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Photovoltaic Systems: A Practical Guide

Grid-tie Systems

grid-connected-solarGrid-tie systems are connected to your utility company via your electrical service panel. In this way the utility company becomes your storage system. When you are a net producer of electricity the excess is fed into the utility grid; when you are a net user, you draw the amount needed from the grid. Net metering refers to the way the utility company credits and charges you for your electricity. In the State of California, all utility companies, save the Los Angeles Department of Water and Power (go figure), are required to credit you at the full retail price you are charged for electricity.

You will need to sign a Service Agreement. This is basically a contract between you and your utility company, spelling out your mutual obligations.

Basically, you agree to install the system with approved equipment and according to the National Electrical Code and local building and safety codes. You are required to maintain an insurance policy. Finally, you are required to provide a visible, lockable disconnect switch in close proximity to the electrical meter.

The utility in turn agrees to install a reversible meter and to credit you for electricity you supply to the utility company. Some companies allow you to carry a credit forward indefinitely (LADWP), while others zero out a credit balance every twelve months (Edison).

All grid-tie systems that use inverters and charge controllers that pass the UL 1741 test standards automatically shut down if the grid itself shuts down or goes out of 'spec'. Once shut down, the equipment will test the grid, and if conditions are back in spec for an uninterrupted five minutes then the equipment begins to operate again. The ability of inverters to shut down when the grid goes down is called "anti-islanding".

Back-up Systems

Since a UL 1741 inverter will shut down if the grid goes down, your PV system will not be able to operate and you will lose power in your home or business. This is true even if the sun is shining and the PV modules are producing electricity. In order to provide backup in case of power outages you will need another source of electricity besides the PV modules.

The most common choice of backup (primary) is batteries. Batteries also require a charge controller to keep them fully (but not over) charged. There must also be a mechanism to disconnect them from the rest of the system should their charge level (voltage) drop below a certain value. The reason for this disconnection requirement is that batteries can be irreversibly damaged if allowed to be completely discharged. Batteries are expensive, require small amount of power to maintain their charge and last about 5 years even under meticulous maintenance. Therefore, a battery backup system is kept as small as required.

The strategy for sizing a battery back-up system is two-fold. First, decide what critical systems need uninterruptible power and isolated them on a separate sub-panel. This can be as simple as moving existing circuits and their breakers to a new panel. Second, decide how long an outage you want to protect from. This length of time and the power consumed by the critical systems will determine the number of batteries needed.

In Southern California, our power outages are usually measured in minutes and a two-hour outage is very rare. In other areas of the country, tornadoes, hurricanes and ice storms can produce outages measured in days. Of course there are also utility-based outages, such as the one that occurred on Thursday August 14, 2003 on the East Coast.

In order to provide protection for longer periods of time than the batteries can provide, you can purchase a generator fired by propane, gasoline or diesel. Although generators are fairly inexpensive to purchase, they are very costly to run and maintain. The operating life to an overhaul is measured in hundreds of hours. Nonetheless, they can be integrated into a system and they will take over the backup function when the batteries become discharged.

You can purchase backup systems that are completely integrated with you PV system and we can help you design one that meets your specific needs.

Finally, integrated PV/backup systems look very much like "off-grid" systems, and share a large number of components in common.

Off-grid Systems

Off-grid Systems are More Complicated:
The lack of the connection to the grid means that we can no longer draw from it and store energy back into it. Batteries and fossil fuel generators are needed to supply electricity when the PV system cannot. Since generators are noisy, polluting and costly to operate and maintain, batteries are usually the primary electrical storage sub-system. There are some ingenious storage mechanisms for storing excess PV-derived electricity such as heating water for domestic use and small hydroelectric systems.

The Bare Bones Off-grid System:
The basic off-grid system has a PV array, charge controller and battery sub-system. All electrical loads are based on the battery voltage, which can be 12, 24 or 48 volts. There are DC refrigerator/freezers, TVs, radios, laptop computers and lighting. A generator is needed to provide back-up during overcast, rainy days.

Adding an Inverter for AC Loads:
If AC loads are required, then an inverter is added to the system. As in the grid-tie systems, the inverters can produce 120/240 single phase or even 208V three phase. The big differences are the fact that the inverters draw their input current at the battery voltage (12-48 V), and there are no anti-islanding electronics. The generator is still needed, and a properly designed system has a by-pass capability whereby the PV/battery system is disconnected and the generator alone powers the AC loads. This feature is particularly useful when the PV/battery system fails or needs to be taken down for maintenance

Off-grid Design Scenario:
The standard scenario off grid system design is as follows:

Determine the average energy consumption per 24 hour period. This involves a careful inventory of electrical equipment, their power consumption and hourly usage per day. Obviously this is much more complicated than reading one's utility bill and looking for the "average energy usage per day (kWh/d)".

  1. Size a PV system that will match that energy usage based on the number of "peak sun hours per day" for your locale, taking into account several system inefficiency factors. More on determining this number later.
  2. Size the battery bank to provide several times the average 24 hour usage number. This is the product of the number of batteries, their voltage, and their capacity in Ah at a specified discharge rate.
  3. Choose a backup generator to provide power when the batteries become discharged. This can happen during the Winter, or occasionally during a prolonged period of no sunshine.

Other Considerations:

Battery systems need monitoring, automatic control of charging and discharging, and periodic maintenance. Both over-charging and over-discharging can lead to rapid degradation or outright failure of battery sub-systems. Modern charge controllers with battery temperature sensors and low-voltage sensing and disconnect systems can prevent most of the over-charging/discharging problems, and sealed batteries eliminate the need for periodic topping off battery cells due to electrolysis and evaporation.

Generators need auto-start capability, in order to cut in and provide power when needed. Like any internal combustion engine, they need periodic checks of oil levels, fuel systems, ignition systems and starters. They need to be overhauled every thousand hours or so of operation. Diesels are more trouble free, but have their own quirks, like difficulty starting in sub-zero temperatures (perhaps just when they are needed most). Generators should be fired up periodically, to insure that they are ready to go when needed.

Tracking Systems

Tracking systems rotate the PV modules to maintain an optimal orientation with respect to the Sun path through the sky. Increases in energy production of over 30% can be achieved compared to a stationary PV module. There are one-axis and two-axis systems.

The one-axis system maintains a constant elevation or angle from the horizon, and rotates in azimuth (the familiar compass angle). The elevation is usually set at the local latitude plus 15 degrees. A single axis tracker can raise "noon-equivalent" hours in Southern California from 5.5 hours to 7.3 hours (33% increase).

The two axis tracker maintains an optimum azimuth and elevation throughout the Sun's path through the sky, and can raise the Уnoon-equivalent hours to 7.6. As you can see, from a total energy production point of view, the second tracking axis produces a smaller increase in performance and is less frequently employed. The one advantage of two-axis trackers is the ability to maintain a high degree performance nearly from dawn to dusk. In some applications, for example off-grid applications using only PV modules, two-axis trackers may offer a critically-needed functionality.

Trackers are expensive, costing roughly $2 per watt. Since they employ motors, gears and bearings, they need maintenance and occasional repair. Don't look for a 25 year warranty on this type of equipment.

There is one tracking system, offered by Zomeworks, that uses non-motorized technology. By employing a closed liquid-vapor system, the heat from sunlight can differentially shift the liquid and vapor components causing torque that causes the system to track the Sun.

Sizing a PV System

Estimate Usage:
The first step is to determine how much energy you consume averaged over a year, and what fraction of that energy consumption you wish to offset with a PV system. Express this number as an average daily energy production target for your PV system.

Estimate Insolation:
The second step is to understand how much sunshine you get where you live. The usual figure of merit is 'noon-equivalent' hours per day. In Southern California we get about 5.5 hours per day. Near the coast or other places with morning fog, the number might be closer to 5.3 and in inland regions the number might be 5.7. The best source for this information is Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors.

Design Your System:
The third step is fairly involved. The following is an abbreviated version, but it will give you a rough idea. It assumes that the PV modules will be placed on a south or nearly south-facing structure, with a tilt from the horizontal roughly equal to the local latitude (34° for much of Southern California).

A PV system should be designed to offset or completely eliminate the need for utility supplied power. A rough estimate of the size of a PV system needed is given by the following equation for P, the required system power measured in Watts-STC:

Where:

  • E is the energy used per day in Wh (watt-hours), averaged over one year
  • H is the number of 'noon equivalent' hours of sunshine per day, roughly 5.5 h for much of Southern California.
  • STC/PTC is the ratio of STC power to PTC power, typically 1.1
  • η is the efficiency of the inverter used, typically 0.94
  • L represents the rest of the system losses. In a well-designed system this will be 0.92 to 0.98 (8% to 2% loss). Let's use 0.96 for the purposes of this calculation.
P=E * 0.235 (Watts-STC)

STC power is also called the 'maximum power' by module manufacturers. It is measured under tightly-controlled conditions, and the PTC power (about 10% less) is a better indication of actual performance in the field.

For example, if the average energy consumption is 26 kWh/day the PV system requirement is 6.0 kW-STC.

High quality commercial PV modules will produce about 10 W per square foot, installed. In other words, your PV system will require 100 sq ft. of south-facing, pitched roof space for every 1,000 W-STC. This calculation is approximate, but it is accurate to within +/- 10%.

Various Factors Impacting Energy Production of a Practical PV System:

To have a realistic system configuration, you need break down the system power P into a permissible combination of modules per string and the number of strings, for your inverter. This in turn, depends on the operating temperature ranges experienced by the modules. Two major inverter manufacturers have string sizing programs that allow an installer determine the number of modules per string and the number of strings.Go to SMA America web-site for their string sizing program, or contact Xantrex for a copy of theirs.

As mentioned earlier, there are several factors which reduce the actual AC power delivered to your service panel. These are module orientation, shading, soiling and wiring losses.

Orientation: The table below shows how a combination of compass and tilt angles affect the annual energy production in California.

ROOF PITCH

FACING

Flat  (0°)

4:12 (18°) 7:12 (30°) 12:12 (45°) 21:12 (60°) Vert.
(90°)
South 0.89 0.97 1.00 0.97 0.89 0.89
SSE, SSW 0.89 0.97 0.99 0.96 0.88 0.59
SE, SW 0.89 0.95 0.96 0.93 0.85 0.60
ESE, WSW 0.89 0.92 0.91 0.87 0.79 0.57
E, W 0.89 0.88 0.84 0.78 0.70 0.52

Shading: Shading is highly variable and the magnitude of the consequent impairment is generally misunderstood. The current output of a PV cell depends on the intensity of the sunlight (watts per square meter). The degradation due to shading depends on the number of cells in a string, the number of strings (in parallel) per module, and the use of by-pass diodes. Bypass diodes are wired in shunt around a group of cells. If one or more cells within that group are shaded and produce less current, the bypass diode shunts currents in excess of that lessened current, and allows the module to operate at a higher current level. Bypass diodes reduce the voltage output, which produces it own set of problems in multiple strings of modules wired in parallel into one inverter. If this seems complicated, it is. An accurate estimation of power reduction due to shading is best left to a professional installer.

The Solar Pathfinder is a passive imaging device that allows an installer to asses shading and resulting energy reduction due to objects such as trees, chimneys, and other buildings. The Los Angeles Department of Water and Power requires a Solar Pathfinder (or equivalent) tracing as part of their rebate application package.

Soiling: Soiling refers to the accumulation of dust, dirt, leaves, and bird droppings on your PV modules. Dirt and dust alone can degrade the output of a PV panels by as much as 6% over the period of a year. Leaves can cause a sudden drop in PV output, and pine trees which shed their needles all year long are particularly bothersome. It is a good idea to check your PV system operation once a week for sudden drops in power output or to check the modules themselves. In the absence of leaves or other nuisance, wash the modules once a year in early Summer with a sweeper nozzle or other hard spray.

Wiring Losses: All wire, regardless of its composition (copper, aluminum) or gauge (size or cross-sectional area), exhibits ohmic loss. Ohmic loss causes a voltage drop in the wiring proportional to its length and the current being carried. For example #12 AWG 7-strand copper wire exhibits a resistance of 2.05 ohms per 1,000 ft. at 75°C (167°F). A single PV modules operating at 34V and 4.4A and connected via 100 ft. of #12 wire will suffer a 0.9 V voltage drop, and a 3% drop in power. Be careful when making these types of calculations, most PV arrays are connected in series, as a string, and then wired into an inverter. An inverter placed 100 ft. from the PV modules will have approximately 200 ft. of wiring (round trip). So a series-connected string of the same 10 PV modules connected to an inverter 100 ft. away will suffer a same voltage drop of 1.8V. In turn this causes only a 0.6% drop in power. This is because the string of series-connected modules are now operating at 340V, not 34V as in the single module example.

Ohmic losses argue for higher voltage, lower current systems. Modern grid tied inverters operate as high as 550V DC input, so strings of 10-12 modules are fairly standard configurations. As long as #12 or larger wire is employed in PV designs, ohmic losses should only be a problem when wiring lengths exceeding 150 ft.

The following chart is a fairly accurate estimate of AC output power and AC energy produced per day for an average system configuration.

Module
PTC
rating
N, of
Modules
Inverter
Efficiency
Orienta-
tion
Shading Soling Wiring
Losses
AC
Output
134.9 20 94% 97% 99% 97% 99% 2,339
W each Percent Percent Percent Percent Percent Watts
               
Insolation Energy
Product'n
           
5.5 12.9            
hours/day kWh/day            
               
Electricty
Cost
Savings Savings Savings        
$0.160 $2.06 $62.64 $751.72        
$/kWh $/day $/month $/year        

The California Energy Commission website lists efficiency ratings for Eligible PV Modules and for Eligible Inverters

Utility Bill Savings, ROI, Payback

The utility bills savings depends on the rate structure and the energy production per day. The simplest calculation involved a constant rate, both over a 24 hour period and over a yearly period. This simplified situation is shown as part of the above calculator.

Time-of-Use is a rate structure that charges most for electricity in the weekday afternoons, less in the late morning and early evening, and least in the remainder of the evening and early and mid morning. All weekend, the lowest rate applies. For the Los Angeles Department of Water and Power, these periods and their rates are as follows:

Independent of a contemplated PV system, ToU encourages shifting electricity usage into the evenings and early morning. Designing a PV system to work in conjunction with a ToU rate structure requires a detailed analysis of electricity consumption and generation by time-of-day. Generally speaking, PV power and electricity consumption can peak in the Noon to 3 PM period, and under these circumstances, PV systems and a ToU rate structure can provide a very attractive combination.

Further discussion of Time of Use and PV systems is beyond the scope of this introduction. However, a competent PV integrator/installer should be able to discuss strategies for employing ToU together with a PV system.

ROI and Payback:

After installation and receipt of your utility rebate, a PV system generates a positive cash flow in two ways. The first is energy bill reduction, discussed in some detail above. The second is a combination of tax credits and depreciation.

There is a one-time Federal Tax Credit of 30% (capped at $2,000 for residences) and if the PV system is powering a business or home office, a one-time Federal Tax Credit and an Accelerated 5-year Depreciation schedule can be applied as well.

These tax related issues are very important as they can accelerate the break-even point, when your PV system will have paid for itself. The spreadsheet below demonstrates a typical financial analysis scenario for a business:

 Current annual avoided electricity cost $ 1,099
 Estimated annual increase in electricity cost 7%
 Total Project Cost $ 21,249
 Utility Rebate $ 9,500
 Total Cost After Rebate $ 11,749
 Percentage of system allocated to business 100.0%
 Federal tax credit, first year 30.0%
 Depreciable basis $ 9,282
 Depreciable basis each year for 5 years $ 1,856
 Tax rate 28%

 INVESTMENT ANALYSIS Year 1 Year 2 Year 3 Year 4 Year 5
 Annual Electricity Cost $1,099 $1,176 $1,258 $1,346 $1,441
 Federal Tax Credit $3,525        
 Depreciation $520 $520 $520 $520 $520
 Cash Flow $5,143 $1,696 $1,178 $1,866 $1,960
 Cumulative Cash Flow $5,143 $6,839 $8,617 $10,483 $12,444

 INVESTMENT ANALYSIS Year 6 Year 7 Year 8 Year 9 Year 10
 Annual Electricity Cost $1,541 $1,649 $1,765 $1,888 $2,020
 Federal Tax Credit          
 Depreciation          
 Cash Flow $1,541 $1,649 $1,765 $1,888 $2,020
 Cumulative Cash Flow $13,985 $15,634 $17,399 $19,287 $21,308

The point in time where the "Cumulative Cash Flow" equals the "Total Cost After Rebate"x is your "break even" point. With many commercial systems the payback can occur in less than eight years and with many residential systems the payback can occur in less than ten years.

With Time of Use or Tiered rates, these types of calculations become more complex. Again, a competent PV integrator/installer should be able to perform an ROI and break even analysis for you under these more complicated conditions.

Your Past Utility Bills


Gather up a year of utility bills, LADWP, Edison, or PG&E, and write down the usage (kWh) for each billing cycle. Total it for one year. Divide the result by 365.25. This is your average daily electricity usage. The number typically lies between 10 and 50 kWh for residential use. If you don’t have your records for the past year handy, you can go online or call Customer Service. It is important to get an average from one entire year as usage varies greatly with each Season.

LADWP customers:
Click here to access your LADWP account from their web site. There you can register, log onto your account, and view images of your bills. Carefully write down your total usage (kWh) for each 2-month period in the past year, add them up, and divide by 365.25 to get your daily average.

You may also call LADWP customer service at 818 342-5397 and ask them what your Average Daily electricity consumption (in kilowatt-hours) was for each billing period of the past year.

Edison customers:
Click here to access your SCE account from their web site. Click on "Register" (below the Log In box in the upper Left corner) and provide the information required to establish an on-line account. Find your Average Daily Usage in kWh for the past year on your bill. If you don’t see your Average Daily Usage, carefully write down your total usage (kWh) for each month of the past year, add them up, and divide by 365.25 to get your daily average.

You may also call SCE customer service at 800 655-4555 and ask them what your Average Daily electricity consumption (in kilowatt-hours) was for each month of the past year.

Your Electrical Usage


Don't know your Average Daily Electrical Usage? The best estimate of future electrical usage is past history. For commercial accounts, this is the only way. For residential accounts, this way is the most accurate, but I have also included below a table which should give you a fair estimate and is useful for new construction or a recently purchased home.


DC Rating 
(kW-STC)
Production 
(kWh/day)
Family 
Size
Home  Details
12 .0 51 6+  4,000 sq. ft.  + large pool + spa +  heavy A/C
9.0 38 5  3,500 sq. ft + small pool + light A/C
6.0 26 4  3,000 sq. ft. + medium A/C
4.0 17 3-4  2,000 sq. ft.
3.0 13 2-3  1,200 to 1,500 sq. ft.

As we have said, your Average Daily Usage is the starting point for sizing a PV system. A properly engineered PV system will produce approximately 4.25 kWh/day of electricity for every 1 kW-STC of installed PV. Therefore divide your daily usage by 4.25 to get the size of a PV system which will provide 100% of your anticipated electricity usage and therefore eliminate your electricity bill!

In the table above, typical family descriptions are listed with usage and PV system size.

Roof Space for your PV system

How much roof space is needed? Typical pitched roofs can produce about 10W/sq ft of roof space. Flat roofs can produce a little less, about 8W/sq ft. For example, a 6.0kW system requires 600 sq ft of pitched roof space. And a 4.0kW system requires 500 sq ft of flat roof space.

 
 
 
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