Solar Training Education and Learning Text

COVID-19 Assistance

Everyone is feeling the effects of the corona virus, and most people are concerned for their family and friends and anyone affected by COVID-19. It has adversely affected the health of many and the economic position of almost everyone. We understand that many find themselves in a position of reduced income, a surplus of time, and a restricted social agenda.

Solairgen would like to offer some assistance by reducing the costs of our online training courses until this health crisis is contained. If you have time and you would like to expand your career, or get some continuing education credits, we hope our price reductions will make it easier to achieve.

For the months April and May, we are reducing the prices listed on the website for online classes by 40%. Your discount will be shown and taken at checkout.

Stay safe.

Mr. Kelly Provence
Certified Master PV Trainer
Solairgen School of Solar Technology
706-867-0678

www.solairgen.com

 

Dunkelflaute

At first glance I thought this term was describing a dark German beer that had lost its carbonation. I was wrong but it is actually almost as bad. Dunkelflaute translates to “dark doldrums” and refers to a period when there is no sunlight or little wind for renewable energy generation. This is particularly serious in areas where solar and wind energy provide a substantial portion of electrical energy generation and is even worse at latitudes above 40˚. Germany faces this problem on a regular basis but are very creative in the ways they deal with it. The first solution may appear to be batteries or other means of storing electrical energy which is a partial solution; they also store energy by other means and incorporate smart energy efficiency.

There are solutions that don’t include conventional energy sources such as nuclear and fossil fuels, and thermal energy (steam driven generators) has been the workhorse of the electrical energy industry for the past 100 years. Electrical energy from renewables can be converted to thermal and stored for a short period of time. Electrical energy can be converted to any number of energy stores such as water reservoirs for hydroelectric and compressed air for air turbine generators. It is also possible to convert renewable electrical energy to hydrogen through electrolysis. This process is fairly easy but storing it as a condensed fuel is energy intensive. Smart energy efficiency is another way to reduce dependence on electrical energy when it is in short supply.

Smart energy efficiency sounds good and it is very doable in this age of smart computers but in order for this to work, we have to give up at least some privacy. Every appliance that uses energy will need to be electronically visible and controllable by the smart energy network (SEN). In the U.S. we are seeing this occur with the new listing for interactive inverters. They will be smart in their own accord with electrical energy fluctuation and controllable by the utilities for further control features to make the electrical grid more reliable. California and Hawaii have already implemented these rules and the rest of the U.S. will follow within a few years. Home monitoring of electrical circuits is becoming an affordable norm. It is reasonable to assume that these devices will eventually be required to communicate with a smart energy network. Everything that uses energy will communicate with the network. This may sound futuristic but it’s not and I don’t think it will turn into some dark dystopian landscape of existence where machines control humanity.

A more conservative but less desirable option is to use conventional energy sources such as nuclear or fossil fuels for base level energy demand. Nuclear would get the vote if CO2 emission reduction is the primary concern, but I think what we are going to see is a blend of renewable energy, energy storage, smart energy management and clean conventional energy production.

There is no way to prevent Dunkelflaute but we can progress to a cleaner and stable electrical grid – and the best way to prevent a good beer from losing its carbonation is to drink it faster.

On Grid Solar Farm

Working with Electric Utilities

Balancing the Needs of PV Customers with Utilities

Solar contractors and solar customers often find themselves on the adversarial side of utilities when interconnecting PV systems to the electrical grid. Most of the time this is due to a lack of seeing the other’s side and/or understanding of the other’s position.

Let’s first look at the utility’s position regarding PV systems. Many utilities offer incentives or encourage their residential customers to have PV systems installed on their properties. The reason for this is almost always for the purpose of appeasing the wishes of their customers. This an effort of goodwill because they know that electrical customers like the idea of energy coming from a clean renewable source and most of them also like the idea of owning their own energy. The benefit to the utility is not financial unless the PV system energy reduces load demand during periods when the cost to generate that power from other sources is high.

With residential customers, those periods are mostly in the afternoon and evening and then again in the morning as the sun starts to rise. This being the case, the PV systems with smart energy storage are the only systems that could financially benefit the utility as well as the owner of the system. They store the energy produced during the middle of the day and then feed it into the home and grid during the peak load periods.

With commercial customers the situation is different. The peak load period may be during the normal business hors of the day or 24 hours a day. These customers with PV systems may be shaving loads throughout the day or they may need to shift loads to a time of day other than the solar hours of the day. For the utility to benefit financially, a load demand study must be made for the specific customer. These customers with PV systems will also need energy storage for load shifting, peak load demand reduction and to provide consistent load support during climatic events that vary frequently during the day. The truth is that PV systems without smart energy storage do not save the utility money.

The exception to this would be with smart inverter technology that is controlled by the utility. California and Hawaii are both working through this process and it will become a more universal option in the very near future. However, these smart inverters and smart energy storage systems don’t address another issue for the utility. They understand how to regulate and shift energy supply with the energy sources they now have, the difficult task is how do they do this with customer-based power systems; the customer has far too much independence for their comfort. Customers would need to become liable to the utility for failure problems or let the utility have operational control of the system performance and operation.

Now let’s look at the customer’s position. The customer doesn’t have a choice when shopping for a utility; moving to another location is the only option if you don’t like the utility in your area. This is often a source of discontent for the customer. Customers depend heavily on electrical power for almost every appliance in the home and business. Natural gas, propane and fuel oil for heat are the exceptions.
Electricity seems expensive because we use so much of it; in fact, it is pretty cheap. In order to generate all your own electrical energy on-site, the cost will be excessive. In order to meet the cost per kWh from the utility, the PV system cost needs to be amortized over a 25-year period. Most residential customers do not appreciate how difficult it is to provide constant electrical power and how reasonable the price actually is. Commercial customers are more aware of this yet they would still like to be more independent for the utility if it is possible.

An important factor to both residential and commercial PV system customers is the return on investment. The customer wants the highest price for the kWh generated from their PV energy and they feel justified because they are generating clean energy and contributing to the reduction of conventional fuel sources with known hazards to our environment. This actually makes sense when looking long range. If the utility can absorb some of the cost of integrating PV energy into the grid, the industry will continue to grow and reduce initial coasts and therefore operating cost. Eventually the cost of PV systems with smart energy storage will be cheaper than any other energy source. It is rational to expect a public utility or a customer owned utility to invest in solar for the future by offering premium rates for the solar kWh. The problem with this position is that the rate to the customer will need to decrease over time with greater solar penetration, otherwise the investment from the utility will not have a positive return and the cost of electricity will have to go up. This is a difficult point to make to customers; someone needs to educate the electrical customer on how this will work to their advantage. The salesman for the solar contractor is unlikely do it and the customer will be much less likely to trust that information coming from the utility itself.

Effective open communication between the solar customer, the solar contractor and the electric utility is essential. Avoid adversarial positions and look for positions that benefits each entity.

 

Kelly Provence
Solairgen School of Solar Technology

 

Reliable Resources for New Solar Contractors

Online information is abundant these days and misinformation tends to dominate many or most search parameter queries. This seems normal and the information appears to be free but it really isn’t. Usually it’s either written to attract people to buy a product or to attract people to someone’s misinformation-laden ego. Either way, it is difficult to filter the good information from the bad. The purpose of this blog is to provide some basic sources of reliable information for the new solar contractor.

Below I’ve listed the resources needed and information to be a well-informed, competent solar contractor.

  1. Training for the solar contractor. Knowing the design and installation trade and standards is most important and it can be achieved through good accredited training providers and certified trainers. If the training provide is not IREC accredited with IREC certified trainers, don’t waste your time with them, https://irecusa.org/. The only exception to this is manufacturers of solar equipment. They will provide good training, but only on their own equipment.
  2. Suppliers of solar equipment. There are many suppliers of solar products and they all differ in the range of products and depth of support they provide to their customers. Small to medium scale PV contractors will need suppliers who offer a wide selection of products from several manufacturers. Large scale contractors may go straight to the manufacturers. Follow the link provided below and select your country. They list wholesalers and distributors of solar products. https://www.enfsolar.com/directory/seller

  3. Leading solar equipment. One way to determine which solar equipment to purchase is to look at a leading resource such as EnergySage. They have a solar panel database with consumer ratings, https://www.energysage.com/solar-panels/. They also have an inverter database with consumer ratings, https://www.energysage.com/solar-inverters/ and a database on batteries as well, https://www.energysage.com/solar-batteries/. If you would like a more comprehensive list of solar equipment, go to the California Energy Center’s (CEC) data base, https://www.gosolarcalifornia.org/equipment/index.php. You can also review the products that the leading suppliers carry; this may be best since this will be your primary resource.
  4. Solar energy resource data. There is only one source for historic solar resource data and that is the National Solar Radiation Database (NSRDB). It has been developed from data collected by NREL, NASA and NOAA over the past 50 years. There is no other resource that can provide this data. The best tool to calculate solar irradiation for a tilt and azimuth of a specific site is PVWatts developed by NREL https://pvwatts.nrel.gov/. Several companies and organizations use this data in their shading analysis tools. If you use one of these tools, check to verify that their irradiation data is from the National Solar Radiation Database.
  5. Financing for the customer. The list of lenders can be long since the solar industry is growing at a rapid pace and has been providing owners a good return on investment. A good place to start is with the list on the EnergySage website, https://www.energysage.com/solar/financing/loan-providers/.
  6. Certification and licensing. These terms should not be confused. Certification is an industry merit that is earned by very competent solar workers; the organization that tests individuals and issues certifications is the North American Board of Certified Energy Practitioners, https://www.nabcep.org/.  The certification adds value and credential to the individual and solar contractor. Certification is often required by solar PV system owners and/or utilities who are offering a financial incentive to their customers. Licensing is a requirement by the state and is a separate issue. The purpose of licensing is to make sure contractors have met the state requirement for competence in their field of construction. A licensed electrical contractor is responsible for all the electrical work performed on the solar installation. The license holder must be a permanent part of the solar company such as an employee or partner.

Kelly Provence
Solairgen School of Solar Technology

 

Residential Solar PV Ground Mount System

Starting a Solar PV Installation Company

Like any new adventure, starting a solar PV design and installation company seems simple at first and then as you learn more about it, the more complicated it gets. If you are already a construction contractor, it is a lot easier to see where you are going; if not, it can be a difficult undertaking.

The first major obstacle to overcome is knowledge about the solar PV industry. Most people start by browsing the internet. There is an almost endless abundance of material to be found when you start your search. As with all internet searches, some of the information is good, some bad and some has nothing to do with what you really need to know.

Costs are a major consideration. The initial investments can be high. You must purchase the necessary equipment and tools to get the job done. You will learn which tools are essential when you are taking the training classes. The basic tools can start as low as $1000 and go up considerably depending on the scale of solar PV systems you plan to install.

It isn’t a bad idea to work for someone else for a while so you can learn the ropes under the supervision of someone who has already gone through this process; this is a good idea if you are not already a contractor. Even if you are a licensed contractor, it can be a good idea to sub-contract the first few installations to an experienced solar contractor. You can learn a lot from this method even if you only break even on the jobs.

I recommend finding an accredited school that offers an introductory course to PV design and installation. Some state technical colleges have solar PV included in their electrical programs but most do not. The best source of education in this field is a school that is accredited specifically for the solar PV technology through the Interstate Renewable Energy Council (IREC). If the school is not accredited, don’t waste your time with them. Taking this entry level course should provide you with enough information about the industry so you can determine if you want to continue, and in which direction you will want to go next.

Once you have completed the basic solar PV course, the next steps will be more obvious to you. If you are a contractor, you are already registered with the city or county as a contractor. If not, you can get a business license with the city or county where you plan to do business; it is a simple process and the fees are usually nominal. Once you have your business license, you can apply to one of the many distributors who sell all the solar products you will need. Not all distributors require you to have a business license to purchase from them, but the better ones do.

Most states require that you or someone within your company be licensed through the state for the type of work you will be providing to the customer. A licensed general contractor meets the requirements, but you will need to hire a licensed electrical contractor before the installation begins. If the owner of the business is not a licensed contractor, either general or electrical, they must meet the state requirements by hiring a licensed contractor. Most states require this license holder to be a permanent employee, not a subcontractor.

Now that the local and state license requirements have been met, you should consider insurance options and requirements. If you hire employees, worker’s compensation is required by law. If everyone who works in the company is a partner, it is not required. However, if you sub-contract under another contractor, they will require it even if it doesn’t cover you.  Liability insurance is also a good idea and is required by most customers. The amount of liability insurance should be balanced with your actual liability if something should go wrong on one of your jobs. However, some commercial contracts will specify the minimum amount of liability insurance.

If you have gotten this far, you will want to advance your knowledge with solar PV system design. If you are a contractor, you know that there are two ways to learn advanced principles, through formal education and by making mistakes. I recommend formal education to lessen mistakes. There are two primary sources for advanced level training; they include advanced online and hands-on courses offered by IREC accredited schools and manufacturer’s training resources.

Manufacturer’s provide design and installation videos and webinars for their products alone, ignoring other products on the market. They often combine the sales aspect with the technical design during webinars and a lot of their installation videos are very educational but, limited to their own best interests.

The quickest and best way to get comprehensive advanced training of design and installation principles is by taking advanced level courses from an IREC accredited school. The benefit is they are impartial when it comes to the market products, so your education is broader which will build your confidence and abilities for when you are on your own.

Your first installation is the point where the risks jump to a high level so make it easy on yourself and first do an installation on your own property or the property of an associate. You are bound to make a few mistakes with the first few installations, so it is a good idea to keep the stress and liability as low as possible in the very beginning.

In review, first estimate the financial investment to get started, second educate yourself on the basics of solar PV design and installation, third get a local business license, fourth address the state contracting license requirements, fifth secure your insurance needs, sixth get advanced level training and experience on solar PV systems and finally acquire the necessary tools and equipment to properly perform the job.

Kelly Provence
Solairgen School of Solar Technology

 

Correct Sizing of a PV Array

Residential Solar PV Ground Mount SystemThe question of optimum PV array size is the first question to be answered when designing the PV system. There are several factors that have to be considered to arrive at the best size. The customer budget, the available area for the array and the purpose of the PV array are always primary considerations. The budget is usually the most flexible since the system can be financed. The second may be difficult to get around if the location options are limited and restrictive. The third part is where most mistakes are made. There are generally two types of systems, (1) stand-alone and (2) utility grid interactive.

The stand-alone system must be designed to serve the electrical loads throughout the year. The PV array is usually sized to serve the loads in the month with the highest ratio of electrical load demand verses available solar energy. As long as the electrical load demand is known and the solar resource is known, the calculation is made for each month of the year (Avg. daily loads kWh ÷ Avg daily insolation ÷ Power conversion efficiency % = ratio) The ratio is the exact size of the PV array necessary to offset 100% of the energy consumed in that critical design month. The hard part is usually determining the kWh of electrical consumption.

The stand-alone system can be off-grid with generator backup or grid supported. If it is grid supported a portion of the house electrical loads are served by the stand-alone system. The PV array is sized for the stand-alone portion of the house.

The graph below show a typical PV system output capacity compared to the electrical consumption for each month of the year. The PV array is exaggerated in size to cover the worst ratio months of the year.

Graph 1

Grid interactive systems must consider how much power can be fed into the electrical system and remain compliant with the utility interconnection agreement and the best power offset value. The most common interconnection agreement is Net Metering for the billing cycle. It is typically set to offset up to 100% of consumption with solar PV generation during the monthly billing cycle. Energy generation in excess of 100% is usually compensated at avoided cost, about 1/3rd the rate of retail. The obvious objective here is to not generate more than is consumed for each month of the year. The challenge is to project generation and compare it to consumption throughout the year.

The months that usually control the PV array size are in the spring and fall. We consume less energy during these periods because of the reduced need for heating and cooling. Coincidentally, these are usually the best two periods for solar PV generation because of clear skies and lower temperatures, April and May are typically the best generation months.

The graph below shows the typical electrical usage for residential customers. The PV system that is designed to offset 60% of the annual electrical consumption is generating 100% of that consumption during the month of April.

Graph 2

A third type of system is a self-consumption type system that requires energy storage and is designed to connect to the grid but not sell into the grid. The PV array size usually follows the same rules as the interactive guideline above, but it can be more complicated and requires a smart control system with consumption and generation metering to keep everything in check.

It is OK to oversize a stand-alone system but with interactive systems keep an eye on the low-consumption vs. high generation months.

Kelly Provence
Solairgen
www.solairgen.com

706-867-0678
info@solairgen.com

Graphis of Excess Energy to the Grid

How Solar PV Can Become the Primary Electrical Energy Source

In order to have this discussion, it is necessary to look at electrical energy consumption patterns. Our energy sources have to match the timing of our consumption. The graphs below show the typical daily electrical energy consumption patterns for residential and commercial consumers.

Time of Day End of Hour

 

Overall peak energy consumption is from 2:00 PM to 6:00 PM. Residential peak consumption is from 4:00 PM to 10:00 PM. Peak solar PV production is typically from 9:00 AM to 3:00 PM. The majority of the solar PV energy is being produced during non-peak consumption hours. If the solar PV system is sized to provide base-loads only, this addresses part of the issue. However, it changes the load demand on the utility grid and makes it difficult to maintain stability of the grid voltage and frequency. The chart below shows the changes to the load demand when solar is added to the grid during the peak solar hours of the day.

2012 represents very little solar generation. Each year shows the actual added solar generation effect and projected solar generation effect on total load demand.

It becomes increasingly difficult and expensive for the utility to meet this type of demand load variation. One solution is to face the PV array toward the afternoon sun. This would help with overall consumption patterns but not for residential consumption patterns. It doesn’t address the issue of cloudy days with little or no solar PV generation. This brings in energy storage as the best solution. The energy storage capacity needs to be large enough to absorb the excess solar PV energy produced during peak solar hours.

Overgeneration risk

Energy storage is the only real solution for solar PV to become a primary electrical energy generation source (EEGS). There are several types of energy storage: Batteries are the fastest growing type of energy storage with Lithium-ion being the leader of that group; the conversion efficiency is pretty high (80% to 90%). Pumped water storage has been used by utilities for this purpose in some sectors of the electrical grid; the energy conversion efficiency is a little lower (70% to 80%). Other forms of energy storage such as fuel cells and capacitors may develop further in the near future but they are not competitors at this time.

Battery storage is the most versatile means of energy storage and it is crossing the barrier of cost effectiveness for shifting loads to a different time of day. To effectively compete with the other EEGS, solar PV with energy storage must have a levelized cost of energy (LCOE) that is lower than the other sources. To calculate the LCOE, divide the total cost of energy by the energy generated during the life-cycle of the EEGS. Twenty-five years is the typical life cycle used for PV systems. The cost of energy includes initial cost and all operating costs during the period of analysis. The kilowatt hour of generation during the period of analysis must be de-rated for non-operational hours and the warranted degradation of the PV output.

Examples:

Residential:

8kW PV system with energy storage:       Net Cost: $3.30/W = $26,400

8kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 279,225 kWh

$26,400 ÷ 279,225 = .094/kWh

Commercial:

500kW PV system with energy storage:  Net Cost: $2.60/W = $1,300,000

500kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 17,451,562 kWh

$1,300,000 ÷ 17,451,562 = .0745/kWh

Utility:

20MW PV system with energy storage:  Net Cost: $2.00/W = $40,000,000

20,000kW x 5 sun-hours x .85 x 365 days x 25 years x 90% = 698,062,500 kWh

$40,000,000 ÷ 698,062,500 = .0573/kWh

The above examples show LCOE rates that are competitive in some markets but not in all. The installed cost of solar PV is continuing to drop and the energy storage industry continues to improve and reduce costs. As these trends continue into the very near future, solar PV with energy storage will meet and beat the LOCE of most of its competitors in all markets.

Kelly Provence
Certified Master PV Trainer
CEO, Solairgen School of Solar Technology

Power Conditioner Image

Failure Causes in Solar PV Systems

During the first 10 years in service, the chance of failure within a PV system is approximately 10%. Inverters and other electronic devices account for 85% of all those PV system failures. Only about 1 in 2000 modules will fail during their warranted 25-year life. The system components most likely to fail are the ones with complex electronic circuitry.  The graph below identifies this type of equipment as Power Conditioning; it includes monitoring equipment, inverters, PV optimizers & other DC to DC conversion devices. The causes of failure are from manufacturing defects, improper installation, operating stress and accidents. The first of these is out of the control of the installation and maintenance contractors and the last of these, accidents, is usually outside of their control; the second two, improper installation and operating stress, are not.

 

Failing Power Conditioning equipment will perform at a lower efficiency than warranted; failed equipment will stop performing its function. Failures due to improper installation and operating stress are both avoidable. Causes of these failures are: Inadequate wire terminations, undersized conductors, environmental conditions that are outside of the equipment rating, inadequate protection from surge voltage and inadequate protection from physical damage. Once the equipment is operational, an infrared camera can detect damage that is occurring from heat at the terminals and inside the equipment. Terminal temperatures should vary no more than 3°C from one to another under the same current load. The internal parts of the equipment should not exceed the max rated temperature listed by the manufacturer. If equipment is failing but still operating, the conversion efficiency can be measured by reading the (Volts x Amps) at the input and output of the equipment. The difference should be no less than the rated efficiency of the equipment. Test six months after installation and then annually thereafter.

Module failures get most of the attention since they don’t usually fail completely but continue to function at a lower output; the causes of failure are numerous. A module is considered failed when one of the following occurs: Breakage, delamination, burned solder joints, browning of a PV cell due to overheating, bad bypass diode or degradation in performance beyond the warrantied percentage. These are covered by the manufacturer’s warranty; depending on the failure mode, proof of failure can be verified with a standard photograph, infrared photograph or by inspection test with a calibrated meter such as an I-V curve tracer. A module is considered failing if it performs lower than other modules of comparable design and type. An IR camera may detect hot spots on PV cells that decrease performance –  an IV curve tracer can determine whether the module is operating below its warranted performance. Test with an IR camera annually, and with an I-V curve tracer for large commercial projects.

The most common long-term failures are: Hot spots due to manufacturing defects in the cells, hot cells caused by high current flow in a de-energized state, potential induced degradation (PID) caused by leakage currents to earth ground, low cell conversion rate due to cracks within the cell, delamination caused by extreme heat and humidity and current loss due to shorts (shunting) between cells where the module substrate is damaged by wind and dust or other natural causes. An I-V curve tracer and an IR camera can identify most of these problems. For residential and some commercial projects module level monitoring will identify problems; it is easy to compare each module performance to the others around it.

The other failure category shown in the graph is balance of system components (BOS). This is generally all the wiring, conduit, switchgear, junction boxes and module support structure. Failure is usually due to improper wire sizing or termination, galvanic corrosion with incompatible metals, materials installed in environments beyond their rating and improperly installed components. Prevention of failure includes testing conductor resistance before startup, capturing IR images at terminals when equipment is operating, measuring resistance to ground on all grounded metal parts and making a visual and torque inspection of all physical and electrical connections. Test six months after installation and then annually thereafter.

The most valuable tool for inspecting PV system performance is a module level monitoring system; most new residential and small commercial projects now have this level of monitoring. The next best tool is an IR camera; since there are now IR devices that connect to smart phones this is a very affordable option. The next best tool is an AC/DC volt-amp meter. You can take readings on the input and output of any equipment and determine the loss factor for that piece of equipment. If you are working on commercial systems, you will want a conductor resistance tester to measure the conductor insulation resistance to leakage currents at the maximum operating voltage. The I-V curve tracer is unnecessary for residential and small commercial projects. They are usually required for large commercial and utility scale projects.

Kelly Provence
Solairgen School of Solar Technology
IREC Certified Master PV Trainer
NABCEP Certified Professional Installer

Calculating Voltage Drop in PV Systems

Voltage drop (VD) is the loss of voltage in a circuit due to the resistance in the electrical circuit. To determine the amount of voltage lost in a circuit, we need to look at three parts: 1. Resistance of the conductor in Ohms (Ω), 2. The length of the circuit conductor, 3. The current flowing through the conductor. A forth component is to compare the VD to the operating voltage in the circuit to see the percent of voltage drop.

  1. The resistance of the conductor per 1000’ (Ω/kFT) can be found in Table 8 and 9 of Chapter 9 in the National Electrical Code (NEC). Table 8 is for DC and Table 9 is for AC. Divide by 1000’ to get the resistance of the conductor per foot (Ω/FT).
  2. The length of the conductor is the full circuit length; for DC and single-phase AC circuits, multiply the one-way distance by 2. For three-phase AC circuits, multiply the one-way distance by the square root of 3 (1.732).
  3. The amount of current flowing in the circuit directly affects the amount of voltage drop in the circuit; e.g. 2 amps of current will double the voltage drop of the circuit with 1 amp of current. Current is represented as intensity of current (I) in the formula.
  4. To determine the VD%, the operating voltage must be used. The operating voltage is dependent on the equipment and how it is connected. The PV source circuit voltage may be the product of modules connected in series or it may be controlled by DC-DC conversion devices and an inverter. The AC operating voltage is simply the nominal utility voltage at the premises.

DC and single-phase AC formula:  VD  =  I  x  Conductor length  x  2  x  Ω/FT

 VD%  =  VD  ÷  Operating voltage

Example: A PV source circuit operating at 9a and 400v using a #12 conductor in a circuit with 50’ of length one-way. Table 8 shows a #12 (7 strand) to have 1.98 Ω/kFT; 1.98   (.00198Ω/FT)

9a  x  50’  x  2  x  .00198  =  1.782 volts dropped or lost in the circuit (VD)

1.782v  ÷  400v  =  .004455  or  .44%VD

If you double the length of the conductor or double the operating current, the voltage drop would also double.

NOTE: This calculation is accurate using current and voltage data either from the PV module Standard Test Condition (STC) or the Normal Operating Condition (NOC). NOC test conditions represent the irradiance and cell temperature during the six critical sun-hours of the day. Both current and voltage are lower than STC. Since they are both lower, the VD% is close to the same as if you used STC current and voltage instead.

The AC side of the calculation is the same as The DC example above, however it may not be as accurate since the inverter rating can be sized from 80% of the PV rating to 135% of the PV rating. This affects the average operating current and the amount of voltage drop. We typically use the inverter’s listed max operating current if the PV array is sized 125% to 135% of the inverter rating. If the inverter is rated the same as the inverter, use 80% of the listed inverter max operating current.

Example: An inverter rated 6000 watts, 240v and 25a is connected to a 7200 watt PV array. The inverter AC output is located 30’ from the AC interconnection point. The conductor used is #10 copper rated 35a with resistance shown in Table 9 of 1.2 Ω/kFT   (.0012Ω/FT)

25a  x  30’  x  2  x  .0012Ω/FT  =  1.8 VD  ÷  240v  =  .0075 or .75%VD

The combined VD% of the DC and AC circuit is .44%  +  .75%  =  1.19%

If the inverter and PV array are the same size, multiply 25a  x  80%  =  20a. This reduced the VD by the same proportion.

We usually consider more than 3% VD in the entire circuit (DC and AC) to be excessive. Increasing conductor size reduces the Ω/FT and reduces the voltage drop in the circuit. The voltage drop percent is a loss factor with energy production. The example above reduces the PV system production by 1.2%.

Three-phase AC formula:  VD  =  I  x  Conductor length  x  1.732  x  Ω/FT

 

Kelly Provence
Certified Master PV Trainer
Solairgen School of Solar Technology

Is everyone a potential solar PV customer?

The short answer is yes since electrical utility companies generate a portion of their electrical energy from solar energy; indirectly everyone is a solar PV customer if they purchase electrical energy from one of these utilities. The better question is who is a direct customer of a solar energy system; the type that will offset or replace electrical energy that would have been purchased from an electrical utility. To clearly see who these customers are, we need to break them down into categories. The first breakdown is between residential and commercial electrical consumers. The second breakdown is stand-alone off-grid customers and grid connected customers. For this blog we will look at the residential customer, both grid-connected and off-grid.

Residential: To be a potential solar customer they must be owners of the property and they must have enough sun exposed space on the roof or ground to support the PV array relative to their level of electrical consumption. They must also have the financial capacity to purchase the system. The four basic criteria are, (1) ownership of the property, (2) extent of solar exposure for a PV array, (3) % of monthly/annual electrical consumption to be offset by solar and (4) the financial capacity to purchase. With evidence of ownership established, the sun exposed space is compared to their electrical consumption to determine the size of the solar PV system and the approximate electrical consumption it will offset. If a method of payment can be established, this is a potential solar customer. There are however four other factors that should also be considered (5 – 8), (5) utility interconnection limits, fees and purchase rates, (6) rate of financial return on investment and cash flow, (7) building covenants or restrictions and (8) understanding the customer’s motivations. Contact the customer’s utility company and find out all requirements, fees and rates that will be offset by solar electrical production; this affects the type of system, cost of the system and the return on investment. Find out about homeowner’s associations or local ordinances that may restrict or even prohibit the installation of the PV array where it is publicly visible. Most solar customers want to offset their energy bills and save money in the process but there are many other motivators such as environmental concerns with conventional energy, independence from the utility or to be gird connected but consume all the energy generated from the solar PV system and have some off-grid capacity during utility power outages. Look at all eight of these considerations to determine who is a potential solar customer.

Residential off-grid: These customers are typically located in rural areas without utility access. That being the case, we can eliminate considerations 5, 6, and 7 above but #8 become much more important. This customer needs a high level of self-reliance and independence with the solar system. Other forms of power generation should be considered as well such as wind, hydro and a fuel generator.

In a follow-up blog, we will look at the criterial of a potential commercial customer.

Kelly Provence
Solairgen School of Solar Technology