Energy Matters: Not all solar panels are the same

It is now almost a year since I wrote my first column for the Bermuda Sun addressing topics that are relevant to how we use energy in our homes and businesses and looking at more global issues that can impact our domestic energy market. 

Over the year I have covered a wide variety of subjects ranging from how the cost of electricity is calculated on our local BELCO bills to how political unrest in other parts of the world can dictate the direction of Bermuda’s future energy profile.

I have received a lot of feedback from my readership, including lots of suggestions for future articles, for which I am very grateful, but what I have also discovered is that there are still some fundamental subjects that cause confusion for many of you. 

So today I think it would be appropriate to do a recap of some of these concepts. Without a doubt the term ‘solar panels’ is the one that causes the most confusion so let’s deal with that one first. 

There are two distinct types of solar panel. The first is what we call solar ‘thermal’ and the second is solar ‘electric’, also known as ‘photovoltaic’ or just PV for short. 

Once we’re clear on the difference we, can look at ‘net metering’ and how it is used in a solar PV system.

Solar thermal systems

Solar thermal panels, as the name implies, absorb the thermal energy (heat) of the sun into a matrix of copper pipes filled with fluid and enclosed in the panel beneath a glass plate. 

As the temperature of the fluid increases, it is circulated back to the water storage tank in the building, where the heat in the fluid is ‘exchanged’ into the water in the storage tank. 

The cooled fluid now returns to the exterior thermal panel to be reheated and so the process continues until all of the water in the storage tank reaches a preset maximum temperature. 

As water is drawn from the storage tank, it is replaced with cold water thus lowering the overall temperature and causing the heating cycle to recommence. 

Solar thermal systems are connected to the buildings water supply via copper piping from the storage tank to the (typically) roof mounted thermal panel(s) and usually use a small circulation pump to move either water or a heat exchange fluid around the loop. 

Thermal panels are significantly larger and heavier than solar electric panels and they operate most efficiently at the hottest times of year. Solar thermal systems are part of the buildings plumbing system and provide no electrical output. 

Solar electric  photovoltaic (PV) systems

Solar electric or PV systems consist of dozens of small individual photovoltaic cells that absorb light energy from the sun and use it, in conjunction with a semiconductor material (silicon), to create a small electric current. 

These individual cells are then connected together to make a solar module (commonly called a panel). 

The modules are then combined in series to form strings that are capable of producing significant electrical output. 

The strings are further combined into an array that can be sized to take care of some or all of the electrical needs of the building.

Solar PV systems are part of the building’s electrical system and are usually connected directly using standardized wiring. 

There are no moving parts in a PV system and they operate most efficiently during the sunniest times of the day/year. Unlike solar thermal, they do not require thermal energy and achieve their highest efficiency at lower temperatures.

Net metering

Net metering is a facility associated with solar PV systems that is used throughout the world to allow residential and commercial customers who generate their own electricity to feed any electricity they don’t use back into the grid (BELCO). 

It involves the installation of a special metre that can account for electricity flowing in two directions. 

For example, if a residential customer has a PV system and is working during the day it is likely the system will generate more electricity than the home is using. 

With net metering, the meter will ‘run backwards’, creating credits, while the surplus electricity is being ‘exported’ to the grid. In the evening, when there is no solar production, the customer will ‘import’ electricity from the grid, creating debits.

At the end of the month the credits and debits are balanced and the customer is only billed for their “net” energy use. 

Nick Duffy is the divisional manager for Bermuda Alternate Energy.

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