Let's look at the way we develop a renewable energy (RE) system design for a specific site.

We start by knowing our goals, which will guide the entire process. Having clear achievable goals will keep projects on track and moving forward. Working without clear goals will often sidetrack or derail good work. Through questioning we provide a sound basis for moving into an appropriate use of technology and a more sustainable way of life.

• Why are you using renewable energy?
• What do you need the system to accomplish?
• What is your budget?
• What is your timeline?

These are just a few of the questions that we need to confront when we start the design process.

The basic RE system designs fall into three categories:

1. Stand-alone battery systems
2. Grid inter-tie battery back-up systems
3. Grid inter-tie battery-free systems

Stand-alone battery systems

Stand-alone battery based systems are used primarily where the utility power grid is not available. Other applications include: sites where costs associated with utility usage are prohibitive; sites where owners do not want any association with their local utility; small applications where its use eliminates utility connection; and applications that are portable (as in highway construction signs).

Grid inter-tie systems

Grid inter-tie battery back-up systems currently comprise the majority of residential systems. These systems provide the user with both clean renewable energy and the ability to power critical loads (refrigeration, lighting, water, heat source controls, communications, etc.) during utility power outages. These systems provide a sustainable and reliable source of electricity and also allow the user to have stable energy costs over time. Because the system cost is up front, the overall cost of electricity that the system provides will be consistent over the life of the system.

Grid inter-tie battery-free systems represent the area of greatest growth in the PV industry. Because of the simplicity of both design and operation, these systems are beginning to get more attention by people interested in using clean electricity.

Many potential users are daunted by both the complexity of battery-based systems and the amount of space these systems require. With the advent of affordable compact battery-free systems, users now have a choice. These systems do not provide any back-up power for utility outages nor do they have any batteries to maintain or replace.

The components that make up a system include:

1. The energy source: photovoltaic modules (PVs), wind turbines, hydro turbines etc.
2. Mounting structures: PV racks, PV tracking racks, wind turbine towers, hydro structures.
3. Balance of system components (BOS-electrical equipment for conditioning and controlling the energy): collection centers, charge controllers, over-current devices and disconnects, meters, inverters, DC and AC load centers
4. Energy storage: batteries, hydrogen electrolizers and fuel cells.
5. Efficient loads

Because each of the different system types has different design considerations, we will discuss each design separately.

Stand-alone battery-based systems present some of the greatest challenges to the system designer. Providing a reliable solar electric system requires that enough generation and storage exist to provide for the specified loads during the darkest periods of the year. The designer must take the utmost care to make sure that all of the owner's desires regarding system performance are understood before proceeding. Therefore, system design begins with a thorough "load profile."

Once an accurate load profile is generated and site parameters are known, the design process moves into specification of equipment. Here close consideration toward the system uses and owner/operator interests can greatly affect the design. How the system will be used and what the owner's expectations are helps determine how much redundancy should be included and what back-up measures will need to be included.

In addition, system expansion should be considered in the start-up design so that upgrading a system does not make certain components redundant or obsolete.

By creating a load profile and average daily watt-hour consumption figure, the designer can take into consideration how much sun, hydro or wind is available at the site to determine the number and type of sources that will be required to generate the desired loads.

Often the desire for completely clean generation sources is confronted by a budget too small to afford all the PVs, wind turbines or hydro turbines required for minimal back-up generator usage. In this case, people will often opt for buying fewer RE sources than are required and put up with increased back-up generator use until they can afford more sources.

Back-up generators are the bane of and a blessing to stand-alone systems. We generally are not building renewable energy systems in order to run fossil fuel generators. These devices are noisy, polluting, self-destructing and a lot of fun to start when it is 20 below zero and you really need them. However, if planned for appropriately and run conservatively these devices will extend battery life and allow users to start up a system on a small budget. However, back-up generators can help a system get going and we can wean ourselves from them by replacing them with more PV modules or wind and hydro turbines.

Choosing charge controls involves knowing how much current the RE sources will produce. This will be determined when the type and number of RE sources are chosen. The amperage output and the type of source determines the required size of the charge controller. For most applications a charge controller that fits the system will be readily available. If the RE sources will grow over time, a charge controller that will accommodate the largest system amperage should be chosen in order to avoid replacing it later.

Storage

Choosing storage begins with our daily watt-hour consumption figure and then estimating how many days' worth of storage you will need. A general rule of thumb suggests that for stand-alone systems, four days of storage (daily watt-hour consumption x 4) provides for a healthy reserve during periods of cloudy weather. Many other factors can be considered including nearby lake cloud effects, unusual consumption patterns, weekend vs. full time use, and operator/maintenance compatibility.

Inverters

Choosing inverters requires that you know what the largest AC appliance is and how long it will run. This data is in the detailed load profile.

To run a one horsepower well pump, you will need an inverter that will provide enough capacity to start and run it. Starting an appliance under a load (the weight of the water above the pump) requires a large "surge" capacity. Be sure to consider the inverter's surge capacity when specifying it for a job. Another consideration is which appliances may be running at the same time. If the well pump runs while a washing machine is running, the inverter will need the capacity to do both.

System expansion needs consideration also. If the load profile will expand into larger appliances, eventually it will pay to choose an inverter larger than initially needed or to choose an inverter that can be expanded (stacked).

Loads (i.e., the appliances), need to be as efficient as possible in order to keep system costs down. Conservation through use of super efficient appliances will allow the user to run a system at the lowest cost with the greatest reliability. Efficient appliances are more readily available and a new group of appliances are available that are designed specifically for the renewable energy market.

Load expansion = capacity expansion

Once a system is working, users often expand their load profile. It must be made very clear during the design process that expanding the load profile requires expansion of the capacity of the generation and storage system. Adding PV modules and battery capacity will accommodate greater load demand and can be planned for the same way that the system was originally planned. Failure to expand capacity when adding load to the system will prematurely age the battery and can lead to system failure. This is a very critical consideration when building stand-alone RE systems. As we will see in the next Solar Cowboy planning for grid-tied systems is a little more flexible and a lot more forgiving. Remember, with grid-tied systems we always have the utility when we lack (or can't afford) enough RE resources!

Since the last installment some folks have asked how they can get going with small systems on a budget.

An inexpensive stand-alone system would consist of:

1-50 watt PV	$350
1-5 amp charge control	$28
1-Deep cycle 12V DC battery	$75
1-DC disconnect (2 pole)	$27
Total:	$480

This would run small DC loads that consumed about 200 watt-hours a day in our region of northern Wisconsin.