# How to Build a Model Solar House for a Kid's Project

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Solar power is part of a 21st-century wave of "clean" or "green" energy sources, that is, those that do not emit the products of carbon combustion into the environment in significant amounts or at all. These energy sources are also called renewables, although this can be confusing because nuclear power, while "clean," is not technically derived from a renewable energy source.

Solar power, wind power and an increased emphasis on geothermal power and hydroelectric power are all part of a mostly concerted worldwide effort to reduce the emission of greenhouse gases such as carbon dioxide (CO2) and thus reduce the expected impact of climate change on global civilization before the end of the century.

A great idea for a kids' science project is demonstrating the practical application of renewable energy using a real or model solar house, or by showing a class one of the working components of a solar collection grid.

## What Is "Power" in Physics?

The reason for the panels on a solar house is to generate power, usually for both electricity and hot water. This power somehow comes from the sun. But how, exactly?

Power in physics is energy per unit time, or equivalently, work per unit time. Energy appears in physics in many forms, including thermal, gravitational-potential, kinetic, electrical and sonic; the standard unit is the joule (J), often expressed equivalently as the newton-meter (N⋅m). Other units are calories, ergs and British thermal units (Btu).

When energy is used to do work, such as powering an electrical generator or activating the heating coils of a hot-water producer and reservoir, the rate at which that work is done is called power. The standard unit is the watt (W) or J/s. 745.7 W = 1 horsepower (hp).

• When you pay your utility bill, you may have noticed that the units of consumption are measured in kilowatt-hours (kW⋅hr). This unconventional unit looks like power, but since it has units of power multiplied by time, it is in fact energy.

When you run all-out for as long as you can, which is about a minute, you are capable (depending on your mass) of generating about 800 W of power, or close to 1 hp – good enough to keep a mid-sized microwave oven going. But for protracted forms of human exercise, such as walking or cycling, an output of 250 to 400 is more typical.

## Overview of Solar Power

Solar power is the most abundant and pollution-free source of energy available, and in fact is the ultimate source of energy of all biological processes on Earth. Its primary uses are for generating electricity, generating heat or both. Solar power has uses not only in the home but in an increasing number of commercial and industrial settings as well.

There are three basic ways to capture solar power: solar heating and cooling (SHC) applications, concentrating solar power (CSP) applications and photovoltaic (PV) cells. The former two are used mainly to generate heat in industrial or larger settings, whereas PV cells are the main elements in the characteristic arrays you see on the roof of a solar house, or sometimes in fields alongside sites that make use of solar power.

A solar panel exposed to direct sunlight can generate up to 1,000 w/m2. The total amount of power generated is a function of the number of PV cells and the time of exposure as well as the angle of incidence of the sun's rays, which on average are more direct at latitudes closer to Earth's equator.

## The Photovoltaic (PV) Cell

Photovoltaic cells are readily identifiable as parts of solar panels, but they are also found in miniature in solar-powered calculators and other portable devices. They make use of the photoelectric effect, which is the ability of photons ("packets" of light) to knock electrons free of the atoms to which they belong. The subsequent flow of these charged and energized electrons can be used to generate electricity for immediate use or storage.

The element silicon lends itself to PV cells because it can be made to act like an insulator, which is a poor conductor of electricity, or it can be made to act like a conductor, depending on the needs of the engineers working with it. This makes silicon a semiconductor and thus a critical component of contemporary PV cells.

Importantly, the electrical current (flow of electrons) created by solar power and PV cells is direct current (DC). This is in opposition to the alternating current (AC) running into most modern homes. Thus a device called an inverter is available to solar house owners or teachers seeking to show students how solar homes work.

## The Passive Solar House

It makes no sense to build a solar home if you don't set the whole house up to maximize energy efficiency, as solar power can be hard to come by all the time even in sunny climates. Clearly, geography is important. Students can show how the angle of solar panels do not face straight up in the U.S. but rather up and to the south; why would this be, and how would this change for a solar home in, say, southern Argentina?

Solar homes are designed to passively store heat thanks to having high thermal mass, for example, and lots of stone masonry. In addition to properly aligned windows, darker colors should be chosen to make the most of thermal mass present.

You may show different styles, such as direct gain, in which the collected energy and power flow from south, such as from south-facing windows, to the rest of the house) and indirect gain through the use of features such as Trombe walls, in which spaces between the walls are used to store energy in the short term.

## Solar Power Demonstrations for Kids

All of these concepts can be shown to kids of most ages, and the older ones can use the general information to embark on their own projects. Building a PV cell is probably too ambitious for most; a better idea would be to have students try to find, with the help of the Internet and perhaps their parents, how many PV cells are in use in devices in the home or at least in frequently seen places.

It is important for students to recognize the challenges inherent in storing large amounts of electrical power, in comparison to, say, data, or the material used in nuclear power plants. If batteries could hold nearly infinite amounts of electricity, how might the landscape of the world change? Would solar houses need to be in especially sunny places?

## Solar and Renewable Energy in the 21st Century

As of 2019, solar power accounted for only 1 percent or so of U.S. energy. On the other hand, in 2017, solar power amounted for one-third of all "new" energy consumption, making it an industry on the rise.

Meanwhile, wind power accounted for about 6 percent of U.S. energy nationwide, and was expected to be overtaken in this regard soon by hydroelectric power. Finally, biomass is another newer player in the renewables game; the burning of material from dead animals and plants can power turbines, but is not considered clean.

These sources of energy are expected to become increasingly popular as the many-pronged bane of climate change becomes increasingly impossible for people and governments around the world to deny.

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