Solar Radiation and PV Generation

The installation of a Davis Weather Station in November 2019 with automatic data collection via Weatherlink means I can investigate how solar radiation measurements correlate with the electricity generation from our PV solar panels.

The solar radiation sensor on the weather station measures the sun's power (W/㎡) over the wavelength range 400 - 1100 nm which, coincidentally and fortunately, also happens to be the region of the electromagnetic spectrum used by silicon PVs to generate electricity. On this basis, we might expect the two parameters to correlate reasonably well.

On the other hand, the solar sensor is fixed in a south-facing position but pointing upwards (on the left above the small solar panel) whereas the PV solar panels are in two locations, one facing SEE and the other SSW. Some variability (scatter) in the electricity generation data might be expected depending on when the sun was strongest (morning or afternoon).

 In the graph below, I have plotted monthly PV generation data (kWh) against the Monthly Solar Radiation data (W/㎡).

An excellent correlation (correlation co-efficient = 0.96), probably better than I expected!

End-of-Season Polytunnel Harvest & Cleaning

September 20th and the nights are getting noticeably cooler. There are reports of night frosts from the borderlands of England & Scotland. Over the next 2 to 3 weeks, it is time to harvest the end-of-season cucumbers, tomatoes, chilli & sweet peppers and aubergines. [Note to grammarians: I tend not to use the Oxford comma in lists where there is no ambiguity). 

I systematically harvest each Quadgrow and Vegetable Planter as I work my way round the polytunnel. The central planters, filled with chillies, peppers and aubergines, are harvested first to free up space for dealing with the Quadgrows (cukes and tomatoes) around the edges.  Non-edible bits are shredded and added to my hot composting bins.  The coir-based 'soil' is sieved and bagged for reuse, mainly throughout the garden but also for potting up and growing winter salad.

By October 10th, the bulk of the harvesting is done.

Empty planters are hosed down ready for winter storage and scrubbed with Citrox P. The polythene cover is washed down with Citrox P and any repairs carried out; I've noticed a small hole in the roof - probably pigeons. Finally, the polytunnel will be fumigated using Garlic Candles to leave it ready for the next growing season. A small area will be reserved for growing salad leaves into the winter but it is important to start them now to make use of any autumn warmth and the limited and decreasing daylight hours.

 By October 15th, most of the work is done. The capillary matting used for feeding/watering the Quadgrows and Planters is hung out to dry on the aluminium frame (left-hand side). Once dry, any roots and soil can be rubbed off with gloved hands and the matting can be re-used next season (and maybe the season after that).

After dismantling the aluminium framework and cleaning out the water butts/pipework, it will be time to get out the seed catalogues and plan next year.

Off-Grid PV Installation

 Back in February (2021), we ordered a summerhouse (Shire Hampton Summerhouse 7x7) so we could enjoy the garden in all-weathers. It took about a week to build including a wood preservation stage.

During Spring, Summer and, now, Autumn it has been well-used especially during the morning and evening when not in direct sunlight (the doors face SW). By the end of August, the clematis (Summer Snow) had covered the roof and sides and the wood colour had mellowed.

We did install some solar LED lights but, whilst atmospheric, don't provide enough light for reading. So the next consideration was to add more power and this seemed like a good opportunity to install an off-grid solar panel and battery.

There are a number of companies selling PV packages for off-grid use. I selected a 185W system from Sunshine Solar available on special offer (£399 reduced from £599) which included the solar panel, charge controller, inverter, battery and all the necessary cabling. It also came with plastic side and corner mounts used to install the solar panels on a motorhome roof.

First, I had to clear the clematis from the roof of the adjoining shed as this is where the solar panel would be located, facing SSW and with a slight inclination.

A couple of treated timber planks were screwed to the shed roof and the PV panel positioned on the plastic side and corner mounts. I used a sealant/adhesive (SikaFlex EBT+) to fix the plastic mounts in position but you could drill holes and screw them down.

For the time being, the panel sits loosely on the plastic mounts; I think it is secure enough and, of course, it will be easy to remove/replace at a later date if necessary. However, I may screw it to the plastic mounts later using self-tapping screws.

The battery and charge controller are located in the shed supporting the PV panel; I had fully charged the battery using a battery charger. The inverter (with its 1 x 230V plug socket and 1 x USB socket) is on the backwall of the summerhouse. A few holes needed to be drilled through shed walls to connect all the wires.

The moment of truth came when the solar panel had to be connected to the system - the final act! The solar panel was covered with black plastic so it could not generate any voltage, the last two electrical connections were made and the black plastic sheet removed from the PV panel. The green lights on the solar charge controller came on showing everything was working as expected.

One final test, plugging the DAB radio in and listening to Radio 4 over lunch.

Overall, the installation was technically very easy with all the necessary cables (colour-coded or labelled) supplied. There are plenty of YouTube videos to help with the installation process. My one complaint is that no battery terminal fixtures were supplied so the current battery connections are a bit Heath-Robinson until I can get to Halfords tomorrow. I also need to optimise the settings of the Solar Charge Controller; a job for tomorrow after I've read the manual.

Final cost of the system was about £420.


Estimating Sunshine Hours using the Davis Weather Station

 Our Davis Weather Station (Vantage Pro 2 Plus) is equipped with both solar radiation and UV sensors. In principle, this should allow estimation of the number of sunshine hours over a specified period (e.g. per month, per year) using an empirical model derived from local data. Meteorological weather stations generally report sunshine hours as one of a suite of climatological data series.

I contacted my supplier, Prodata Weather Stations, to see if the manufacturer (Davis) offered this option via their software (WeatherLink). Apparently, they do but only using their Windows version of WeatherLink which I do not have. Furthermore, the Davis algorithm is relatively crude with a tendency to overestimate. Third-party software for Davis Weather Stations (e.g. CumulusMX) uses a more sophisticated method for estimating sunshine hours and maybe an option.

The standard instrumentation for measuring sunshine hours is the Campbell Stokes recorder, a real piece of art invented in the Victorian age.

The Ross-on-Wye weather station changed from the Campbell Stokes recorder to an automatic Kipp & Zonen sensor in 2019. For my purposes, the changeover came at the right time; I could just use the most recent K&Z results from Ross-on-Wye and not have to worry about converting CS sunshine hours to K&Z sunshine hours. The Davis solar radiation sensor works on the same principle as the K&Z sensor, as far as I can tell (both are pyranometers). Sunshine hours determined by the Campbell Stokes recorder and the Kipp & Zonen sensor are broadly comparable though the relationship is not simple.

The Davis radiation sensor measures the sun's power in W/㎡ and covers the visible and near infrared parts of the electromagnetic spectrum (400 to 1100 nm). The WeatherLink software records the 

daily average solar radiation which I have summed to give monthly solar radiation data for Hereford. These data are then plotted against the Ross-on-Wye sunshine hours (Kipp & Zonen) provided by the Met Office.

The correlation coefficient (0.862) indicates a good fit for a simple linear model to estimate sunshine hours in Hereford:

Monthly Sunshine Hours (Hereford) = 0.03*Total Monthly Solar Radiation (W/㎡) + 25

Interestingly, if UK sunshine hours (rather than Ross-on-Wye) are used then the fit (correlation) is better although the model (equation) is very similar.

The weather station also has a UV sensor monitoring the 280 to 360 nm region of the electromagnetic spectrum.  This covers both the UVA (400-320 nm) and UVB (320-280 nm) parts of the spectrum; some UVA is needed to produce Vitamin D while UVB is harmful (skin cancer). UV intensity is reported as daily average UV Index in the WeatherLink software - each day has a value of 0, 1 or 2. Daily values were summed to give monthly totals and plotted against the monthly sunshine hours for Ross-on-Wye.

Again, an excellent fit though there is a 'threshold' issue with winter months that record zero UV Index values but have non-zero sunshine hours.

In summary, I have a reasonable and simple linear regression model for estimating monthly sunshine hours using the daily solar radiation values recorded by the Davis Weather Station. In the future, I'll look at non-linear and multiple linear regression techniques to see if a better model is possible.


Davis Weather Station

 The wireless Davis Vantage Pro 2 Plus weather station was installed on November 8th 2019 and includes anemometer (wind speed and direction), temperature, humidity and solar/UV sensors and rain gauge. An AirLink air quality (particulate matter) detector was added on 3rd November 2020. Data collection are via the WeatherLink Live subscription service using the WeatherLink app for real-time observations.

All the equipment was bought from Prodata Weather Systems - a company with the highest quality customer service offering friendly and helpful advice.

Unfortunately, there is no ideal location in the back garden for the weather station. Located in a built-up area there is an obvious and observable urban heat island (UHI) effect; typically between 1 ℃ and 2 ℃ on the sunniest and hottest days of the year. The sensor suite has a standard passive radiation shield - there are options for fan-assisted radiation shields but these are significantly more expensive. The main sensor suite is about 2 metres high with the anemometer atop a pole approximately 5 metres above the ground.  The surrounding buildings provide some shelter from the wind but appear to have minimal effect on wind direction.

Current weather conditions can be viewed by clicking the cross in the top right-hand corner.

Dishwasher or Hand-washing? Which is better for the environment?


The internet has been awash (pun intended) with the revelation dishwashers are better for the environment than handwashing your pots and pans in the sink. This 'news' also made it onto one of my favourite Radio 4 programmes: More or Less. It seems the 'study' revealing that even a part-loaded dishwasher is more efficient resource-wise than handwashing came from a division of Proctor & Gamble that makes dishwasher consumables. This does not necessarily mean the report is untrustworthy but it should be treated with some skepticism until its claims can be justified or validated.

My first port of call for information on carbon footprints was Mike Berners-Lee's book "How Bad are Bananas?" I have used the data from the 2020 edition which is different to that given in the 2013 edition.

Things we might want to consider:

1. Carbon footprint: This is mainly down to the embodied carbon in manufacturing/maintaining the dishwasher plus the energy needed to heat the water. Dishwashers have an embodied carbon dioxide equivalent (CO2e) = 100 g per wash but they do use less, though hotter, water than handwashing. 
(i) Handwashing in warm water (30-40 degC): 360g to 3000g CO2e depending on whether you use the water sparingly (bowl) or extravagrantly (under a running tap). Most people will be somewhere in-between these extremes.
(ii) Dishwasher in eco-mode (50 degC) = 470g CO2e. Dishwasher in professional mode (65-70 degC) = 600g CO2e. These data include the embodied carbon.
Conclusion: If you are extremely frugal with the amount of water you use then the carbon footprint of  handwashing could be a little better than the dishwasher but I suspect for most people it isn't. These carbon footprint calculations assume a relatively long lifetime for the dishwasher (12.5 years) so buy a machine that is efficient and well-built and look after it.

2. Energy use: Predominantly the energy needed to heat the water. Dishwashers use hotter water (50-70 degC) but much lower volumes (50 - 90% less depending on the energy rating of the machine) and they only heat the water they need. Handwashing typically uses water from the hot water cylinder (probably at 50-60 degC) so the volume of water heated is much more than that needed for washing the dishes. Conclusion: Dishwashers, on average, use about 1 kWh per wash so are very economical to run. If you were to boil water in a kettle for handwashing, the same 1 kWh would boil about 15 litres, enough for 5 washing up bowls (see Water usage below) of warm water. Running a full dishwasher likely uses less energy but the difference will not be large. 

3. Water usage: As noted above, handwashing uses considerably more water than a dishwasher to clean the same number of dishes assuming you change the dishwater before it starts recoating the dishes with grease and food particles! According to Which, you can wash 2 standard meal sets (12 pieces of crockery plus associated cutlery) by hand in a bowl containing 9 litres - it is not clear whether this includes rinsing. A full-size and filled dishwasher will wash between 9 and 19 standard meal sets, depending on its efficiency, in the same amount of water (9 litres) and definitely includes rinsing.
Conclusion: The dishwasher wins hands-down. No contest! Important note: I know some people who pre-wash or rinse plates before putting them in the dishwasher. Never do this - just scrape the plates into the kitchen compost caddy.

4. Cleanliness/hygiene: Handwashing in warm water is less hygienic than a dishwasher (400 times higher bacterial count in the case of the former) because of the higher water temperatures, longer contact times and natural air-drying in the latter. Drying with a tea towel is only going to make things worse.
Conclusion: The dishwasher wins by a country mile.

5. Cost: A reasonable quality and efficient dishwasher can be had for £400 though it is possible to spend much more. The performance and energy efficiencies appear to be quite similar (e.g. using less than 10L water and around 0.75 kWh/wash in the Eco-mode) so you may be paying for longevity and ease of repair. Even if you use the dishwasher everyday for 10 years that still works out at 11p per wash for the £400 machine assuming no repairs. This will be much more expensive than buying a few washing-up bowls and pan scrubbers.
Conclusion: Even when taking in the lower running costs of a dishwasher, handwashing is clearly the cheaper option.

6. Time/Convenience: Handwashing may take six times longer than loading & unloading the dishwasher! That could be an hour a day spent on more enjoyable or productive activities. Most people I know do not go back to handwashing after regular use of a dishwasher. It generally takes several hours to run a standard wash programme on a dishwasher so you do need to make sure you have enough crockery and cutlery.
Conclusion: Dishwasher every time!

In summary, the dishwasher comes out on top especially if you only run it when full and use the eco-mode function. Handwashing is the cheaper option overall but loses out on energy use per wash, total water usage, cleanliness/hygiene, convenience and time. Potentially, the carbon footprint of handwashing could be lower largely because of the embodied carbon in the manufacture of the dishwasher; in practice, this advantage is lost by the way most people handwash. Basically, just choose whichever option best suits your lifestyle.

Finally, I devised a frugal method for handwashing, first used when camping, and now used at home also. Camp-site washing-up facilities typically comprise a row of large stainless steel sinks. We tend to wash-up as we go along so there are only the pots, pans and cutlery for two people/one meal to consider. For the washing-up liquid, I use an old spray deodorant bottle filled with eco-friendly washing-up liquid diluted 50:50; this makes it less viscous for ease of spraying and reduces the likelihood of overuse.

After placing the plug into the plug-hole, briefly pre-wet the plate/cup/pan under the tap and then spray with a couple of shots of detergent. Scrub with scrubber of your choice, rinse and repeat if necessary (e.g. for greasy frying pans). The water that collects in the sink can be used to pre-soak items. The concentrated washing-up liquid (c.f. that diluted in a sink full of water) cuts through grease and grime easily and is applied only where required. On completion of the washing-up, there is usually about one inch of dirty water in the sink and very little in the way of suds from overuse of washing-up liquid. This makes cleaning the sink, ready for the next user, quick and simple. It is not uncommon to see fellow campers filling a nearby sink (30 litres or more), adding a 'too large' squeeze of washing-up liquid and then leaving a sink full of suds for the next person!

In the end, whichever method you choose, you can sleep easy at night provided you do your best to minimise the resources you use. We use a dishwasher (full, eco-mode with environmentally-friendly dishwasher tabs) plus occasional frugal handwashing as that suits our lifestyle best.

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