collect well only during the morning hours. This orientation is best for locations with cloudy and/or rainy afternoons.
The best tilt for a solar collector on the equator is no tilt at all: a horizontal collector. This causes problems, however, with naturally circulating systems such as food dryers and thermosiphon water heaters: the air or water doesn't know which way to flow; it doesn't know which way is up. As soon as you tilt and orient a collector, it will only work half of the day or year.
Therefore, the tilt and orientation of a solar collector near the equator depends on what time of year the solar energy is needed and what part of the day is sunniest in the microclimate of the collector.
The orientation of a solar collector in the Southern Hemisphere should be toward the north, toward the equator. The tilt of the collector should be the same as for a Northern Hemisphere collector -- latitude plus 10°.
HOW TO SIZE A FLAT PLATE COLLECTOR
To properly design and construct a passive solar water heater, one needs to know the amount of energy required in the form of heated water and the amount of sunlight available on an average day during the time of least sunshine.
By simply dividing the energy required per day by the energy available per day per area, one can determine the area of collector aperture needed.
1. How much energy is required?
Average hot water usage per person per day
40 liters hot water per person per day
2. How much energy is available?
Clear day winter insolation for the desired tilt
2700 Kgcal/m² day
3. By dividing the amount of energy required (Step 1) by the amount of energy available (Step 2) one can get a very good approximation of the collector aperture required to provide the desired temperature and volume of water on an average day during the period of least sunshine.
5.1 m² of collector aperture is needed to provide 120 liters of water at 45°C if the incoming water is at 15°C and the insolation is 2700 Kgcal/m² day.
(Notice how a complex fraction -- a fraction over a fraction -can be simplified by "inverting and multiplying." Also note how the units will always cross out to leave 2 just the units needed: in this case, square meters, or m .)
4. Once this ratio of aperture-to-volume is found, it can be used to size a collector for any size hot water tank, assuming all of the variables remain the same.
If the system will not be asked to provide hot water during the period of least sunshine (if, for example, there is virtually no sun for six months of the year), the clear day summer isolation for the desired tilt and the summer percent possible sunshine must be substituted for the winter data used in Step 2. Summertime system efficiency is also much greater than wintertime efficiency because there is less heat loss in the summer.
Care must be taken not to ask too much of a solar collector system: If a system is sized to provide hot water in the winter, it will probably produce very hot water in the summer, which is potentially dangerous (scalding occurs at water temperatures of 60 C).
HOW TO PRESSURE TEST A PLUMBING SYSTEM
1. To pressure test with water only:
Cap or plug all openings in the system, except two. Of these two, loosely cap or plug one of them and attach a garden hose or some other water source to the other. Make sure that the loosely capped or plugged opening is near the top of the system.
Begin filling the system. When water begins to leak from the loosely capped or plugged opening, tighten the cap or plug so that no water can escape. Inspect all joints in the system for leaks by looking for obvious ones and feeling each joint for any sign of moisture. Mark any leaky joints. Drain the system, fix the leaks and re-test.
2. To pressure test with water and compressed air:
Cap or plug all but one opening near the top of the system. Fill the system with water using this upper opening. Attach a compressed air source to the system and compress to 50 pounds per square inch (3.5 Kg/cm²) pressure or to the pressure at which the system will be operating, whichever is greater. Tap each joint with a wooden or rubber mallet to simulate expansion and contraction stresses. Look for water leaks at each joint. Mark all leaks, drain the system, repair the leaks and re-test. (If a pressure gauge is available, attach it to the system and test for 24 hours.)
3. To pressure test with air only:
Cap or plug all but one opening of the system. Attach the pressure gauge tester to the remaining opening and compress with air to 50 psi or 3.5 Kg/cm² or the pressure at which the system will be operating, whichever is greater. Tap all joints with a wooden mallet to simulate expansion and contraction stresses. Listen for leaks. Leave the gauge on the system for at least 24 hours. If the gauge shows ANY decrease in pressure, there is a leak in the system. Leaks can be found by applying a soap-and-water solution to each joint and watching for bubble formations. Mark any leaks. Release the pressure from the system, fix the leaks and re-test.
DIRECT CAIN SIZING PROBLEMS
Given the following information, decide whether or not a direct gain solar water heater will be effective and, if so, find the number and size of tanks needed and the aperture needed to raise the water to the desired temperature.
FLAT PLATE SIZING PROBLEMS
Given the following data, determine the size of storage tank and area of collector required. Assume a 50% efficiency on the collector.
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