Positioning Your Solar Devices

DIY 3D Solar Panels

Do It Yourself Solar Energy

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It is important to note that the position of the sun in the sky changes from hour to hour, day to day, and year by year. While this might be interesting, it is not very helpful to us as prospective solar energy users, as it presents us with a bit of a dilemma— where exactly do we point our solar device?

The ancients attributed the movement of the ball of fire in the sky to all sorts of phenomena, and various gods and deities. However, we now know that the movement of the sun through the sky is as a result of the orbital motion of the earth, not as a result of flaming chariots being driven through the sky on a daily basis!

In this chapter, we are going to get to grips with a couple of concepts—that the position of the sun changes relative to the time of the day, and also, that that position is further influenced by the time of the year.

How the position of the sun changes over the day

The ancients were aware of the fact that the sun's position changed depending on the time of the day. It has been speculated that ancient monuments such as Stonehenge were built to align with the position of the sun at certain times of the year.

The position of the sun is a reliable way to help us tell the time. The Egyptians knew this, the three Cleopatra's needles sited in London, Paris, and New York were originally from the Egyptian city of "Heliopolis" written in Greek as H^fou no^tQ. The name of the city effectively meant "town of the sun" and was the place of sun-worship.

It sounds like the destination for a pilgrimage for solar junkies worldwide!

We can be fairly sure that the obelisks that they erected, such as London's Cleopatra's needle (Figure 3-1), were used as some sort of device that indicated a time of day based on the position of the sun.

If you dig a stick into the ground, you will see that as the sun moves through the sky, so the shadow will change (Figure 3-2). In the morning the shadow will be long and thin; however, toward the middle of the day, the position of the shadow not only changes, but the shadow shortens. Then at the end of the day, the shadow again becomes long.

Of course, this effect is caused by the earth spinning on its axis, which causes the position of the sun in the sky to change relative to our position on the ground.

We will use this phenomena to great effect later in our "sun-powered clock."

How the position of the sun changes over the year

The next concept is a little harder to understand. The earth is slightly tilted on its axis; as the earth rotates about the sun on its 365^-day cycle, different parts of the earth will be exposed to the sun for a longer or shorter period. This is why our days are short in the winter and long in the summer.

Figure 3-1 Cleopatra's needle—an early solar clock?

Figure 3-1 Cleopatra's needle—an early solar clock?

Cleopatra Needle Shadow Down The Well
^ Figure 3-2 How shadows change with the time of day.

The season in the northern hemisphere will be exactly the opposite to that in the southern hemisphere at any one time.

We can see in Figure 3-3 that because of this tilt, at certain times of year, depending on your latitude you will receive more or less sunlight per day. Also if you look at your latitude relative to the sun, you can see that as the earth rotates your angle to the sun will be different at any given time of day, depending on the season.

We can see in Figure 3-4 an example house in the southern hemisphere—here we can see that the sun shines from the north rather than the south . . . obviously if your house is in the northern hemisphere, the sun will be in the south!

This graphically demonstrates how the sun's path in the sky changes relative to your plot at different times of year, as well as illustrating how our rules for solar positioning are radically different depending on what hemisphere we are in.

What does this mean for us in practice? Essentially, it means that we need to change the position of our solar devices if we are to harness the most solar energy all year round.

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Diagram Earth Illustrating Seasons

Figure 3-3 How the earth's position affects the seasons.

Figure 3-3 How the earth's position affects the seasons.

Solar Position With The Seasons

Project 1: Build a Solar-Powered Clock!

You will need

Tools

0 CM

• Photocopy of Figure 3-5

• Matchstick

This is a dead-easy and quick sundial for you to build. Take a photocopy of Figure 3-5. If you want

Solar Clock Sundial

CM Figure 3-5 Template for our "solar-powered clock.

Online resources

Sundials are absolutely fascinating, and a cheap way to investigate the properties of the sun. The dial presented here is just one type of sundial; however, there is a lot more information out there, a lot more to explore—here are some links, some of them with printable plans that allow you to make different types of sundial that you might like to investigate!

www.nmm.ac.uk/server/show/ conWebDoc.353

www.liverpoolmuseums.org.uk/nof/sun/#

plus.maths.org/issue11/features/sundials/

www.hps.cam.ac.uk/starry/sundials.html

www.sundials.co.uk/projects.htm

www.digitalsundial.com/product.html This page is well worth checking out . . . very ingenious this is a "digital sundial," yes you read correctly—a digital sundial.

to be really flashy you can stick it to a piece of cardboard in order to make it more rigid and durable.

You need to cut out the dial that relates to the hemisphere that you are in—north or south. Then, you need to think about your latitude in degrees north or south. You will need to fold the sidepieces at the same angle marked in degrees as your latitude.

Stick a matchstick through the point at which all of the lines cross. What you should be left with is a piece of cardboard which makes an angle to the horizontal.

Now take your sundial outside and point the matchstick in the direction of due north (or south). You should be able to read the time off of the dial—compare this to the time on an accurate watch—remember you might have to add or take away an hour!

Rules for solar positioning

It is an artist's rule that you look more than you paint—for solar positioning this is also true. You need to look carefully and make observations in order to understand your site. Look at how objects on your plots cast shadows. See where your house overshadows and where it doesn't at various times of the year—remember seasonal variation—the position of the sun changes with the seasons and won't stay the same all year round (Figure 3-6).

Also, just because an area is shaded in one season, doesn't necessarily mean that it is shaded in all seasons. In fact, this can often be used to

Dibujos Tmbrl

Figure 3-6 How seasonal variation affects the optimal position of solar collectors.

Figure 3-6 How seasonal variation affects the optimal position of solar collectors.

your advantage. For example, in summer, you don't want too much solar gain in your house as it might overheat; however, in winter that extra solar energy might be advantageous!

Think carefully about trees—if they are deciduous, they will be covered with a heavy veil of leaves in the summer; however, they will be bare in the winter. Trees can be used a bit like your own automatic sunshade—in summer their covering of leaves blocks the sun; however, in the winter when they are bare they block less sun.

Make a record of your observations—drawings are great to refer back to. Keep a notebook where you can write any interesting information about what areas are and aren't in shadow. Note anything interesting, and the time of day and date.

Make sure that you are on the lookout on the longest and shortest days of the year—the first day of summer and the first day of winter. This is because they represent the extremes of what your solar observations will be; therefore, they are particularly useful to you!

Think about when in the day you will be using your solar device. Is it a photovoltaic cell that you would like to be using for charging batteries all day. Or, is it a solar cooker that you will be using in the afternoon? Think about when you want to use it, and what sunlight is available in what areas of your plot.

Work out which direction is north—try and find "true north" not just magnetic north. A compass will veer toward magnetic north so you need to find a way of compensating for this. Having a knowledge of where north and south is can be essential when positioning solar devices. Note which walls face which cardinal directions (compass points). If you are in the northern hemisphere, site elements where coolness is required to the north, and elements where heat is required to the south.

Think about the qualities of morning sun and evening sun. Position elements that require cool morning sun to the east—and those elements which require the hot afternoon sun to the west.

Project 2: Build Your Own Heliodon

You will need

For the cardboard heliodon

• Three rigid sheets of corrugated cardboard, 2 ft x 2 ft (60 cm x 60 cm)

• Split leg paper fastener

For the wooden heliodon

• Three sheets of 1/2 in. (12 mm) MDF or plywood 2 ft x 2 ft (60 cm x 60 cm)

• Countersunk screws to suit hinge

• Lazy Susan swivel bearing

For both heliodons, you will need

• Clip on spotlamp

• Large blob of plasticine/modeling clay

Tools

For the cardboard heliodon

• Protractor

For the wooden heliodon

• Protractor

We have already seen in this chapter about the sun's path—and we have learnt how we can use the sun to provide natural lighting and heating.

We saw in Figure 3-3 how the position of the sun and the earth influences the seasons, and how the path of the sun in the sky changes with the seasons. This is important to us if we want to design optimal solar configurations, as in order to maximize solar gain, we need to know where the sun is shining!

A heliodon is a device that allows us to look at the interaction of the light coming from the sun, and any point on the earth's surface. It allows us to easily model the angle at which the light from the sun will hit a building, and hence see the angle cast by shadows, and gauge the paths of light into the building.

The heliodon is a very useful tool to give us a quick reckoning as to the direction of light coming into the room, and what surfaces in that room will be illuminated at that time and date with that orientation.

A heliodon is also very useful for looking at overshadowing—seeing if objects will be "in the way" of the sun.

With our heliodon, it is possible to construct scale models that allow us to see, for example, if a certain tree will overshadow our solar panels. The heliodon is therefore a very useful tool for solar design, without having to perform calculations.

In this project, we present two separate designs. The first is for a cardboard heliodon, which is simple if you just wish to experiment a little with how the heliodon works. The design requires few materials and only a pair of scissors—but, it may wear out over time. This does not mean that there is any reason for it to be less rigid than its sturdier wooden equivalent. The second design is for a more rigid permanent fixture which can be used professionally, for example if you are a professional who will routinely be performing architectural design or using the heliodon for education.

Our heliodon will consist of three pieces of board. The first forms a base; on top of this base, we affix a second board which is allowed to swivel by way of, in the wooden version, a "Lazy Susan" bearing. This is a ball-bearing race that you can buy from a hardware shop, which is ordinarily used as a table for a "Lazy Susan" rotating tray.

In the cardboard version, we simply use a split leg pin pushed through the center of both sheets, with the legs splayed and taped down.

The third board is hinged so that the angle it makes with the horizontal can be controlled, it is also equipped with a stay to allow it to be set at the angle permanently and rigidly. And that is just about it! With the wooden version, a length of piano hinge accomplishes this job admirably, and with the cardboard version, a simple hinge can be made using some strong tape.

The other part of the heliodon is an adjustable light source. This can be made in a number of ways. The simplest of which is a small spotlamp equipped with a clip that allows it to be clamped to a vertical object such as the edge of a door. Slide projectors are very good at providing a parallel light source—these present another option if their height can easily be adjusted. If you will be using the heliodon a lot, it would make sense to get a length of wood mounted vertically to a base, with the dimensions given in Table 3-1 marked permanently on the wood.

Heliodon experiments

Once you have constructed your heliodon you can begin to perform some experiments using it.

Table 3-1

Lamp heights for different months of the year ti 0 -ü 0 ■h h 0) H

Table 3-1

Lamp heights for different months of the year

January 21

8 in.

20 cm

from floor

February 21

22 in.

55 cm

from floor

March 21

40 in.

100 cm

from floor

April 21

58 in.

145 cm

from floor

May 21

72 in.

195 cm

from floor

June 21

80 in.

200 cm

from floor

July 21

72 in.

195 cm

from floor

August 21

58 in.

145 cm

from floor

September 21

40 in.

100 cm

from floor

October 21

22 in.

55 cm

from floor

November 21

8 in.

20 cm

from floor

December 21

2 in.

5 cm

from floor

These measurements are assuming a measurement of 87 in. between the center of the heliodon table and the light source

These measurements are assuming a measurement of 87 in. between the center of the heliodon table and the light source

You need to be aware of the three main adjustments that can be made on the heliodon.

• Seasonal adjustment—by moving the lamp up and down using the measurements listed above, it is possible to simulate the time of year.

• Latitude adjustment—by setting the angle that the uppermost flat sheet makes with the base, you can adjust the heliodon for the latitude of your site.

• Time of day adjustment—by rotating the assembly, you can simulate the earth's rotation on its axis, and simulate different times of day.

The two table adjustments are illustrated in Figure 3-7.

In order to secure the table at an angle, probably the easiest way is to use a length of dowel rod with a couple of big lumps of modeling clay at each end. Set the angle of the table to the horizontal, then use the dowel as a prop with the plasticine to secure and prevent movement.

There are a couple of simple experiments that we can do with our heliodon to get you started. Remember the sundial that you made earlier in the book? Well, set the angle of latitude on your table to the angle that you constructed your sundial for (Figure 3-8). You will see that as you rotate the table, the time on the sundial changes. You can use

Figure 3-7 Heliodon table adjustments.

Compass points

Remember to think carefully about where north and south are in relation to your modeling table. Consider whether the site you are modeling is in the north or south hemisphere and adjust the position of your model accordingly.

this approach to calibrate your heliodon. You might like to make some marks on the cardboard surface to indicate different times of day.

The next stage of experimentation with the heliodon is to look at modeling a real building.

Online resources

Read more about heliodons on the web

www.pge.com/003_save_energy/

003c_edu_train/pec/toolbox/arch/heliodon/ heliodon.shtml arch.ced.berkeley.edu/resources/bldgsci/ bsl/heliodon.html en.wikipedia.org/wiki/Heliodon

Construct a model from cardboard (Figure 3-9), and include for example, window openings, doors, patio doors, and skylights. By turning the table through a revolution, it is possible to see where the sun is penetrating the building, and what parts of the room it is shining on. This is useful, as it allows us to position elements of thermal mass in the positions where they will receive the most solar radiation.

We can also make models of say, a solar array, and cluster of trees, and see how the trees might overshadow the solar array at certain times during

Basic Electrical Insulation

the year. Use the heliodon with scale models to devise your own solar experiments!

Now with modern computer aided design (CAD) technology, the heliodon can be replicated digitally inside a computer. Architects routinely use pieces of CAD software to look at how light will penetrate their buildings, or whether obstructions will overshadow their solar collectors. However, heliodons are still a very quick, simple technology which can be used to make a quick appraisal of solar factors on a model building. A professional, more durable heliodon can be seen in Figure 3-10.

Project 3: Experimenting with Light Rays and Power

You will need

Small torch Length of string Tape

Big sheet of paper Bunch of pencils Elastic band

Attach the large sheet of paper to the wall using the tape. Then, take the piece of string, and attach one end roughly to the center of the paper with the tape. Now hold the string to one side of the piece of paper, and attach the torch to the string so that the bulb of the torch falls within the boundary of the paper.

We are going to see how angle affects the light power falling on a surface when the distance from the surface remains the same.

Now imagine our torch as the sun, hold the torch to face the paper directly keeping the string taught. You should see a "spot" of light on the paper.

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Ht h

Hg h rt

Draw a ring around the area of highest light intensity. Now, hold the torch at an angle to the paper, and again with the string taught, draw a ring around the area of high intensity. Repeat this at both sides of center a few times at different angles.

Figure 3-11 shows us what your sheet of paper might look like.

What can we learn from this? Well, the power of our torch remained the same, the bulb and batteries were the same throughout the experiment, the amount of light coming out of the torch did not change.

However, the area on which the light fell did change. When the torch was held perpendicular to the paper, there was a circle in the middle of the page. However, hold the torch at an angle to the page and the circle turns into an oval—with the result that the area increases. What does this mean to us as budding solar energy scientists? Well, the sun gives out a fixed amount of light; however, as it moves through the sky, the plane of our solar collectors changes in relation to the position of the sun. When the sun is directly overhead of a flat plate, the plate receives maximum energy; however, as we tilt the plate away from facing the sun directly, the solar energy reaching the plate decreases.

You might have noticed that as you angled the torch and the beam spread out more, the beam also became dimmer.

Remember the bunch of pencils? Well grab them and put an elastic band around them. Imagine each pencil is a ray of light from the sun. Point them down and make a mark with the leads on a piece of paper. Now, carefully tilt all the pencils in relation to the paper and make another mark with all the pencils at the same time (Figure 3-12). As you can see, the marks are more spread out. Remembering that we are equating our pencil marks with "solar rays," we can see that when a given beam of light hits a flat surface, if the beam hits at an oblique angle, the "rays" are more spread out. This means that the power of the beam is being spread out over a larger area.

It is important that we understand how to make the most of the solar resource in order to make our solar devices as efficient as possible.

Figure 3-11 Light ray patterns drawn on paper.
Figure 3-12 Bunch of pencils experiment.

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