May 8, 2012 | Engineering + Technology, Solar Resource Measurement

When the lawyers and bankers get involved, you know you’re playing in a whole new ballgame.

John Goglia is the technical lead on solar resource assessment for NRG Systems

As Dave Simkins pointed out in a recent post, utility-scale solar is following a similar path as utility-scale wind. Costs of solar PV are coming down, technologies are improving, risk-averse lenders are investing more resources, and developers are getting more sophisticated about driving uncertainty out of their projects.

Last week, I had the opportunity to sit down with John Goglia, Senior Technical Support Engineer, to talk about trends in utility-scale solar and what they mean for the industry.

Can you walk me through the utility-scale solar development process?

It isn’t much different from the wind energy development process. First, a developer needs to acquire some land – or rights to develop that land – and determine if it’s suitable for a solar farm. This includes permitting, access to transmission, finding a buyer for the power, and understanding the solar resource. Historically, most developers did early stage prospecting by looking at solar irradiation satellite data available online. But today, more and more are doing onsite measurement.

What does an onsite solar measurement campaign look like?

Again, it’s very similar to wind resource assessment. Most developers measure for 6-12 months and then correlate to long-term satellite data to improve the accuracy of their estimates. The basic process is to develop a measurement plan, verify the accuracy of your sensors, document the site and its characteristics, collect the data, and work with an analyst to come up with a resource projection. Most developers use one solar resource assessment (SRA) system per solar farm, though if the farm spans many kilometers, they might use more. There is no hard and fast rule for this; it’s landscape specific, and based on their budget and risk tolerance.

Why is onsite measurement better than using just satellite data?

I like to compare satellite data to digital photos from ten years ago. When you enlarged those photos to 8x10, the quality was terrible – they were all pixelated. Using satellite data alone is like trying to find a 10-acre property using pixelated, fuzzy photos. Satellite imagery can provide fairly good landscape-level data, but it can’t focus on a particular solar farm. Using onsite measurement brings the pixels into focus. Once calibrated to on-ground measurements, satellite data becomes even more useful.

It allows a developer to add more value to their project. In finance speak it reduces the “supply risk” – the risk associated with the variability of the resource itself. Satellite data is based on models, not direct measurements, so there is error associated with it. It’s ideal to use onsite measurement in conjunction with the models.

How does the approach to solar resource assessment differ between utility-scale PV and thermal concentrating solar power (CSP)?

For utility-scale PV farms, it’s important to understand the Global Horizontal Irradiation (GHI): light that comes directly from the sun and light that is scattered throughout the atmosphere. CSP projects use mirrors to focus the sunlight on water, which creates steam, which runs a turbine. It’s a more complicated form of solar energy production because the mirrors must be curved and constructed just so. For CSP, it’s important to understand the Direct Normal Irradiation (DNI) – the light that only comes directly from the sun.

There are different types of pyranometers available to serve each market: one type is static, the other incorporates a mechanical device to track the sun. Measurements for CSP plants are very site specific.

What types of sensors are required for onsite measurement and what do they tell us?

Pyranometers – two on each system – measure the Global Horizontal Irradiation. They are attached to a small tower with booms that can be fixed at a 90 degree angle or tilted to place the pyranometers at the expected plane of array. Some developers refer to “plane of array measurements” as tilted GHI, which is preferable at latitudes further from the equator. The global standard for pyranometers, ISO 9060, provides guidance on sensor classification.

Solar PV operates most efficiently in temperate climates – excessive heat can damage the panels. That’s why wind vanes, anemometers, temperature sensors, and rain gauges are also included in an SRA system. Wind and water are cooling and cleaning agents for solar PV, helping to optimize production of the solar farm.

Where do you see solar resource assessment in five years?

For megawatt-scale development it’s going to be the standard, not just an optional practice. The financial community is going to ask for onsite measurement to reduce project risk, plain and simple. The more ground-based measurements developers collect now, the better. Not only can these measurements be used to improve the accuracy of satellite data, they can also improve the accuracy of resource estimates now, thereby reducing risk and cost for everyone involved.

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