Advanced Design Steps System Design Steps
System Design Steps
System sizing and design can be broken down into a number of logical steps. The Solar Design book uses 11 steps which are:
- Energy Audit
- Determine Loads
- Account for Losses
- Size Battery
- Size Water Pump and Storage Tank
- Site Survey
- Choose Modules
- Choose Equipment
- Size Conductors and Overcurrent Devices
- Documentation
- Costing Calculations
1. Energy Audit
The energy audit is very important. Depending on the solar system, utility cost, and cost of upgrades, for every dollar spent in improving energy efficiency, about four dollars can be saved in the cost of the solar system. For a battery system the ratio is even greater. You can hire a professional or do the energy audit yourself. Do the energy audit before talking to a professional installer.
2. Determine Loads
If your house has been connected to the grid for a while, you can use the electricity bill to determine the energy usage. The Solar Design book goes into detail on how to do this and the SolarSizer software will do the calculations for you. If you have not saved your bills you can usually get them online or from the utility company records. If the house is new or the loads unknown, they have to be listed one by one and the energy consumption added up. The SolarSizer software does this for you.
3. Account for Losses
All systems have losses. There is power loss in the wiring. The modules get dirty and produce less over their life. There are losses in the equipment such as inverters and charge controllers. The SolarSizer software allows you to account for these losses. The Solar Design book explains what the typical losses are and how to account for them.
4. Size Battery
If the system does not have batteries, you don't need to do this step. If the system does have batteries, you will need to determine what size and how many to buy. The size of the battery bank is determined by how long you need to go without charging and the loads. If the system includes a motor driven generator, you don't need as large of a battery bank. The generator does not have to be run very often because the solar system is usually supplying the power. The combination can reduce the overall cost life-cycle cost of the system. The same is true if there is a wind or water turbine. The SolarSizer software is used to size the battery bank and plot the battery recovery curve for different months of the year.
5. Size Water Pump and Storage Tank
If the system does not include a water system, you don't need to do this step. The water tank is determined in much the same way as a battery bank, by the number of days it needs to supply water without being replenished and the water use. There is usually an added reserve for fire fighting or emergencies. The size of the pump is determined by how many days are allowed to replenish the water. The SolarSizer software calculates the size of the water tank and pump. The sizing process and an example are in the Solar Design book and more information about solar water systems, including pumps, is in the Solar Basics book.
6. Site Survey
If the array is going to be mounted on a roof, it needs to be measured, including the slope, size, and orientation. If multiple roof surfaces are going to be used, they each have to be measured. Usually one surface will produce more energy from each module than other roof surfaces. The Solar Design book explains how to handle different roof surfaces and has an example of a grid-connected residence with 3 roof surfaces, each pointing in different directions and each with different shadowing. If the system will be ground mounted, it usually should be pointed towards the equator, south in the northern hemisphere, and tilted approximately equal to the local latitude. The tilt and orientation can be optimized using the SolarSizer software. Tilting an array to the local latitude is normally not the optimum. Structural considerations are covered in the Solar Basics book. If the system is going to mounted on a tracker, the particulars are entered into the SolarSizer program. Usually a tracker will add about 25% more energy production, depending on the local climate, but adds cost and complexity to the system.
7. Choose Modules
Modules are chosen based on cost, availability, application, warranty, and esthetics. They have to supply the energy required by the system. The voltage and current requirements of the inverter, charge controller, or other down-stream equipment will determine the wiring configuration of the modules. Basic information about modules is covered in the Solar Basics book and more advanced information is in the Solar Design book.
8. Choose Equipment
Depending on the system requirements, you will need to size inverters, charge controllers, and other peripheral equipment such as DC to DC converters. Inverters for grid-connected systems are chosen based on the array output. Inverters for battery systems are chosen based on the AC loads. Charge controllers are chosen based on the array output. Charge controllers are adjusted to provide the proper voltage and current for the type and number of batteries used. Wiring of the array is determined based on the voltage and current requirements of the equipment. Charge controllers normally need a higher array voltage than the battery bank.
9. Size Conductors and Overcurrent Devices
Wiring and over current devices (breakers or fuses) are determined first by safety based on the National Electric Code (NEC). The second consideration is voltage drop. If there is too much voltage drop in the system, the loads could be damaged or not work as designed. The third consideration is power loss which leads to inefficiency of the system. It is less expensive to increase conductor size to decrease power loss than to increase the number of modules.
An excellent guide on how to interpret the NEC for solar systems is provided by John Wiles of the Southwest Technology Development Institute. The guide is called "Photovoltaic Power Systems And The 2005 National Electrical Code: Suggested Practices." It can be obtained at http://www.nmsu.edu/~tdi.
The WireSizer program, supplied with the Solar Design book, will calculate the correct conductor size and breaker size according to the 2005 NEC rules. It is meant as a guide only and the results should be checked using the NEC. The WireSizer program also calculates the voltage and power losses.
10. Documentation
The system design should be documented using drawings, engineering caculations, and bill of materials. The complexity of the documentation is determined by the complexity of the system and the requirements of the building department. Accurate documentation will make the job go easier and reduce cost. The bill of materials should be as complete as possible, especially if the job site is far away from a ready source of materials.
11. Costing Calculations
The last task is to determine the cost. This can be simple or complex depending on the requirements. A simple system can get by with the initial cost determined by the bill of materials. A large project will need life-cycle cost, cost of electricity, and payback period. Utility scale projects will require exacting detail. Information about costing is in the Solar Basics book and software to help you calculate cost is in the Solar Design book.