Introduction to Solar Energy

In this modern world of Global Warming and Green Mania, we're bombarded with acronyms and terms that we may not be familiar with. A good citizen wants to know about alternatives to foreign oil and the technologies that might free us from it, and we want to be able to talk about alternative energy and think about it with some confidence that we understand it. PV, CSP, STP - what are they? What does grid tie mean? Net metering? Well, fear not, we're going to cover those and more.

First things first, though. You may already know that "solar energy" is the term we use to describe the conversion of the light the Sun shines on the Earth into energy that we describe as useful, as in useful for human purposes. We might use it to heat our homes, run water pumping stations, or heat our water. Solar energy is generally perceived to be non-polluting energy, in that its direct use creates no emissions or waste specific to solar energy. Some people use a process called "full cost accounting" that includes the emissions of the industrial chain that culminates in the product at hand, and in those cases solar emissions are not zero, but in most implementations they do not increase over time. They don't increase after the solar energy system is installed.

Two common terms you'll hear bandied about are "passive solar energy" and "active solar energy". Passive solar energy applications are those that operate usefully without mechanical or electric devices to cause them to operate. Initially, it can describe houses that are designed to maximize the solar input by their design, for heating purposes, by the familiar black-painted solar water heater, or a thermosiphon water heater. All of these devices convert solar input into useful energy (usually heat) without the input of external energy.

A house built in the Northern Hemisphere with passive solar energy in mind might have large, south-facing windows that would allow sunlight to fall on a dark colored stone floor. South windows because the sun is in the southern sky for most of the Northern Hemisphere - and noticeably so as one gets into latitudes that need heat the most. A thick stone floor would be chosen for the thermal mass it provides - "thermal mass" in this case being the dense stone that will store a lot of heat for some time, re-radiating that heat after the sun has gone down. The stone should be a dark color because dark colors absorb and radiate heat more efficiently than light ones. This house might have a passive water heating system, which usually consists of a water reservoir exposed to solar heat through a black coating. It can take many shapes, but a black tank is the simplest form, which can then be directed to the conventional water heater, which will have to do less work since the water coming into it is much warmer.

"Active Solar Energy" describes systems that have active components. Systems that use heat engines to drive generators are active systems. Active systems require monitoring and state change to maintain their function in an optimal state. For instance, an active solar thermal installation might use a large black panel insulated from the outside air by glass, with a fan to circulate air through this collector. The black panel will heat up, and as the air flows over it, the air will be warmed. This is a type of heat exchanger - any device that moves heat from one media or substance to another is called a 'heat exchanger'. They are very important to active solar energy systems. Another active solar thermal system might use panels with black pipes inside, again insulated from the outside air via glass or plastic. The sun heats the pipes, while a pump pumps some fluid through those pipes to collect the solar heat. The fluid can then be pumped through radiators ( a liquid to air heat exchanger) or through another sealed reservoir that's used to store, say, heated water.

Another, more complex system uses CSP - concentrated solar power. CSP is a system that uses mirrors or lenses (usually mirrors) to concentrate solar energy on a small area, raising the temperature in that area to extremely high levels. This is then used to drive heat engines via steam or direct heat, which in turn spin generators that produce electricity. This type of system requires heliotropes, which are devices that track the sun, keeping the sun's light concentrated in the right spot even though it is moving across the sky. Some utility-level CSP generators ( that means an installation large enough that an electric company might build or use it) store solar heat in thermal mass and use it to drive generators even at night. The thermal mass can be molten salt or other types, but this is an approach to providing power from CSP even when the sun has set or is obscured by clouds.

The most common form of active solar that we encounter in residential areas is called "PV panels". PV stands for photovoltaic. These are large panels that produce electricity (volts) directly from light (photo). There are no mechanical moving parts, but the sunlight falls on them, and electricity comes out. They're fairly expensive, but they're very low maintenance and very durable. They just sit there, on top of your roof, or your shed, and generate electricity whenever the sun shines. Their efficiency can be improved using a heliotrope to keep them perpendicular to the sun's rays, but most installations adopt a compromise position that allows them to generate the most electricity with the simplest installation.

Often in residential areas we find these systems set up in with a 'grid tie system'. This means that their output is tied to the 'grid', the existing electrical infrastructure - the wires that come from the electric company. A device turns the DC generated by the PV panels into AC that matches the phase and voltage of the grid, and this allows your PV system to provide electricity to other people (if it produces more than you need while the sun is in the sky). In many areas, the law has enforced "Net Metering", which basically lets you sell your extra output back to the utility. In most instances, this helps offset the energy you use when the sun isn't in the sky.

This should introduce you to most of the basic concepts of solar energy systems. If you read and understand it, you should be able to follow most of the discussions you're likely to encounter in the news. There is a lot more to know about solar energy, and I highly recommend that find out more, because solar energy is going to become more and more important in our lives as the price of oil and other fossil fuels continues to increase.