Focus Solar

What is the difference between your data and those of NASA and NREL?

What is the accuracy of your data?

Which are the sunshine cities in the US?

Which market studies have been done on solar system performance?

How do different market support programs compare?

When will solar energy be economical?

How is the performance ratio defined?

Why is my performance ratio not 100%?

Why is the performance ratio in summer lower than in winter?

What can the performance ratio tell me about my PV system?

Is there any record of how much energy I am feeding into the grid?

Is it worthwhile to invest in solar when I am living in a region "less favored" by the sun?

	NASA
URL	http://eosweb.larc.nasa.gov/sse
Data Source	satellite
Pro	long history (10 years) worldwide coverage
Con	coarse spatial resolution (100 km x 100 km) not real-time (unsuitable for monitoring)
Purpose	system sizing site evaluation

 

	NREL
URL	http://www.nrel.gov/gis/solar.html
Data Source	ground measurements combined with cloud coverage data
Pro	very long history (30 years) high accuracy at measurement points
Con	few measurement points (10 points in California) cloud data have coarse resolution (40 km x 40 km) not real-time (unsuitable for monitoring)
Purpose	system sizing site evaluation

 

	Focus Solar
URL	http://www.thinksunsmart.com
Data Source	satellite
Pro	real-time data  spatial resolution is 1,000 times better than NREL and 10,000 times better than NASA
Con	short history (begins in 2006)
Purpose	advanced site evaluation microclimatic optimization system monitoring

Why is the data quality important to you?

It's all about your expectations how reliable you want to know your financial returns from a solar investment. If a 30% uncertainty is acceptable to you, freely available information from NREL or NASA will serve the purpose. The rather large uncertainty is due to limited spatial resolution. Within a 40 km x 40 km grid cell the amount of solar energy can vary by as much as 30%. Existing local structures are washed out and microclimates remain invisible. There is, however, no good reason to stop short if imagination and computing power are not the issue. The only true boundary is the resolution of the satellite. Focus Solar unlocks the full potential as it processes satellite imagery in its full resolution yielding a factor of 1,000 to 10,000 improvement over conventional sources.

The data set comprises half-hourly values for global, direct normal, and diffuse irradiance with a spatial resolution of 0.6 x 1.0 square kilometers at the sub-satellite point. The spatial resolution varies somewhat with the distance to the sub-satellite point depending on the viewing angle of the satellite. In any case the resolution is well below 2 km.

Satellite derived estimates of solar energy have been extensively tested against ground measurements with pyranometers. It was found that satellite data unfold their strength in mid to long-term estimates where the time period ranges from a day to a couple of years. The longer the time period over which energy is accumulated, the more satellite pictures will be available, and the better will be the accuracy. For daily values of global irradiance the accuracy is normally in the range of 8-10% comparable to the accuracy of solar cell radiation sensors. A monthly irradiance estimate has an error of only 5% and annual irradiance values reach an accuracy of 2-3% coming close to the precision of pyranometers.

The quoted error margins correspond to normal circumstances, but there are few exceptions where satellite estimates can fail almost entirely. These special cases occur for highly reflecting ground surfaces such as snow and salt-beds. The high reflectivity is falsely interpreted as being due to clouds resulting in untrustworthy irradiance estimates. These topics are subject of ongoing research.

However, as long as our users do not live in snow-covered areas or in the vicinity to a salt-bed in the desert, there are no problems. In the absence of snow, the accuracy is as good as stated above. It's actually easy to identify the "problem regions" in our irradiance maps. They show up as local minima at the snow-covered peaks in the mountain ranges. A snow detection routine is part of our algorithm, yet, we cannot guarantee that it treats the effects on irradiation absolutely perfectly.

The snow detection routine only works in the United States, but not in Canada. This is because we receive data on snow coverage from the US weather service which do not cover territory outside the US border. Therefore irradiance estimates for the Canadian provinces in the winter period shall be considered with some degree of limitation.

fancygens.com

Which city was the most sunny in 2007? Here is the result ranking 20 selected cities by their solar energy values from the year 2007. The most sunny city was Phoenix, AZ, and the least sunny city was Seattle, WA.

Position	City	State	Annual Solar Energy in 2007 (in kWh/m2)
1	Phoenix	AZ	2169
2	Las Vegas	NV	2132
3	Albuquerque	NM	2093
4	Santa Fe	NM	2024
5	San Diego	CA	1966
6	Los Angeles	CA	1909
7	Miami	FL	1909
8	San Jose	CA	1883
9	Salt Lake City	UT	1804
10	New Orleans	LA	1793
11	Atlanta	GA	1780
12	Denver	CO	1749
13	San Antonio	TX	1734
14	San Francisco	CA	1713
15	Houston	TX	1698
16	Washington	DC	1573
17	New York	NY	1477
18	Chicago	IL	1438
19	Boston	MA	1437
20	Seattle	WA	1195

Please have a look at the following studies:

• Berkeley Study on Effectiveness of State Incentive Programs
• Performance Program of the International Energy Agency

The Berkeley report of G. Barbose, R. Wiser, and M. Bolinger (first reference) notes that systematic studies of PV system performance have just begun and performance problems have been an issue though the extent and specific causes of poor performance are not always understood. The most common causes for poor performance are identified as shading, equipment and installation defects, inverter failure, lack of basic maintenance, and deviations from module manufacturers' specifications. A recent evaluation by the California Energy Commission is cited which revealed that 3% of systems, in a sample of 140, were not operating at all or were operating well below expected output. 7% of systems, in a sample of 95, were found to have lower-than-expected power output.

The performance monitoring group of the International Energy Agency (second reference) maintains a database on operational performance and economic cost consisting of more than 600 PV systems from 17 countries including freestanding, rooftop, and facade integrated systems. This group, headed by U. Jahn from Germany, has presented the most comprehensive surveys available to date, yet, also notes that information on system failure is hard to come by. They cite a study from Japan where 12% of replies, in a sample of 725, have reported failures, and that the most common failure is a failure of the inverter which is most likely to occur before the second year of operation. Module failures and failures in the balance of system components such as cables or switches are reported to be less likely.

Read more about net metering, feed-in tariffs, and the California Solar Initiative here.

It's true, solar energy isn't really cheap. But the good thing is, with solar energy you are on the winning side. The price of conventional energy will only go up and the price of solar energy will only come down. It's just a matter of time when the price curves will intersect and solar energy will become economical. Most people are unaware how close that break-even point already is. Looking at the latest gas prices gives you an idea:

Experts from the European Photovoltaic Industry Association (EPIA) forecast that in sunny states like California the break-even point will be reached between the years 2010 and 2020. That's when the price of solar-generated electricity will fall below retail utility prices. In other words, almost everybody who is alive today will experience the rise of the solar age in their lifetime. That's not something that's going to happen in a far distant future. Grid parity is within reach today and will be tomorrow's reality. Mr. Hoffmann, the president of EPIA, predicts the break-even price for one kilowatt-hour to be around 15 to 25 cents for California and around 20 to 30 cents for less sunny states. That's good news for people who are afraid of skyrocketing energy prices. When the price will be that high, solar energy will begin to stabilize the price. As the potential of solar energy will unfold, the trend of rising energy cost will reverse and energy will become what it is meant to be: clean and affordable.

The performance ratio is defined as the electrical output per kW peak system size and per unit of solar energy input:

kWh produced
-------------------------------------------------------
(kW peak) * (kWh/m2 solar energy)

where the unit of solar energy is kWh per square meter. It expresses how much of the available solar energy is converted into electrical energy. The performance ratio was introduced to compare PV systems independent of size, mounting, and location.

Let's look at a typical example: In California, a customer might have a 3 kW peak system and the annual solar energy is 2000 kWh/m2. If the system were perfect, it could potentially produce 3 x 2000 = 6000 kWh per year. When the actual production is 5000 kWh, then the performance ratio is 5000 kWh / 6000 kWh = 83%.

The performance ratio can also be calculated for time periods other than a year. Interesting time scales are for instance a month or even a day. It's only important that the time scale is the same for both, the generated electrical energy and the incoming solar energy. Otherwise it's like comparing apples with oranges.

We can distinguish two types of losses: "hard" and "soft" losses. Hard losses are the losses with a physical cause and which can potentially be removed. Soft losses are also real losses, but they are influenced by human perception. Soft losses are actually much harder to remove than physical losses. In practice the owner of a PV system can't do anything about them.

Hard losses:

The most important physical loss factors are: losses in the inverter (typically 4%, some brandnew inverters reach 2%), losses from shading (0-100%) and soiling (0-25%) which simply means that PV panels have accumulated dust and soil. Shading and soiling are site-dependent conditions. Dry and desert-like locations with little rain have larger soiling problems. Some locations in California have up to 25% soiling losses. There are also minor losses from cabling, diode and connection losses, mismatching, inefficiencies in the tracking of the maximum power point, etc, which typically add up to less than 5%.

Soft losses:

Soft losses relate to the definition of the standard that is applied in the nameplate rating of PV modules. To say it politely, the standard was chosen "optimistically". Soft losses occur whenever a PV module is operated at weather conditions that result in a lower efficiency than the efficiency at standard test conditions (STC). Unfortunately this is almost always the case because standard test conditions are defined for the lab environment and not for the real world. STC defines the irradiance as 1000 W/m2 and the module temperature as 25°C. At these conditions PV modules reach their highest efficiency. In a sunny location, however, the temperature is typically much higher than 25°C because PV panels heat up in the sun and in a colder location, the illumination level is typically lower than 1000 W/m2. In either case, the operating conditions are not the ideal conditions from the lab environment. STC performance can only be achieved if PV modules are cooled during operation. In the lab calibration process, modules are only briefly flashed by so-called solar simulators, so that they do not have a chance to heat up.

In summary, soft losses are weather-related losses in which PV modules do not reach the nameplate efficiency. They are highly site-dependent and it is not unusual to have soft losses of 10% or even higher. When we add up all losses, we arrive at a typical number of approximately 20%. Therefore a good performance ratio should be 80% or better.

By the way, soft losses could be reduced if the industry would decide to change the standard for nameplate rating. This would require, however, to relabel a 100 W module into a 90 W module, for instance. Obviously, the sales people of PV module companies will not be too happy about this prospect.

Temperature is the most crucial factor influencing performance of PV modules. The efficiency decreases with rising temperature. In summer times a PV module can reach temperatures of 50°C degrading the performance by approximately 10 to 15%. It is recommended to have a layer of air circulate between the PV modules and the roof to keep the temperature as low as possible. When solar modules are integrated into the facade the losses due to temperature rise are largest.

In winter times, the performance is less influenced by temperature. There is a small effect from irradiance. If the irradiance falls below levels of 300 W/m2, the efficiency decreases as well. Overall, however, the reduction is relatively small in winter times. A yearly graph of the performance ratio typically shows a minimum in the summer and a maximum in the winter.

It is important to keep in mind that the performance ratio can identify the existence of a problem, but not the cause. The cause of the problem requires further investigation, which may include a site visit by service personnel. Performance ratios are useful for determining if the system is operating as expected and for identifying the occurence of failures. Large decreases in performance ratio indicate events that significantly impact performance, such as inverter faults, circuit-breaker trips, or loss of a whole module due to connection failures inside a PV junction box. Small or moderate decreases indicate a less severe problem. Intermittent problems such as shading, snow coverage and soiling show up as fluctuating performance numbers with an on/off characteristic. Long-term degradation appears as a small continuous decline in performance values.

For the calculation of the performance ratio it is necessary to know how much energy your system was producing. Unfortunately the utility company is not recording these numbers. Your monthly statement does not give you information on the number of generated kWh. This is not happening because the utility wants to withhold this information from you. They simply don't know it. When you have only one electricity meter, as most people do, you can only record the net flow of energy which is the difference between generation and consumption. If you want to separate generation from consumption, you need two meters. However, there might be a simpler solution.

Most modern inverters offer a display of the daily production. You might want to record those numbers in a book. A more advanced option would be to subscribe to one of the internet platform services offered by inverter companies. Your individual production data are recorded by a central server and displayed on the internet. This is probably the most convenient option available to date. If you make these numbers available to us, we can compute the performance ratio for you.

Example internet platform: Sunnyportal of SMA

Latest research indicates that sunny and hot conditions are not everything when it comes to solar. The electric yield of PV modules is also favorable in cold and windy climates, as long as the skies are clear. Solar modules just love a fresh breeze coming in from the ocean. A steady breeze acts like a cooling fan improving the conversion efficiency of solar modules. Of course, nothing works without solar energy because it's the energy in sunlight that is converted in solar modules. Experience from Germany, the leading PV market in the world, has shown that the yield in cold and windy regions can exceed original expectations by up to 12%. That's because it's relatively cold in Germany. The short summary is, that sunshine is good, but sunshine in combination with cold temperatures is even better. These ideal conditions can be found high-up in the mountains or in areas with a moderate breeze.