Low/Zero-Carbon Office


Posted on Feb 4, 2014

I don`t want to use carbon offsetting, eg planting trees that nominally soak up some agreed amount of carbon/CO2 (for a while), but rather I want to genuinely and measurably reduce consumption and/or (micro)generate some of the energy that I consume, causing a net reduction in energy actually imported to the house each year. I`d already taken all my office lighting off-gridv by the start of this project in 2007/2008, so other than embodied manufacturing energy costs, I`m already zero-carbon there. The Internet Server is sometimes taken off-grid and powered from the system used for the office lighting (eg ~5% of December, ~10% of Oct/Nov) so I have put a slightly lower mean power (25W) than the measured 27W. As of the start of 2009 the separate SDSL router and WiFi are now one ~8W box (up to 10W with my laptop plugged into a LAN port), so daily consumption on the above metrics is probably below 1kWh/day. Thus my average daily energy consumption in the office is ~1. 1kWh/day, reasonably constant all year round. (This is down from ~16kWh per day before my efforts to eliminate waste. ) If I could attach fixed (non-tracking) panels to a completely unobstructed south-facing roof at optimal pitch (~37 °), and if I could regard the grid as effectively a 100%-efficient storage for power from a grid-connected PV system, and if for the southern UK I assume a year-round average of about 2. 47W/day per Wp of installed panel, then to cover the 1. 1kWh/day of consumption I`d need ~450Wp of panel. (PVGIS roughly agrees with this. ) The roof on our house is at about 23 °, but faces east on one side and west on the other, not south anywhere. This knocks about 15% off year-round collected energy requiring a boost of the panel to ~550W to cover 1. 1kW/day average. The collection reduction is especially savage in winter (~50%) when the sun is low in the sky and stays more or less due south. As discussed in Towards a Real LZC (Low/Zero-Carbon) UK Home and Saving Electricity it is probably not reasonable to regard the grid as a perfect energy store, since I will export most power in the summer when UK demand is low and can probably be largely met from low-carbon fuels such as natural gas and nuclear (as well as renewables such as other solar PV), so I probably end up preventing the burning of some natural gas. In winter, when I would need to import, UK demand is high, probably with a higher-carbon-intensity fuel mix, and more strain on (and losses in ) the grid. If I wanted to cover each day`s consumption locally without any imports even in mid-winter, I`d probably need at least 3kWp of panels on the roof (east- or west- facing) and a more complex system including at least about 3+ days` worth of energy stored for consecutive overcast days. (If the roof were south-facing, 1. 1kWp of panel would probably suffice. ) Thus, realistically, aiming to cover a reasonable chunk (but not all) in winter, and significantly over-exporting in summer to compensate for the difference in carbon-intensity of summer-displaced exported units vs winter/night-imported units (say 0. 22kgCO2/kWh for efficient natural-gas-generated electricity now and near-future projected UK electricity carbon intensity, vs 0. 43kg+CO2/kWh average UK electricity carbon intensity circa 2007), is probably a reasonable balance and close to carbon-neutral. A 1. 1kWp system, ie twice the minimum computed for year-round zero-electricity reflecting the export/import CO2 ratios above, is probably reasonable. This margin can also be taken to cover the embodied e


Low/Zero-Carbon Office
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