Shades and screens

Solar shade cloth works very nicely, and you can remove it in the summer.

Solar shade cloth is similar to conventional screens, but it’s thicker and has

a specially designed mesh to inhibit infrared and ultraviolet light, neither of

which do you any good in your house other than heating (since you can’t

see either wavelength). Solar shade cloth also has more integrity, and lasts

longer, so the added expense isn’t as bad as it may seem. Newer fabrics allow

light to pass through without too much blur. On hot summer days, a lot less

light is desirable — even if the room temperature is the same, the room feels

cooler with less sunlight. Solar screens work better than sunlight-inhibiting

blinds because they’re on the outside, and the heat isn’t dissipated on the

inside as it is with blinds.

 A good do-it-yourself project is to upgrade your existing screens from the

cheap bug screens that are so common, to the solar shade cloth type materi-

als. You don’t need to change all the windows, just the ones where you want

to inhibit sunlight. Big picture windows facing south are the best candidates.

You can get screen at most hardware stores, but you can get the better per-

forming stuff online or in specialized solar catalogs. It’s a very good cost/

benefit tradeoff, perhaps one of the best.

Automatic retractable shades are available from specialty suppliers. In my

area, the big expensive houses generally come with electrically retractable

shades mounted outside on roller tracks. Many of these houses have two

floors of continuous windows facing west. On hot summer days, the sunlight

can be murder, and without a shade the house would be uninhabitable. Plus

in the winter, they can act as insulators.A bit of light interaction

Radiation reacts with matter in a number of ways, all of which can play a role

in your solar-power system:

 ✓ Absorption: When a photon is absorbed, it’s energy is changed into a

different form, either heat or electrical energy. Every solar system is

primarily concerned with absorption, for until that happens there is no

capture of energy.

 ✓ Transmission: When light simply goes right through a medium, it’s

called transmission. Windows are good transmitters, and the atmo-

sphere is a rather good transmitter when the air’s clear and smog free.

 ✓ Scattering: Scattering occurs when photons interact with molecules, the

result being that a “secondary” form of radiation emits in many direc-

tions at once. Scattering is responsible for such diverse phenomenon as

rainbows and the dusk/dawn reddening of the sky.

 In the atmosphere, light scatters off individual molecules, and scattering

occurs much more readily with blue light than red. Hence, when you look

up into the sky, you see blue because blue light is bouncing around all over

the place. On the other hand, when you see light that has traveled through

a lot of atmosphere (in the evenings and mornings) it often appears red

because that wavelength passes through the atmosphere the most easily.

 ✓ Reflection: Mirrors are reflectors. In general, shiny, hard surfaces are

much better reflectors than porous, dark surfaces. Reflection differs

from scattering in that the direction of a reflection is a mirror image of

the incident (the original) photon.

Some mediums pass radiation, but also insulate heat very well. This idea is

useful with solar collectors because you want sunlight to enter your system,

but after it’s converted into heat, you don’t want that to get back out. Glass

impregnated with iron silicon transmits wide spectrums of light and also

insulates heat very well.

When a medium doesn’t transmit or reflect all wavelengths equally, it’s called

a filter. People can make windows that pass visible light very well but reflect

ultraviolet. Sunglasses are a perfect example of filters. By selectively filter-

ing certain wavelengths, for example, ultra-violet, visibility can be enhanced

while cutting out heat. Filter performance also comes into play when design-

ing glazing for solar water heating collectors (glazing is the material that

covers the collector and allows sunlight to enter the system; see Chapter 12).

In this case, it is desirable to allow the maximum amount of sunlight to enter

the system, and so the filter properties are centered where the most sunlight

is, namely the visible and near infra-red regions of the spectrum.Incident sunlight

The amount of sunlight incident on a module directly determines how much

power output the module will be capable of (go to Chapter 5 to get into more

details about how to best locate a module).

Cell temperature

Cell temperature affects the power output of a PV module more than you

might expect. In general, heat causes inefficiencies in electronics. The output

of a solar PV module goes down with increasing temperature, which is kind

of ironic if you think about it since the goal is to maximize the amount of sun-

light on a panel. But with more sunlight comes more heat, and as a module

heats, it becomes less efficient. In practical terms, you can get more output

from your solar PV system on a partly sunny, cold day than you may get on a

very sunny, very hot day. A good breeze also helps to cool a module, so the

best conditions are cool, breezy, sunny days.

The fact that solar production decreases with temperature causes many

new solar customers concern. They intuitively expect their outputs to track

the intensity of the sun, but because on really hot, sunny days they end up

seeing their system outputting less, so they call their contractor and claim

something’s wrong. A good contractor will explain this effect to you up-front,

so you’re not surprised.

Shading

Shading is the bane of all solar PV systems, some more than others. No

surprise there, but the magnitude of the effect is more than you’d probably

expect: A panel with only 5 percent shading may experience 50 percent

reduction in power output. Therefore, positioning panels so that the shade

is reduced as much as possible is extremely important. But in a typical

application, some shading is inevitable. That’s why the goal is to optimize

module placement by taking into consideration all the conditions that may

be expected over the course of a day and a year. Given a choice, shade in the

morning is better than shade in the afternoonOpening and closing windows and doors

In the same way that you can save by programming your HVAC system to

operate at varying levels at different times of day or night, you can also

achieve cost savings by strategically opening and closing windows and doors

over the course of a day.

When you want breezes through your house, open windows and doors only

on the windward and leeward sides of the house, and try to balance the open-

ings. Don’t just open every window you can. Experiment a little and you’ll see

the different results.

 In addition, follow these tips:

 ✓ For best effect, use blinds and window coverings in conjunction with

opening and closing windows. (See “Window Covers: Blinds, Awnings,

and Shades,” earlier in this chapter.)

 ✓ For cooling purposes, open windows that aren’t in direct sunlight.

Windows on the north are the best candidates.

 ✓ If you have multiple stories, opening windows on different floors can

increase the flow of breezes. Experiment to see what works best.

 ✓ In the winter, open all blinds that are exposed to the sun. Let the sun

shine into the room instead of striking the blind. Close the blinds when

the outside is cold and lightless in order to insulate the window as much

as possible.

 ✓ Use blinds and drapes wisely. Open them when you want sunlight and

heat to come in. Close them when your air conditioner is running. And

most people don’t do this, but close them on cold nights to add an extra

layer of insulation to the windows.