Image credit: Modified from trafficticketteam.com
“Power cap worry” and “BLPC limit threatening” were two of the headlines a few months ago. They were raising the concern that the amount of solar PV that the Barbados Light and Power was accepting to flow onto the grid was too small. At that time it was capped at 7MW of installations but has now been raised to 9MW. Mind you, this is for residential and commercial installations. The current overall capacity limit for solar PV in Barbados is 9MW + 8MW (utility scale, in this case BL&P’s) = 17MW. With a peak demand of approximately 170MW why stop at 17MW (10%) for solar? Why not just go 100% solar right now? The foreign exchange savings would be tremendous if fuel did not need to be purchased, imported and burned. Without going into the technical aspects, in its simplest form it boils down to intermittent supply and constant demand. The following analogies may explain the idea better.
| Solar PV system | Start up business | Farmer | |
| Intermittent supply | Solar generation during the day and sunny periods | Income from new clients or sold products | Harvested crops at a particular time of the year |
| Constant demand | Regular electrical loads | Bills, expenses, payroll, mortgage | Buyer’s demand at a farmers’ market |
| Storage | Batteries | Bank | Freezer |
| Base load supply | Electricity production from other sources i.e. steam turbines and low speed diesel engines | Stable source of income | Supermarket |
| Perspective | Utility (i.e. BL&P) | New business owner | Buyer |
| Worst case scenario which is trying to be avoided | At the moment, if solar made up 100% of the island’s generation (without storage or source of base load power), the system could not meet loads at night or during long cloudy/rainy periods. | If all of the new business owner’s income is from a start up business, (without a loan, savings, or spouse’s income) cash flow is unpredictable and it can be difficult to pay expenses | If a buyer wanted to buy all their food fresh from a farmer throughout the year, (without storing any in a freezer or going to the supermarket) drought, excess rain or damage to the crop could limit the amount of food which could be bought |
As you can see from the examples above, these are not insurmountable challenges, we must simply learn from known areas and apply the knowledge to new ones. Storage and a diversified base load are key components. For the new business owner, having savings in the bank and a diversified income stream from various clients and products reduces the strain of paying bills and payroll from unpredictable cash flow. A proponent of farmers’ markets can maximize the amount of food purchased by storing some in a freezer, going to various markets and occasionally going to the supermarket if a particular harvest was low yielding.
How does this apply to the capacity limit for solar PV? From the utility’s point of view they have the responsibility of providing a safe, reliable and stable electricity grid to the people of Barbados. Over the course of the day electricity demand rises and falls creating peaks and troughs. This load profile must be carefully monitored and the necessary power generated to suit. If demand is low you don’t want to produce too much power, and vice versa, if the demand is high you don’t want to produce too little power. Back to the example of the farmer, if the demand for mangoes is 20 but he brings 100 to market they either go to waste or he stores them. On the other hand if the demand for mangoes at the market is 100 but he only brought 20, he cannot meet the demand alone and may have to ask another farmer to help him supply the need.
Part of the medium to long term solution for increasing the percentage of solar PV in Barbados is implementing storage and diversifying the energy mix. The cost for storage will continue to drop in the coming years at which point homeowners, businesses and BL&P can assess the best solution available. In the mean time, incorporation of other energy sources such as wind, biomass and waste-to-energy can be integrated into the energy mix of Barbados. By doing so, the effect of any one weakness is minimized i.e. long cloudy period affecting solar, high oil prices affecting our conventional generation, weather systems sucking the wind out of the region affecting wind turbines etc.
One near term solution to increase the capacity limit is to reassess the assumptions underlying the existing limit. For Barbados the solar limit appears to be 10% of peak demand, so 17MW. Some utilities have a 7% rule; a more common one is 15%. In other jurisdictions such as Hawaii and California, they have increased the percentage based on a revised model and new assumptions. The original thinking behind the rule was that in the event of a power failure the utility did not want local solar powering an area, but all grid tied inverters are built to a standard which prevents this. The utilities also wanted to prevent a situation where local electricity generation exceeded local electricity in a neighbourhood and flowed backwards further onto the grid. One glaring flaw in logic in the traditional calculation was that the minimum demand on a power line is usually in the early morning (e.g. 4am) which is at night when solar PV systems are asleep like everyone else! Simply put, the utilities were using a model that looked at the scenario where it was 4am and the electricity demand was low then all of a sudden, solar PV systems in the neighbourhood turn on and start producing power in the middle of the night.
Image credit: ILSR.org
Thankfully solar advocates in Hawaii and California, among other things, were able to convince their utilities that solar PV does not produce power at 4am and their caps were raised to 25% and 50% respectively; my cap is off to them. Hopefully Barbados Light and Power is moving in this direction. They are conducting a study due by the end of 2014 which will determine how much higher they can increase the cap.