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close this bookAppropriate Community Technology - A Training Manual (Peace Corps, 1982, 685 p.)
close this folderPhase IV: Solar water heaters
View the document(introduction...)
View the documentPhase IV Calendar
View the documentSession 1. The role of the volunteer in development: international development part 2: the green revolution: successes and failures
View the documentSession 2. Introduction to solar water heaters
View the documentSession 3. Assessing community water needs and uses
View the documentSession 4. Introduction to solar water heating: determining hot water demand
View the documentSession 5. Plumbing a solar water heater
View the documentSession 6. Sizing a solar water heater
View the documentSession 7. Demonstrating a technical concept
View the documentSession 8. Shade mapping and solar siting
View the documentSession 9. Design of solar water heaters.
View the documentSession 10. Construction of solar water heaters
View the documentSession 11. Multi-media standard first aid
View the documentSession 12. Wind technology
View the documentSession 13. Volunteer in development part 2: women in development
View the documentSession 14. House design in four climates
View the documentSession 15. Presentation of solar water heaters

Session 6. Sizing a solar water heater

Total time:

2 hours

Objectives:

* To use and discuss the solar water heating sizing formula and sizing rule of thumb


* To review and discuss the relationships between insolation, collector area, storage size and hot water demand


* To correctly orient a solar water heater


* To discuss how to pressure test a plumbing system

Resources:

* Attachment IV-6-A, "Rules of Thumb: Sizing a Solar Water Heater"


* Attachment IV-6-8, "Rules of Thumb: Orienting a Solar Water Heater"


* Attachment IV-6-C, "How to Size a Flat Plate Collector"


* Attachment IV-6-D, "How to Pressure Test a Plumbing System"


* Attachment IV-6-E, "Direct Gain Sizing Problems"


* Attachment IV-6-F, "Flat Plate Sizing Problems"

Materials:

Newsprint and felt-tip pens

Trainer Notes

This session will require considerable preparation as you will need to make copies of each of the attachments and have them ready for distribution during the session.

Step 1. (5 minutes)
Post the objectives, outline and explain the session activities.

Trainer Notes

* It is helpful for one or more of the participants to volunteer to describe each attachment to the rest of the group as it is distributed.

* Encourage group discussion of each attachment and add your comments if the information offered by the volunteer is incomplete or inaccurate.

* Discuss each of the following questions completely to be certain that every participant understands the principles of each attachment.

* Encourage those who understand the principles to work with those who are having difficulty.

Step 2. (10 minutes)
Distribute, review and discuss Attachment IV-6-A, "Rules of Thumb: Sizing a Solar Water Hearer."

Trainer Notes

To stimulate the discussion:

* Ask for the difference between a direct gain and a thermosiphon solar water heater.
* Ask for the sizing ratio of one group's insolation meter.

It is appropriate here to explain the difference between a "rule of thumb" and a formula and that the reason for having "rules of thumb" is that they are more easily understood by people with no technical background.

Step 3. (10 minutes)
Distribute, review and discuss Attachment IV-6-B, "Rules of Thumb: Orienting a Solar Water Heater."

Trainer Notes

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.

To stimulate and guide the discussion:

* Ask for the orientation of the tilt of a solar water heater if :

1. You are on the equator, it rains every afternoon, every morning is clear and hot water is needed year-around.

2. You are 15° south of the equator and only need hot water from September through March.

3. You are 15° north of the equator, it rains every morning, the afternoons are clear and you need hot water year around.

Step 4. (10 minutes)
Distribute, review and discuss Attachment IV-6-C, "How to Size a Flat Plate Collector."

Trainer Notes

Explain that the variables in each step of the attachment can have an effect on the size of the collector needed: if hot water usage doubles, the collector area and tank volume will I have to double. If the number of people using the hot water doubles, then the collector area and storage tank volume will have to double, and so on.

Here are some sample discussion questions:

* Does each variable have a direct relationship to the collector area and storage tank volume (as in the examples I given above) or an inverse relationship to the area and volume (i.e., if the insolation rate mysteriously doubles, the collector area and storage tank volume can be cut in half)?

* How important is "percent possible sunshine" to the equation?

* What do you think is the range of collector efficiencies?

* How can you determine the insolation rate for your own microclimate?

Be certain that each participant understands the logic behind the calculations, even if they don't understand the math.

Point out that Step 4 of this attachment leaves us with an area-to-volume ratio which is described as a rule of thumb in Attachment IV-6-A.

Step 5. (10 minutes)
Distribute, review and discuss Attachment IV-6-D, "How to Pressure Test a Plumbing System."

Trainer Notes

Allow time for everyone to read the attachment. Refer to the last session, Plumbing a Solar Water Heater, and discuss any questions about the attachment.

Step 6. (30 minutes)
Distribute and review Attachment IV-6-E and have the participants complete the problems. When they have finished, have them share and discuss their answers.

Trainer Notes

One option during this and other such exercises is to encourage people to work together, in groups of two or three. Although it is not the traditional way of taking a quiz or test, it does promote cooperation end learning and reduces frustration and unnecessary competition for "the best grade."

During the discussion of the answers, explain that:

* Direct gain solar water heaters work best when the desired final temperature is not above 55°C (130°F).

* Plumbed water heaters should have at least two tanks to reduce mixing of hot and cold water.

Step 7. (30 minutes)
Distribute and review Attachment IV-6-F and have the participants complete the problems. When they have finished, have them share and discuss their answers.

Trainer Notes

Remind the participants that they can use either the sizing formula or the sizing rule of thumb in this activity. During the discussion of the answers, note any difference between using the sizing formula and the rule of thumb.

Step 8. (15 minutes)
Conclude the session by reviewing the various methods used to size solar water heaters.

Trainer Notes

Explain that the participants will have an opportunity to apply the information from this session during the design and construction of solar water heaters.

Encourage any questions, comments or discussion.

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.

Experimenting

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?

Find:

Average hot water usage per person per day
Number of people using hot water per day
Desired temperature of hot water
Incoming cold water temperature
Density of water (weight per volume)

For example:

40 liters hot water per person per day
3 people per day
45 degrees C desired hot water temperature
15 degrees C incoming water temperature
Density of water is 1 Kg/liter

(energy required)

2. How much energy is available?

Find:

Clear day winter insolation for the desired tilt
Percent possible sunshine
Collector system efficiency

For example:

2700 Kgcal/m² day
65% possible sunshine
40% system efficiency

(energy available)

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.

For example:

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.

Daily hot water volume requirement (liters)

80

200

240

300

400

Desired outlet temperature (0° C)

50

65

40

45

70

Inlet temperature (0° C)

15

20

20

15

20

Insolation available (Kgcal)/(m² day)

2300

3000

2700

2500

2250

Will a direct gain solar water heater be effective? (Yes / No)






Number and size of tanks






Aperture area (m²)






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.

Hot water volume requirement (liters/day)

100

120

40

200

20

Desired hot outlet temperature (0° C)

40

50

55

60

35

Inlet water temperature (0° C)

15

20

15

20

15

Insolation rate (Kgcal)/(M² day)

2700

2500

2000

3000

2250

Percent possible sunshine (%)

75%

70%

65%

60%

50%

Size of tank(s)






Area of flat plate collector (m²)