Developing community scale zero carbon energy scenario’s

Scenario’s such as ZeroCarbonBritain and the analysis in Sustainable Energy without the hot air are often national scale energy scenarios, for the UK this provides the advantage of being able to consider powering a large bulk of UK electricity demand from offshore wind turbines, where wind speeds are generally higher and more consistent, or perhaps focusing particular energy crops in regions most suited to their growing.

Others suggest increasing the scale further to include Europe and North-Africa wide high voltage DC electricity grids to make it possible to harness North African desert solar power, Northern European wind and Norwegian hydro storage. Going even larger there are a number of scenarios that explore global super grids could be used to shift solar power around the world.

Many of these projects are relatively large scale and are perhaps in contrast to more decentralised ideas of a renewable energy supply with high degree’s of household and community ownership. This said there is no reason larger projects cannot be community owned. The Middelgrunden 40MW offshore wind farm off the coast of Denmark is a good example with 50% community ownership. Perhaps the ultimate result would combine benefits across all scales with a degree of national and inter-national grids as well as geographically more local onsite solar or community scale wind and hydro.

Building energy scenarios at national and larger scales can feel quite abstract, the numbers become so large as to become harder to grasp what they mean. When you multiply anything by a large number such as 26 million households it sounds huge and perhaps less feasible.

David MacKay in Sustainable Energy without the hot air breaks everything down into common units of kWh per day per person in order to make quantities more relatable “The kilowatt-hour per day is a nice human-sized unit: most personal energy-guzzling activities guzzle at a rate of a small number of kilowatt-hours per day. For example, one 40 W light bulb, kept switched on all the time, uses one kilowatt-hour per day” http://www.inference.phy.cam.ac.uk/withouthotair/c2/page_22.shtml

To a certain degree models such as ZeroCarbonBritain simply multiply up the energy consumption of a single household by the number of households in the UK to produce a national demand picture, transforming personal and household energy demands in kWh per day to Gwh/day and TWh/year.

An interesting and perhaps useful approach is to take an energy model such as ZeroCarbonBritain and scale it to the size of the geographical area that we live in or most identify with. This may perhaps encompass a city, neighbourhood, town or village. We can then better compare the size and number of wind turbines, solar panels, or the area of land dedicated to biomass energy production to our chosen geographical area.

Small town / village example: 1000 households

The following example builds a zero carbon energy scenario for a small town or large village of about 1000 households and a population of roughly 2400 people (2.4 people per household).

On the surface it may be too easy to dismiss or feel that the following scenarios as too costly or ambitious for such small areas, however that would miss the bigger picture of how much we spend on energy anyway, a quick back of the envelope calculation helps to put the cost of the measures below in some context. Assuming the current average UK household energy expenditure of £2500/year for electricity, gas and petrol/diesel as calculated in Appendix 1: UK Energy 2015. A small town or village of 1000 households will spend £62.5 million on energy over 25 years even if no changes where made and assuming no rise or fall in energy prices.

1. Using the default energy model

To start our example we will use the default settings in the 'full household energy model' which corresponds most closely to the ZeroCarbonBritain solutions mix, including:

For a small town of 1000 households, the model suggests that for all electric, heating and transport demand: 1.9 MW of wind, 1.9 MW of solar pv and 1.6 MW of backup gas turbines would be required.

The default model uses sabatier enhanced methane production using excess wind and solar electricity to produce hydrogen which is then combined with biogas from anaerobic digestion in order to produce a larger amount of methane for a given amount of biomass. Using this process 1 MW of electrolysis plant would also be needed and the equipment needed for the sabatier reaction in addition to the anaerobic digester.

The resulting land area requirements are:

If the village or town has an equal share of UK land area, the land area available would be 923 hectares, which would need to cover a variety of different requirements of which biomass energy is only one of them.

2. Without electrolysis and the sabatier process

The sabatier process used in ZeroCarbonBritian is not yet used as part of today's anaerobic digestion plants, what if we used standard AD instead of the 1 MW electrolysis plant and sabatier reactors? Choosing perhaps to retrofit these options on at a later date, as the technology is further advanced?

We can modify the model to explore the implications of this option by setting the electrolysis capacity to zero and putting a tick in the box for meeting unmet demand for both liquid fuel and gas from less efficient direct biomass methods.

The result increases the land area required for rotational grasses from 34 hectares to 60 hectares and the land required for miscanthus and short rotation coppice from 12 hectares to 20 hectares.

3. Biomass wood chip or pellet boilers instead of heatpumps

Another option could be to use biomass wood chip or pellet boilers instead of or in combination with heatpumps. If all heat demand was supplied with wood chip or pellet boilers the land area requirements change to:

We can see here that the demand associated with providing backup electricity for heatpumps was 21 ha of rotational grasses, with biomass boilers this has changed to 78 ha of short rotation coppice instead. Overall the amount of land required for heat has increased by almost 4 times.

The amount of wind and solar capacity required in such a scenario reduces to 1.2 MW of each and the backup CCGT capacity reduces to 650 kW. The size of the methane stores also reduce significantly from around 5000 MWh to 2000 MWh.

4. Heatpumps with direct biomass backup

In the dedicated section on renewable heat different mixes of biomass and heatpump heat are explored. One interesting option to explore in more detail may be bivalent air source heatpump and pellet boiler backup systems.

Here heatpumps provide 75% of heat demand when wind and solar energy is available, the biomass then kicks in at times of low wind and solar output and extreme cold temperatures. This could reduce the land area required for short rotation coppice in option 3 above from 78 ha to about 20 ha, providing a similar land area requirement to option 1.

Wind and solar capacities would need to be increased back up to 1.9 MW each as in option 1 however the backup capacity would stay at 650 kW.

5. Hydro and Solar

In mountainous areas hydro may be more suitable than wind, how much hydro would our example village or small town require? The difference in capacity factor results in a need for 1480 kW of hydro (45% capacity factor) instead of 1900 kW of wind (35% capacity factor).

6. ?

How would you change the scenarios above?