Friday, 18 March 2022

How Safe is Solar PV?

 Putting the Numbers in Perspective

Comparing the fire risk from solar PV with that from common electrical appliances
Comparing the fire risk from solar PV with that from common household appliances

Recent research has advanced our understanding of the risk of solar PV fires.  The Building Research Establishment (BRE) and the Netherlands Organisation for Applied Scientific Research (TNO) published reports of their investigations of fire incidents involving solar PV installations in the UK and Netherlands respectively.

In this blog we take a look at the numbers and try to put them in perspective by comparing the findings from the reports with statistics for fires started by household appliances.

Fire and Solar PV Systems – Investigations and Evidence, Coonick et al, BRE National Solar Centre, 11th May 2018 (link)

Researchers from the BRE National Solar Centre investigated 80 fires in the UK that involved solar PV systems in some way, either because it was a potential source of the fire or because it was involved in a fire that started elsewhere. 

The solar PV system was found to be the source of the fire in 56 of the incidents.   Of these 22 were classified as serious fires (those that were difficult to extinguish and spread beyond the point of origination), with the remainder being classified as either localized or ‘thermal events’ (smoking, overheating).  The investigation looked at incidents that were both historical (33 that happened before the project start date of July 2015) and live (47 that occurred between July 2015 and February 2018).

It would be misleading to compare the number of fires caused by solar with the number of fires caused by other electrical appliances - because there are so many more of these.  To make the comparison fair we should take into account the number in operation.  I'll be using the number of fires each year per million systems in operation as the benchmark figure.

Taking the overall proportion of fires where the PV system was found to be the source of the fire (56 out of 80) and applying this figure to the 47 live incidents collected over the 30-month project length gives a rate of 13.2 solar PV fires/year.  

In January 2017 – half way through the study, the cumulative number of PV installations in the UK was 904,033 systems.

So our rate of 13.2 fires in 0.904m systems translates into 14.6 fires per million operational solar PV installations per year.

Brandincidenten met fotovolta├»sche (PV) systemen in Nederland. Een inventarisatie.  Bende EE & Dekker NJJ, TNO, 13th March 2019 (link)

The TNO researchers identified 28 incidents in the period 2015-2018 categorised into both residential properties and business (which includes both commercial buildings and ground arrays).  21 of the fire incidents occurred in 2018, and 15 of these were on residential properties.

The Central Bureau of Statistics (CBS) publishes data on the number of solar installations in the Netherlands.  At the end of 2018 there were 720,522 domestic installations, and 67,313 commercial installations.

The calculated fire rate for solar PV systems on domestic properties based on 2018 is therefore 15 in 720,522  or 20.8 fires per year per million systems in operation.

Comparison with Common Electrical Appliances

Data published by the UK Home Office on incidents attended by the Fire and Rescue Service in England breaks out the causes of fires, and from this we can get numbers that allow us to compare with the figures for solar PV.

Workings are detailed below, but what can be seen from the figures is that solar PV systems safety compares very favourably with that for electrical appliances that we wouldn't think twice about having in our homes.

It may be highly reassuring to say that a PV solar system is safer than a toaster or tumble dryer, but that doesn't mean that the solar industry should complacently sit back and do nothing.  Both reports highlighted common faults that can lead to fires.  In my next blog I'll be looking into these to ask what steps the industry can and should take to improve the safety of solar systems further.

Sources and workings on figures for electrical appliances 

In 2020 there were 27,292,000 UK households.  England represents 84% of UK population.  So let's assume 22,925,000 households in England.

98% of UK households own a washing machine, giving a number in use in England of 22.47m

91% own a toaster, giving 20.86m in use in England

58% own a tumble dryer, 13.30m in use

49% own a dishwasher, 11.23m in use

The Domestic appliance fires dataset was accessed from this UK government web page.  The average number of fires each year in England from 2010 to 2020 was used.  This figure is divided by the number of appliances in use (in millions) to get a rate per year per million appliances in use for comparison with the solar PV figures.

Friday, 11 March 2022

The Electric Arc and DC Solar Systems

If you cut a wire in an electrical circuit, current will stop flowing because the air in the gap is a very good electrical insulator so electrons cannot travel across it.

A electric field is formed between the positive and negative sides of the gap (which are called the electrodes), represented by the red field lines in the diagram below.

If you increase the voltage between the two electrodes, or narrow the distance between them, the electric field across the gap increases in strength.

If the electric field is strong enough, it will pluck electrons away atoms in the gas molecules in the field, causing the gas molecules to split into positive ions and negative electrons.  Both electrons and positive ions are attracted towards the oppositely charged electrode, so they accelerate off in that direction.  A current is now flowing in the circuit again.

But as the ions and electrons move in the field they can collide with gas molecules.  The energy from the collisions is seen as a temperature increase.

More electrons moving between the electrodes means there are more collisions with the uncharged gas molecules.  A chain reaction (or avalanche effect) occurs with more collisions leading to more charged particles and more charged particles leading in turn to more collisions.  We now have a high temperature plasma between the electrodes and this is a far better conductor of electrical current than air because of the greater number of charged particles available to move across the gap.

Temperatures are now so high in the gap that an intense white light is given off and the conductor material  may be vaporised away - an electric arc has formed.  

Even though the loss of electrode surfaces means the gap width increases the plasma's higher conductivity means that the current continues to flow.  A much lower voltage is needed to maintain an arc than is required to start it in the first place, an effect is well know in welding where 'striking an arc' refers to tapping the welding stick on the work piece and pulling it away.  The arc forms when the gap is narrow but is maintained as the stick is moved away to the working distance.

Arcs can be useful in applications such as welding, but undesired arcs are a potential source of fires with  the high temperatures in electric arcs easily capable of setting alight combustible material nearby.  Great attention is paid in electrical installations to avoiding arcs forming in the first place.

DC Solar Systems

There are two reasons why arcing is a particularly relevant consideration for solar systems.  

Firstly the voltages in solar systems can be very high compared to the Alternating Current (AC) supplied from the grid, and as we've seen the higher the voltage, the stronger the electrical field across a gap and the more likely is is that an arc will form.

In European countries AC electricity is supplied at 230-240V for domestic and small commercial buildings.  The most common electrical arrangement for the solar panels in a solar PV installation is to connect the panels (which might be 35V per 340Wp module) in a series string with the voltage increasing with each panel added.  For a 4kWp, 12 panel installation the voltage reaches 420V.  In larger commercial and utility scale installations voltages up to 1,000V or 1,500V are commonplace. 

Secondly, an AC electric arc is more likely to self-extinguish once formed because the voltage goes through zero volts 100-120 times a second (50 or 60Hz supply) and each time this happens the arc needs to re-establish.  Because the field strength required to start the arc is much higher than that needed to keep one going any increase in the arc length due to electrode erosion will mean that the arc will not re-establish.  By contrast the DC voltage in a solar PV wiring system is constant and the gap will need to open up much further before the arc is extinguished (this is why most forms of arc welding use DC current).

These factors mean that greater attention must be paid to arc risks in solar PV systems.  Despite this the safety of solar PV systems is very high, and this will be the topic of my next blog.