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