Back Down to Earth

Friends, at the end the last article I wrote:

The combination of 12 solar panels and a Tesla Powerwall battery has been sufficient for us to be practically off-grid for the last 3 months. And that will probably continue for another 3 months.

..it feels astonishing to be sustaining a good quality of life powered entirely by the Sun.

As we approach the summer solstice, I feel like I have reached apogee in a solar-powered rocket, and I am briefly floating weightless.

A week of miserable weather has brought me firmly back down to Earth.

After 87 days drawing no electricity from the grid, as the chart below shows, we have had to re-connect.

Click for a larger version. The graph shows daily electricity drawn from the grid (kWh) since November last year. After the battery installation, this fell to almost zero. Also shown is daily electricity used from the battery and solar panels (kWh). This has risen recently because electricity is now being used for air conditioning, cooking and domestic hot water.

We have now switched the mode of operating the battery so that it charges itself at night using off-peak electricity.

Solar Statistics: Summer Solstice Review

The summer solstice is probably a good point to review the performance of the solar panels installed last November 2020.

The £4200 system consists of 12 Q-Cells Duo BLK-G8 panels tilted at 40°. Six panels facing 25° East of South and six facing 65° West of South. A fuller description can be found here.

Click for a larger version. The graph shows daily solar generation (kWh) versus day of the year along with a 5-day running average. Also shown are two estimates for expected generation (kWh)alongside typical daily consumption.

The last 5 days have seen very poor generation. Last Friday 18th June, generation was just 2.3 kWh – more typical of mid-winter than mid-summer! And a battery with 13.5 kWh capacity is not big enough to see us through this dip.

Click for a larger version. The graph shows cumulative solar generation (kWh) versus day of the year along with a cumulative exports (kWh). Also shown are lines showing the estimated annual and semi-annual generation as specified by the installer.

Total generation so far this year is 1780 kWh – very close to 50% of the installer’s annual estimate.

The system has exported 590 kWh, my benevolent contribution to the grid, and I have used around 1200 kWh saving me around £250 compared to the situation without solar panels and batteries. If the panel’s performance is similar in the second half of the year, this would give a modest 3.5% return on my investment.

Carbon dioxide emissions 

Some fraction of this generation will have displaced gas generation which would have given rise to 0.45 kgCO2 per kWh, and some fraction will have displaced a typical generating mix which would have given rise to roughly 0.2 kgCO2 per kWh.

So depending on the assumptions made, my electricity generation has probably avoided emissions of between 350 kg and 800 kg of carbon dioxide so far this year, and will probably have avoided between 0.7 and 1.6 tonnes of CO2 by the end of the year.

The bigger plan 

The installation last week of the Air Source Heat Pump, a Vaillant Arotherm plus 5 kW model, together with a domestic hot water cylinder, marks the end of my investments in reducing carbon dioxide emissions from the house.

The ‘magic’ of the heat pump is that it uses 1 kWh of electrical energy to extract typically 2 kWh of thermal energy from the air, yielding around 3 kWh of heating.

This is central to reducing my carbon dioxide emissions. It has allowed me to replace the polluting gas boiler.

To compare carbon dioxide emissions with what what would have happened if I had made no changes, I have made a month-by-month estimate of household carbon dioxide emissions over the next 20 years.

These calculations are still preliminary, but the figure below shows their general form. It charts the anticipated carbon dioxide emissions if I had done nothing, alongside the anticipated carbon dioxide emissions in my plan.

Click for a larger graph. This chart shows month-by-month calculations of anticipated household carbon dioxide emissions based my current plan, or the do nothing alternative.

The green line shows an initial rise due to the 10.5 tonnes of carbon dioxide emitted during the manufacture of:

  • External Wall Insulation Boards (1.6 tonnes)
  • External Wall Mortar (1.0 tonnes)
  • Argon Triple Glazing (1.9 tonnes)
  • Solar Panels (1.6 tonnes)
  • Battery (1.4 tonnes)
  • Heat Pump (1.5 tonnes)
  • Air Conditioning (1.5 tonnes)

The green line then shows a much lower slope. The calculations indicate a break-even in terms of carbon dioxide by the end of 2023, and the non-emission of around 60 tonnes of carbon dioxide by 2040 when compared with the ‘do nothing‘ alternative.

So…

It’s disappointing to be back ‘on grid’ for a few days, but overall the solar panels are performing pretty much as anticipated, already avoiding the emissions of hundreds of kilograms of carbon dioxide.

And they are just one part of the plan. The installation of the Air Source Heat Pump is the last part of the plan, and I will now monitor the house to see if my expectations are fulfilled.

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