| Sunlight, Electricity and Heat | | | | collector of all, so the surface of your heat transfer |
| The sun is basically just a ginormous nuclear reactor | | | | devices will be dark. |
| sitting out in space. The sun's nuclear reaction forces | | | | - Transfer the collected heat energy into a medium |
| turn the atoms in its mass into photons, light energy | | | | that is easily used - the simplest heat transfer medium |
| particles that radiate out. These photons travel across | | | | is liquid. Running water through a black metal pipe will |
| space and strike the Earth as light and heat. | | | | take the heat absorbed by the surface and transfer it |
| The sun's energy comes to us in a wide spectrum of | | | | directly to the water. Putting on a tea kettle transfers |
| wavelengths, from ultraviolet to infrared. The amount | | | | energy from the burner through the pan and into the |
| of sunlight that hits the Earth's surface is about 35,000 | | | | water. |
| times more than the total amount of energy used by | | | | - Prevent heat loss through good insulation - you can |
| all humans. This solar radiation, and its interaction with | | | | generate heat all day, but what good will it do you if |
| various materials, is what enables us to generate | | | | the cold air just transfers the heat right back out of |
| power. | | | | your medium? Air inside a closed house retains heat, |
| Turning Sunlight Into Power | | | | but open the windows and you'll need a sweater. |
| There are four basic ways that sunlight interacts with | | | | - Store enough heat - the medium needs to have an |
| matter: | | | | intrinsically high heat capacity. That means that the |
| - Absorption - the energy in a photon is absorbed by | | | | physical structure of the materials you use have to be |
| the material and changed into a different form | | | | able to retain the energy that is transferred into them. |
| - Transmission - the energy goes through a material | | | | Different materials have different physical properties |
| without changing form or losing power | | | | and energy retention capabilities. Here's a quick table |
| - Scattering - the energy splits up into different | | | | of common materials and their heat storage capacities: |
| wavelengths with different properties, and changes | | | | Material / BTUs per cubic foot |
| direction. Rainbows are the most well-know example | | | | Air / .02 Plastic / .6 Fabric / 2 Bricks / 25 Wood / 27 |
| of scattering | | | | Steel / 59 Water / 62 Copper / 78 |
| - Reflection - the energy changes direction without | | | | The two best materials for storing and moving heat |
| changing properties. Mirrors are reflectors. Different | | | | are water and copper. Water is cheap, but it doesn't |
| materials have different ratios of properties. Glass | | | | absorb heat from light because it's highly transparent. It |
| panes coated with iron silicon, for example, allow light | | | | can, however, retain and store heat transferred to it |
| to pass (transmission) and keep heat in (absorption). | | | | from a different collector material. The material most |
| The glass used in fashionable sunglasses, on the other | | | | commonly used in solar heat collectors is copper |
| hand, allows light in (transmission) but blocks (reflection) | | | | painted black. |
| certain wavelengths like ultraviolet (UV). | | | | Photovoltaic Cells Turn Sunlight Into Electricity |
| A photon's energy is changed to heat when it strikes a | | | | Photovoltaic (PV) cells are two layers (positive and |
| surface that absorbs light. Thermal energy (heat) is | | | | negative) of purified silicon making a semiconductor |
| nothing more than molecules moving very rapidly. The | | | | sandwich. The most common example of a |
| faster they move, the hotter the material gets. | | | | semiconductor is the transistor, which is found in all |
| Boiling water, for example, is just liquid whose electrons | | | | modern electronic components. |
| have been so excited by the transfer of energy from | | | | When you hook a closed electric circuit to your PV |
| the burner that they move like crazy. Hot stuff burns | | | | cell, by, say, connecting a lightbulb, you create the |
| you because the excess energy gets transferred into | | | | conditions for electricity to flow. |
| your skin, which is not equipped to absorb it. | | | | When the photons from the sun strike the electron-rich |
| In order to use the heat energy in sunlight, you have to | | | | N (negative) silicon layer surface, they cause some of |
| change it into a usable form, and you have to | | | | those electrons to break free from their atoms, leaving |
| physically move it from where it is collected. There are | | | | holes behind. If the electrons are close enough to the P |
| three ways with which heat can be moved: | | | | (positive) layer, they can jump across the barrier and |
| - Conduction - the heat energy itself moves from one | | | | try to fill holes in the atoms in that layer. |
| molecule to another, while the molecules stay in place. | | | | The resulting electrical imbalance encourages the |
| Heat moves from a hotter surface to a cooler | | | | electrons to flow through the circuit (lighting up your |
| surface. | | | | bulb along the way) and back to the N layer. This |
| - Convection - the heat energy moves from a fixed | | | | electrical current is a direct result of the action of the |
| material to a liquid material in contact with the fixed | | | | solar radiation (photons) interacting with the physical |
| material's surface. Convection also happens as the | | | | properties of your PV cell. |
| heat energy is moved along with the molecules | | | | A PV array is just a whole bunch of cells hooked up |
| travelling within the fluid itself. The heat energy makes | | | | together. There's all kinds of stuff in a solar array |
| the hotter liquid molecules rise, while gravity pulls the | | | | system to keep you safe from getting zapped, |
| colder molecules down. | | | | convert your current from DC to AC (which most |
| - Radiation - All fire emits infrared radiation, and the sun | | | | homes and appliances use), and store what you don't |
| is the biggest fireball of all. The main idea in a solar | | | | use right away for later. |
| energy system is to collect as much solar radiation as | | | | In its simplest, most fundamental form, every solar |
| possible, and transform it into a usable form. You want | | | | energy system is basically just about efficiently |
| your collection system to be able to: | | | | harnessing the action of photon-excited electrons as |
| - Convert solar radiation into heat as efficiently as | | | | they jump from one material to the next. |
| possible - a black surface is the most efficient heat | | | | Isn't science fun? |