Energy in New Hampshire: solar irradiation

Showing posts with label solar irradiation. Show all posts

Sunday, August 6, 2017

Solar Power in New Hampshire – Part 2

Today we see solar power and especially photovoltaic (PV) technology everywhere: it is powering homes and businesses, roadside warning signs, large community applications, and even larger grid-scale operations. PVs generate electricity directly from sunlight using semiconductor technology that is built into the PV panels. The ever-increasing scope of PV applications ranges from small devices that generate tiny amounts of electricity used to power calculators (outputs in the milliwatt (mW) range), to one- or two-panel systems generating 100 to 300 watts (W) to charge cell phones and provide light (often installed in developing countries), to 2 to 50 kilowatt (kW) systems that power homes and businesses, all the way to grid-scale solar farms with ratings as high as 1000 megawatts (MW). Below are photographs of some solar installations that I have recently observed.

There are two kinds of PV systems: grid-connected and off-grid systems. In grid-connected systems, the excess AC (alternating current) output of a solar operation is fed into the electrical grid to supplement the power produced by other power plants. These operations usually do not include any storage so they can only generate and supply power to the grid during daylight hours. The supply from these operations is therefore highly variable: low in the mornings and afternoons, high at midday, and cloud cover significantly reduces their output. The electrical grid needs to be managed to adjust to this variable output. Most smaller residential solar systems in the US are grid-connected, and range from large utility-scale systems to smaller home-based units in which electricity produced during the day in excess of that used by the homeowner is fed back into the grid. These systems are often bidirectional: during the day, electricity is supplied to the grid; during the night, when no solar electricity is produced, power is drawn from the main electrical grid.

The other type of solar system is not connected to the main electrical grid. These are known as off-grid systems and are typically found in off-grid homes or in remote areas far away from the grid in developing countries. These usually incorporate batteries so that any excess energy can be stored for use during evening hours. During the day, the sun generates electricity that is used to power the site, while excess electricity is stored in batteries to provide power for the evenings. Off-grid systems are sometimes combined with other means of electricity generation, such as diesel generators, that can provide backup power during cloudy conditions or when the batteries are depleted. These are referred to as hybrid systems.

Some solar systems combine grid-connected and off-grid approaches. These have battery storage, but are also connected to the grid. Such operations generate some or all of the electricity needed by the homeowner or business during the day and any excess is stored in the batteries (as opposed to sending it out to the grid); however, the grid connection is there to provide any shortfalls in power production from the solar panels or when the batteries are depleted. These systems offer the best of both worlds—they produce and use renewable energy so their electricity purchases from the grid are reduced, but the grid is there as a standby to cover any shortfalls. Solar systems utilizing the much-touted Tesla Powerwall battery systems are of this type and I anticipate that we will see many more of these systems in the future.

In the energy field, one needs to be sure to understand what is meant by the rating of a power plant, whether it be a small residential solar system or a nuclear power plant. For most power plants, say the 1244 MW Seabrook nuclear power plant in NH, the power rating refers to the output of AC electricity. In this case, it is easy to calculate how much electricity a power plant would generate over a certain time period. For example, if the Seabrook plant was running at its rated output, uninterrupted for 24 hours, the yield of electricity would be:

1244 MW x 24 hours = 29,856 MWh.

Solar system ratings are different. The PV modules produce direct current (DC) electricity and the rating of solar PV operations is given as the combined DC output capacity of the panels under the standard irradiation condition of 1000 W/m² at 25^oC – conditions known as one peak sun (see an earlier post for an explanation of irradiation and the peak sun hour concept). As I showed in my previous post, the irradiation levels are only close to one peak sun at around noontime. A solar panel will therefore only produce its rated output of DC electricity for a short period around midday; at other times, the irradiation is lower and the output is commensurately lower. But DC electricity is not particularly useful for powering our existing homes and businesses: we have to convert that DC current to AC to operate our appliances, lights, and devices. During this conversion, there are losses through the electrical system and wiring. These losses are typically of the order of 5 to 10%. There are also performance losses due to dust on panels, degradation of the panels over time, and elevated temperatures.

Intuitively, one would expect hot sunny days to be ideal for solar power generation, but an aspect of PV technology not often appreciated is that the electricity output of PV panels actually decreases as the temperature increases—by approximately 0.5%/^oC. When temperatures are high, panels operating in the New England area can often reach surface temperatures of 140^oF (60^oC), which can cause a 10 to 12% decrease in performance. This problem is even more extreme in the sunny environments of Nevada and Arizona, where the choice of solar power may seem obvious. Furthermore, when clouds roll over the skies during the day, we can also expect a big decrease in electricity production.

Between the conversion, dust, degradation, and temperature losses, clouds, and the limited number of peak sun hours during the day, the AC output of a solar installation is, in fact, a small fraction of its DC rating. For example, the PV calculator from NREL shows that a DC-rated 1 MW solar plant in NH will produce, on average, 3.6 MWh of AC electricity per day. A 1 MW AC-rated fossil-fuel plant operating for 1 day would produce 24 MWh—almost seven times more electricity, This is an important distinction that is often forgotten. Size is important in energy production, but it is important to understand what the rated size means.

Speaking of size, the largest solar plant in NH is presently the 942 kW operation that is powering the wastewater treatment plant and other municipal buildings in Peterborough. Here are some interesting specifics about this plant:

It cost $ 2.4 million. Half of the funds came from the Renewable Energy Fund administered by the NH Public Utilities Commission; the remainder was funded by the developer and builder of the solar array, Borrego Solar.
The array is built on land previously covered by a holding pond at the wastewater treatment pond.
The plant consists of 3088 Canadian Solar modules, each with a rating of 305 W.
The project went online in November 2015.
The benefit to Peterborough is that there was no upfront capital investment and, per the power purchase agreement, the town buys all the electricity produced by the solar array at a cost of 8c/kWh (with a 1%/year increase for next 20 years). Previously, the cost for electricity from the utility was 14c/kWh. It has been estimated that this solar installation will produce savings of $ 250,000 to $ 500,000 over the 20-year term of the agreement.
Borrego Solar gets to sell the associated renewable energy credits and benefits from the 30% federal tax credit.
Based on the NREL PVWatts calculator, the annual output for the Peterborough system should be 1165 MWh; however, as noted above, the output will vary from year to year depending on the irradiation conditions, temperatures, and amount of cloud cover. In 2016, the annual electricity production was 1280 MWh, which was a little better than calculated.
The performance of the plant can be monitored online through a useful solar dashboard link.

Aerial View of Peterborough Solar Plant. Source: Peterborough

The chart below shows the panel temperatures and solar radiance levels for one recent day, 7/20/17, at the Peterborough solar plant. Even though the ambient temperature only reached 90^oF, the panel temperatures rose to as high as 140^oF. The periodic dips in the solar power and irradiance levels are due to passing clouds.

In my next post on solar power in NH, I will look at how the state is doing with respect to solar installations. I will also highlight some recent changes that NH homeowners who are considering solar should take into account.

Until next time, remember to turn off the lights when you leave the room.

Mike Mooiman

Franklin Pierce University

mooimanm@franklinpierce.edu

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Monday, June 19, 2017

Solar Power in New Hampshire - Part 1

In states such as Massachusetts, New Jersey, and California, there is a remarkable roll-out of solar energy generation operations underway, both large- and small-scale, driven by generous solar power incentives as well as improvements in technology and the falling prices of solar panels. In some respect, NH is arriving late to the game, but, as I will highlight in future posts, there are positive steps being taken. In my next series of posts, I discuss various aspects of solar energy, how NH is benefiting from its 2500 hours/year of sunshine, and how the state could further tap into this solar energy potential.

There are several ways to harness the energy of the sun:

Solar thermal uses the heat of the sun to warm up water so that it can be used for showers and other hot-water applications like washing.
Concentrating solar power concentrates the energy of sunlight by mirrors onto a focal point. The focused sunlight heats a fluid, which is used to generate steam, which then turns a turbine to generate electricity.
Photovoltaic generation of electricity through the use of solar panels is the most widely used and most promising approach to tap into the sun’s energy. Its application is growing exponentially in many countries and it represents one of the most significant resources of renewable energy.

Each of these applications requires sunny days and the direct radiation of the sun, so let’s start with some simple measures of solar radiation. The state of New Hampshire has ~2500 hours of sunshine and 90 clear days per year. In comparison, Nevada has about 160 clear days annually and about 3600 hours of sunshine. For a more exact determination of the power available from the sun, we use the concept of irradiance, which is a measure of sunlight intensity on one square meter of horizontal surface at a point in time. This, of course, changes during the day: it is low in the early morning; at a maximum at noon when the sun is directly overhead; and then tapers off towards evening. It also changes depending on the location, time of year, and amount of cloud cover. The typical average peak irradiance at noontime is 1000 watts per square meter (W/m2) for the planet. This value is referred to as the peak sun value. Measurements of irradiance averaged over 30 years for two days in June appear in the figure below. The yellow June 8th data show an essentially cloudless day, whereas the grey June 18th data show a day with a considerable amount of cloud cover. The drop-off in irradiance levels with cloud cover is significant.

Source: PVWatts

Irradiance is a measure of the intensity or power of sunlight at a point in time, but what we are really interested in is the energy that we can harvest over a period of time. In an earlier post, I explained the difference between power and energy. Energy is the amount of power expended over a period of time (an hour or a day). The mathematical relationship between Energy and Power is given by the simple formula:

Energy = Power x Time.

In the solar field, we determine the energy that can be potentially harvested over a day by calculating the area under the irradiance curve, such as the yellow area in the figure above. This measure of energy over a day is termed insolation; it is measured in kilowatt hours per square meter (kWh/m2). Another measure of insolation is to calculate how many hours of peak sun (with a fixed irradiance of 1000 W/m2) will deliver the same energy as the sum of the varying (irradiation x time) values over the day. The hours of peak sun per day is a particularly useful measure and is extensively used in the solar energy field to determine the daily output of electricity from solar panels. Insolation data are often available in tables of peak sun hours for different times of the year and different locations around the world. For example, the figure below shows peak sun hours for Concord, NH, at different times of the year. On average, the annual peak sun value for Concord is 3.9 hours.

Source:NREL

These insolation measures are for a panel lying flat on the ground, but that is not the normal orientation for most solar projects. In the northern hemisphere, solar panels are angled towards the south, so as to capture as much sunlight as possible as the sun rises and sets in the southern skies. Typically, the mounting angle of the panels is equivalent to the latitude of the location: panels in Concord, NH are ideally oriented at an angle of 43o from the horizontal. With the correct mounting angle, the average annual insolation value increases from 3.9 to 4.6 peak sun hours, an 18% improvement over a horizontally mounted panel. This is the average improvement for a fixed-angle array, but further improvements can be achieved by adjusting the orientation of the panels during the year. In winter, panels should have a higher mounting angle to capture the sunlight from the sun sitting lower in the southern skies and, in summertime, the panels should lie flatter to catch the rays of the sun when it sits high in the sky during the day.

Some solar arrays are very sophisticated, with intricate motor drives and control systems that can follow the sun from east to west during the day and also make small daily adjustments in the tilt angle to follow the sun’s seasonal orientation. Dual-axis systems, such as these, can boost the insolation by about 28% or more over a fixed-angle array; however, these systems are expensive and maintenance issues with the drives and controllers often occur. Solar panels are pretty cheap these days, so a 28% gain in efficiency can rather be captured by simply adding more panels and avoiding the maintenance headaches. For this reason, most solar systems are simple fixed-angle systems.

Let’s return for a moment to measures of solar insolation. We have been using units of peak hours, which are particularly useful when calculating the energy that a photovoltaic (PV) panel will generate. Another useful unit is the direct measure of energy per square meter, i.e., kilowatt hours per square meter, kWh/m2. We determined above that the average for a Concord, NH, array mounted at 43o was 4.6 peak sun hours. So, at a peak sun value of 1000 W/ m2, we can calculate the average annual insolation value as:

4.6 hours/day x 1 kW/m2 x 365 day/year = 1679 kWh/year.

Of course, higher irradiation values created by improved mounting angles lead to higher annual insolation values. To gauge the amount of solar energy that can be harvested, maps of annual solar insolation have been prepared for the entire planet. The map below shows the average annual insolation, in kWh/m2, for the USA.

It is clear that New Hampshire is not subjected to large amounts of high-intensity solar irradiation. The choice areas lie in the Southwest, but there is still sufficient solar radiation here in the Northeast to make it worthwhile harvesting.

In an earlier post, I noted that annual electricity consumption for New Hampshire was ~11 million megawatt hours/year (MWh/y). Using the average insolation value of 1680 kW/m2 as determined above, we can roughly calculate how much land area would be needed to generate New Hampshire’s annual electricity demand using the following assumptions:

PV panel efficiency: 15% (a typical value for modern panels)
Electrical and storage system losses: 50%
Panel coverage of land area: 50%

Based on these assumptions, we can determine that an area of approximately 67 square miles would be needed to generate sufficient energy to meet New Hampshire’s annual electricity needs—an area approximately 8 miles by 8 miles. This hardly seems much in state with a land area of 9350 sq. miles. The thought that it would only require ~0.7% of NH’s land area to generate all of the state’s electricity needs is an intriguing one; however, it is important to put this fun-to-do order-of-magnitude calculation into perspective and consider the technical and economic feasibility of this idea.

Let’s start with the size of the plant. Very large solar plants are being built today. There are several in the 500 MW range in the US and some larger ones in India and China. As of the date of this post, the largest solar power plant in the US is the BHE Renewables Solar Star operation in Antelope Valley, California. This is a 579 MW AC output plant, capable of generating ~1, 800,000 MWh of electricity per year or about 16% of NH’s needs. It uses 1.7 million solar panels and is located on 5 sq. miles of land. Technically, large-scale solar plants are feasible, but the costs are high. The cost for a 550 MW plant in California was reported to be $ 2.4 billion. The largest solar farm in the world is presently the Kurnool Ultra Mega Solar Park in Andra Pradesh, India. This monster has over 4 million panels, a nameplate capacity of 1000 MW, a cost about $1.1 billion, and covers a land area of 9.3 sq. miles. It is reported to produce 2.6 billion kWh of electricity annually, which is 24% of NH annual needs.

Cost is an issue, but the bigger problem associated with solar power is that it is an intermittent and variable resource. Unlike a traditional nuclear, fossil fuel, or hydroelectric power plant that can vary its output day or night (within a certain range), a solar resource can only produce energy during the daylight hours and is subject to the whims of cloud cover and passing storms – during which output can drop considerably. To fully utilize solar energy, we need electricity storage in batteries to provide power for the nighttime and when it is cloudy. Unlike grid-scale electricity generation from large PV plants, grid-scale battery storage is still in its infancy. It is complicated and very expensive. To date, the largest grid-scale battery-based storage operation is near San Diego in California – a 30 MW unit with storage of 120 MWh. To store just one day’s worth of electricity for NH would require some 30,000 MWh of storage. This is 250 times the capacity of the largest existing storage plant and is simply not technically and economically feasible at this time. In many respects, generation of electricity from sunlight these days is fairly straightforward; the storage aspect is the bigger challenge that we face this century.

My back-of-the-envelope cost estimates yield a $10 billion price tag for a solar and storage operation to supply all of NH's electricity needs and one full day of storage. This is certainly cheaper than building a nuclear power station and I am confident that with the falling cost of storage and solar panels, we will, within the next decade, be building solar and storage plants of this size. Perhaps one of these will be in NH.

This is the first in a series of posts about solar power and its application in NH. I trust that I have given you a sense of the resource available and the scale that is needed to harness it, but also a sense of the technical challenges, complexities, and costs involved in developing this resource.