Tuesday, February 25, 2014

Selling Value vs. Price - The Eternal Struggle

The mark of a good salesman is identifying customer needs and providing them the products they need to fulfill those needs.  I’m taking a break from design posts to take a look at the sales side of solar and some of its unique challenges.  I've enlisted the expertise of our resident sales guru, Tim McGivern.  Tim is a seasoned veteran of sales with a firm grasp of customer interaction, and he comes with a wealth of knowledge and experience.  Today’s challenge is how to sell quality and value versus pure cost in a solar project.

Find the right solution for your target customers and the products you use, and focusing like a laser on them to efficiently and effectively grow your business while leaving your customers satisfied.  In dealing with your customers, it is your job to make that value proposition and demonstrate the unique quality of your products.  The pitfall in the value vs. price relationship is when you try to sell someone purely on the price of a solar system.  This is a battle that you will never win in the long run.  If your customer is a price shopper, they will have zero loyalty to you and fail to appreciate your integrity and good faith in delivering them a quality product.  If you get the sale, you are left with low margins and no room for error in the installation.  If you don’t, it’s more time spent on someone that’s just going to shop your quote to competitors.  It doesn't make sense to do price-driven sales when you are using high-quality solar equipment. Nor does it make sense to install low quality equipment that will cost you future labor in repairs and stressful customer service.

The value inherent in tier one solar panels, inverters, and racking cannot be overstated.  It can be the difference of all solar cells being lined up on mono-crystalline panels, a sturdy module frame, or higher efficiency cells.  A company like Hyundai offers quite a lot in added value because they are a diversified company that makes an efficient solar panel and isn’t affected by potential tariff issues.  Canadian Solar, while made in China, has unique value that differentiates it from other Chinese modules like Trina or Yingli, for example a high PTC/STC production ratio which points to excellent real-world performance.  While Power One doesn't have the track record Enphase does with Microinverters, they have unique value in being a bankable company that you can have confidence will be around for years to come.  Unirac combines excellent pricing with extremely robust engineering and long-term performance that no competitor can match.  These are just a few examples.

Finding the unique value in the products you use is the best way to compete against cheap competitors that sell lower quality solar equipment.  Let it shine!

Friday, February 14, 2014

Grid Tied Inverter Overloading Analysis (with PVSyst)

For today’s post, I brought out the solar modeling software and gotten myself into trouble (grid tied inverter loading analysis).  System analysis and solar modeling software is a delicate subject, there are dozens of variables and different models can produce different results.  I've tried to keep things as simple as possible for the purpose of this analysis.

When you hear about inverter loading/overloading, it refers to the ratio of Solar DC to Inverter AC power (Watts) that you design into your system.  This can be expressed as a percentage value.  For example, a system that has an inverter that’s “20% overloaded” (or 120% loaded) would mean the DC array size is 20% larger than the AC rating of the inverter.  A system that is 0% overloaded (or 100% loaded) would have a solar array and inverter that equal each other in size.  For the purpose of this analysis, I created three different inverter loading scenarios using Power One string inverters and Canadian Solar panels.

Solar Array
Inverter Load
Power One 6000   (6 kW)
(24) Canadian Solar 250W Poly
2 Strings of 12
Power One 5000   (5 kW)
Power One 4.2     (4.2 kW)

It’s important to keep the string size consistent when making comparisons like this to cut down on loose variables.  In this case, I was lucky to find a situation where an identical solar array can be compared across three different inverter loading scenarios in a fairly linear fashion.  The only difference between the inverters is a slight decrease in the CEC efficiency (0.5%) from the 5 & 6kW inverters to the 4.2kW version, while maximum efficiency and voltage windows are almost uniform across all three.  I corrected for the CEC efficiency loss by increasing the 4.2kW inverter production numbers by 0.5%, but in the end this had little to no effect on the data.

The different loading ratios have been simulated across four different cities for average annual production:

Albuquerque, NM – A good representation of a dry climate with high-intensity sun.

Miami, FL – A sunny site with interesting weather effects and a lot of humidity.

Chicago, IL – A colder site that gets less sunshine with long cloudy winters

Kansas City, MO – An average site that has characteristics of the other three sites.

To estimate production, I’ve used PVSyst (V5.65) for the simulation software.  The system orientation was set at a 30 degree tilt and true south azimuth.  The following results were obtained for average annual production across all sites and loading conditions:

Inverter Size:
% Loss
% Loss
Albuquerque, NM
Miami, FL
Chicago, IL
Kansas City, MO

Production numbers are to be expected for the four cities.  Some may be surprised that Miami is so low while Kansas City is higher.  The reason is Miami has a lot of humidity and a fairly uniform temperature profile year-round.  Kansas City has hot summers, but their winters are fairly cold and that boosts solar production, and their dryer climate helps more of the sun’s rays hit the solar array.  Chicago has the worst solar production as expected, but they are affected more by inverter loading than Miami.  This is due to colder overall temperatures allowing the solar array to produce more continuous power during optimal conditions, which can cause clipping on an overloaded inverter. 

What jumps out immediately is the change in production in going from a 6kW inverter to a 5kW inverter, or lack thereof.  There is virtually zero difference in production for three of the sites by going to an inverter loaded 120%, and for Albuquerque it’s only half a percentage point off the top.  This is negligible, and after the first year or two of service there will be no difference due to degradation of the solar panels (which typically lose ~0.5% of their production per year installed).  It can be seen that overloading the inverter by 20% is something that should be designed into any solar job, while the benefit from matching the inverter rating with the solar array size just isn't there.

The 140% loaded inverter has more significant production losses than the 120% inverter.  For Albuquerque especially, the difference is too large to be ignored, and overloading the inverter that much should not be attempted at similar sites for optimal orientations.  For the other three cities, the power loss is pretty bad but there is still hope.  The simulation parameters are for a system tilted at 30 degrees, oriented optimally with respect to the sun, all with zero shading.  If a system is being considered with sub optimal orientation or shading, anything that might adversely affect production, an inverter loaded to 140% isn't out of reach and in many cases will work just as well.  The production difference is only 0.8% for Miami, so temperate sites without cold winters could have 40% overloaded inverters without any trouble.

What are the biggest takeaways from this?  The better the site is for solar (days of sunshine, low humidity, cold winters), the less the inverter should be overloaded.  The minimum for inverter loading falls around 120% for the United States.  Anything lower and you are wasting inverter capacity, anything higher and you need to consider the site conditions before proceeding.  Temperate climates where winter temperatures do not drop below zero have the most options with inverter loading, and 40% is not out of the question.  Climates where clear and cold conditions occur have to be careful in over sizing the solar array for the inverter past 20%, colder temperatures let the solar array produce more power and can cause production losses if the inverter isn't sized correctly.  It is this designer’s opinion that 120% inverter loading will work just about anywhere, and should be the standard when pairing a solar array with an inverter.

Friday, February 7, 2014

Solar System Design - String Sizing

When designing a solar system, the most important calculation is determining the length of the string of solar panels.  Solar inverters and charge controllers have set voltage windows that have to be met by a string of solar panels whose voltage can vary as much as 40 – 60% throughout the year.  With low string voltages, operation is less efficient and the system can be in danger of shutting off during hot conditions.  Design a string voltage too high and cold sunny conditions could put the inverter into an overvoltage fault mode which shuts the inverter down.  Solar designers have to hit the “sweet spot” where their string voltage will always fall within their equipment’s voltage window while maximizing the string length for more efficient operation.  This is done by designing solar strings based on the upper voltage limit of the inverter or charge controller.

Effect of Temperature on String Voltage

At its basic level, higher temperatures drop voltage and lower temperatures raise voltage in electronics.  For the solar designer, this means string voltage is at its highest when the temperature is coldest, and the extreme low temperature is used to design the solar string.  There are two methods for calculating solar string voltage based on temperature, both outlined in NEC 690.7(A) Maximum Photovoltaic System Voltage:

1)      …Maximum photovoltaic system voltage for that circuit shall be calculated as the sum of the rated open-circuit voltage of the series-connected photovoltaic modules corrected for the lowest expected ambient temperature …. The rated open-circuit voltage shall be multiplied by the correction factor provided in Table 690.7…
2)      When open-circuit voltage temperature coefficients are supplied in the instructions for listed PV modules, they shall be used to calculate the maximum photovoltaic system voltage as required by 110.3(B) instead of using Table 690.7.

The first method calls for using NEC Table 690.7.  To use the table, take your solar panel’s open circuit Voltage rating (Voc), found in the data sheet, and multiply it by the temperature correction factor based on your lowest expected ambient temperature.  The lowest expected temperature can be the record low temperature which can usually be found online.  For example, in Albuquerque, NM, our record low temperature is -17o F. Converting to C puts it at -27o C, with a corresponding adjustment factor of 1.21.  This means for Albuquerque I would multiply the solar panel’s Voc by 1.21 to find the maximum design voltage for string sizing.  Assuming a typical 60-cell solar panel with a Voc of 37V, the maximum design voltage is 44.77V.

The second method requires using an equation and referencing the temperature coefficient of voltage found on the solar panel data sheet, but it gives a more exact answer than using NEC Table 690.7.  The temperature coefficient of Voc is usually between -0.3 and -0.4 % per degree C/K, but it varies from panel to panel.  The equation for temperature effect on string voltage is:

Design Voltage = Voc *(1 + TVoc * (Design Temperature - 25o C))

Using a temperature coefficient of -0.33 %/C and the Voc and low temperature used in method 1 (37 Voc, -27 C), the design voltage becomes:

                Voc * (1  + (-0.0033 * (-27 - 25)) = Voc * (1 + 0.1716) = 43.35V

Note that the voltage determined using voltage coefficient is slightly lower than that found using the NEC table.  The NEC table is the more conservative and less exact method to use, but it’s also a little easier than using the temperature coefficient, which gives an exact answer for the extreme minimum temperature and solar panel.  Per NEC 690.7 (A), the temperature coefficient method should always be used if the temperature coefficient of voltage for the solar panel is known, which it usually is from the equipment data sheet.

Record Low vs. Minimum Dry Bulb Temperature

If you continue reading 690.7(A), there is an informational note on what data can be used for the low temperature in string sizing calculations:

Informational Note: One source for statistically valid, lowest-expected, ambient temperature design data for various  locations is the Extreme Annual Mean Minimum Design Dry Bulb Temperature found in the ASHRAE Handbook — Fundamentals.  These temperature data can be used to calculate maximum voltage using the manufacturer’s temperature coefficients relative to the rating temperature of 25°C.

While it’s a mouthful, the gist of it is if you have access to the American Society of Heating, Refrigeration, and Air Conditioning Engineers (AHSRAE) handbook or to their temperature data, you can use the extreme minimum dry bulb for the low calculation instead of the record low temperature.  This temperature is always higher than the record low.  For example, in Albuquerque, NM, the record low temperature is -17o F, while the extreme minimum dry bulb temperature is 10o F.  If I run it through the voltage coefficient equation again, the design voltage becomes 41.52V.  This is the difference between a string of 13 and a string of 14 on a 600V input solar inverter, so the improvement by using this data can be significant.

Sometimes, your exact location isn’t available in the ASHRAE data tables.  In this case, either select the closest site with similar latitude and elevation, or take an average of surrounding sites to approximate the minimum dry bulb temperature at that location.  If you want to know the minimum dry bulb temperature for your location for solar design but don’t have access to an ASHRAE Handbook – Fundamentals, someone may be able to look it up for you….

Effect of Mounting Method on String Voltage

Sometimes the solar system needs to be designed with shorter strings that are close to the lower bound of the equipment voltage window, and you need to confirm that the system will work in the hottest conditions instead of the coldest.  The voltage coefficient equation and NEC Table 690.7 are both only usable for maximum voltage calculations.  This is because maximum voltage calculations are able to make the assumption that the solar equipment’s temperature is equal to the ambient air temperature, as the low design temperature typically occurs in the hour before sunrise.  For the minimum voltage, the solar array needs to be considered when it’s at its hottest, when it’s producing power and the sun is shining on it.  At this point, the equipment can be much, much hotter than ambient temperature due to the direct solar radiation it receives.

How do you figure out the design hot temperature for a minimum voltage calculation?  To start, go back to weather data for your location and find the average high temperature for the hottest month of the year.  This will become the design temperature in a new temperature coefficient calculation of voltage.  Next, based on the solar mounting method, select the temperature rise that will be added directly to this value.  These are common values for temperature rise that the solar industry uses.

Mounting Method
Temperature Rise
< 10o on a flat roof
36o C
> 10o on a flat roof
34o C
Flush mount, pitched roof
32o C
Ground mount
30o C
Pole mount
29o C

Under optimal conditions (pole mounting), the solar array is assumed to be 29o C hotter than ambient temperature, or 82o F hotter.  It only gets worse the closer you install to the roof, as air circulation decreases and the array temperature steadily climbs.
While the max voltage calculations called for using the open circuit voltage, with minimum the “max power” voltage, or Vmp, needs to be used.  Coupled with the temperature rise factor, the minimum voltage equation becomes:

Design Voltage = Vmp *(1 + TVoc * (Design Temperature + Temperature Rise - 25o C))

Using an average high temperature of 95o F (35o C) with a solar panel Vmp of 30V, here’s an example of the minimum voltage of a solar panel installed on a flush mount:

                Vmp * (1  + (-0.0033 * (35 + 32 - 25)) = Vmp * (1 - 0.1386) = 25.84V

This is much, much lower than the 41.5V calculated earlier for the maximum solar panel voltage.  It highlights the importance of temperature effects on your minimum string size.  Based on these numbers, if a solar inverter with a minimum voltage of 200V were considered, a string of 7 would fail under hot operating conditions, while a string of 8 would continue to work.