After deciding to provide heat through electricity, it quickly became clear that the current generation capacity was not sufficient to provide electricity for heat pumps.

 

The next step was then to decide which renewable technologies would be the best to provide this extra generation. The technologies would have to meet some set criteria:

 

  • Cost effective
  • Reliable resource throughout the year
  • Mature technology
  • Minimal maintenance

 

We then identified seven different technologies which where examined more in depth to assess their potential. The technologies deemed unsuitable were:

 

  • PV
  • Hydro
  • Pumped Hydro
  • Offshore Wind
  • Tidal
  • Wave

 

The chosen technology was wind power, as discussed here. Details on why the different technologies was ruled out will be explained in detail below.

 

 

 

Photovoltaic (PV) panels are already installed on the Isle of Eigg, with a total capacity of 53.4kW. They complement the other renewable sources, particularly hydro, by providing a large portion of the total electricity output in summer. More detail on the available solar resource on Eigg and the panels themselves can be read here.

 

Heating is predominantly required in winter to combat the colder temperatures. Without sufficient long term storage, PV power is perhaps then rather unsuitable for supplying the necessary electricity for heat pumps in efforts to electrify heating on the island.

 

Nonetheless, it is worth analysing increasing PV via simulation in HOMER. To do this, it is useful to first simulate using the scenario with the lowest load ie. upgraded buildings with GSHPs and then work upwards if PV power proves successful. The table to the right shows how PV capacity was increased incrementally. Hydro and wind capacities were left as they are and so are not exclusively displayed in the table.

 

The first row is that of the present day microgrid (read about the current configuration here) which has, on average, a renewable penetration of 90% and has consequently been reduced to only 46% upon the introduction of GSHPs. The last row corresponds to a huge increase in batteries as well as converters to see what overall affect this would have.  The costs for PV panels, batteries and converters can be viewed in the cost analysis section.

 

The renewable penetration increases rather steeply with increasing PV capacity initially but then a great increase to 400kW only results in a small increase. This indicates that the renewable penetration must reach capacity somewhere around these values.

 

The renewable penetration increases relatively steeply with increasing PV capacity initially, but then a great increase to 400kW only results in a small increase. This indicates that the renewable penetration must reach capacity somewhere around these values.

 

A small increase up to, for example, 80kW shows a large renewable penetration increase of 5% and so may be worthwhile. However, looking at the graph opposite, it would appear that PV output still only makes up a small proportion of the total, especially in the winter months. Seeing as this is when electricity for heating is most needed, the PV output was overall deemed inadequate to justify expanding its current capacity.

 

 

Increasing the current hydro generation would be a desirable solution for providing more electricity. As hydro generates more electricity in the winter months, it would be a good match with the heating demand profile. Hydropower is also a well proven technology and usually is dispatchable.

 

The main reason hydropower was discarded is because of the available resource on Eigg. The largest river has already been built into a run of the river hydro scheme. There are however other rivers available on the Island, but the power output from these would be small compared to the 100kWh capacity of the current scheme - some private 6kW schemes have been built but their contribution is minimal. Also the smaller rivers are spread out across the island which would drive up the cost of cabling from the power production sites.

 

Because of these issues, hydropower was ruled out as a potential solution to increase the capacity.

 

 

As the biggest issue with run-of-river hydropower is the resource available, another solution is to create an artificial reservoir. This would serve two main purposes: It would store some of the rainwater, and it could also used to store energy by running the turbine in reverse in a pumped hydro scheme.

 

If using pumped hydro, the excess energy available in the current configuration could be stored and provide electricity for the heat pumps. However, the generation would have to be increased for example through wind to generate power to run the pumped hydro. Intermittency in the new resource would be eliminated, as power could be stored and used when needed.

 

The two biggest issues with pumped hydro are the environmental impact and the associated cost.

 

Pumped hydro would require the construction of a large reservoir on a high point of the island to which seawater would be pumped up. This reservoir would have to be man-made (as in the image to the right), or an expansion  of one of the small-volume reservoirs that already exist. If the latter, sea water would need to be pumped up such that the ecosystem of the existing small lake would be destroyed. Furthermore, most of the possible sites are located in SSSIs (Site of Special Scientific Interest), shown in red on the map to the right. This would make planning permissions hard to obtain.

 

The second aspect is the cost, and constructing such a reservoir would be very costly. Louis Breen carried out calculations in his MSc dissertation, and found that a reservoir with a volume of 3144m3 would be necessary to provide electricity for one day with the current consumption. (2) To provide heating through electricity as well the volume of the reservoir would need to double.

 

 

As such a small island, Eigg has potential offshore resource in close proximity to all residential areas. While mapping resources and narrowing the scope, it was decided not to take any of the following technologies further. Beyond the reasons listed below, ultimately this was a question of cost and safety.

 

 

Despite the extensive wind resource on and around the Isle of Eigg, it was decided that offshore wind would not be explored when scoping possible generation technologies. Reasons for this are predominantly scale and location. One offshore turbine would be enough to supply over 3000 homes, so 60 times as many as on the Isle of Eigg.(3)


Beyond this, with an average offshore wind turbine rated at 3.6MW, major changes to the infrastructure would be necessary to accommodate high voltages and ensure safety of residents. Maintenance would not be possible by members of the community, and so costs would be high throughout the turbines lifetime.

 

Finally, Eigg's location on the west cost gives little room for offshore wind, as the open spaces between its surrounding islands are key ferry routes daily in summer months, as illustrated right. This gives rise to further issues regarding safety and social impacts, and reinstates the decision to scope out offshore wind.


For interest, if  offshore wind were to be pursued in a higher-demand and higher-budget future, the only site options which may avoid ferry routes would be off the west coast of Eigg, between the island of Rum and Muck as highlighted opposite. This would be relatively inaccessible for maintenance,  but does have notable wind exposure.

 

 

There are two core means of exploiting tidal energy - tidal stream and tidal barrage.

 

Tidal barrage involves sectioning off an inlet of water, letting water flow in when the tide is high, maintaining it (similar to a dam), and then releasing it through turbines when the tide is low. There are very few natural tidal barrage opportunities worldwide, and the scale of Eigg simply could not withstand the civil infrastructure changes that are necessary to create a viable barrage. Similarly to the case in pumped/stored hydro, the environmental impact of flooding such a large area would be incomparable to other generation opportunities. (7)


Tidal stream, however, has notably high potential in Scotland. There are already explored usable tidal flows between mainland Eigg and Castle Island, as indicate in the image opposite. These resources have been explored, but are weak in comparison to further opportunities in the UK. Furthermore, castle island is located within view of the only ferry port (*), and the only shop on the island (x), and so there is consistently a lot of activity only meters away from where the tidal stream array would be.

 

 

In the scoping requirements, one of the core considerations was the exclusion of immature technologies. While wave energy is proven, it is still not a major component of the UK grid, and it is not mature enough to be affordable in locations such as Eigg. As seen in the image below, Eigg is relatively sheltered with regards to wave height in its surrounding waters. Therefore, it is unlikely that wave would be economically viable currently, and if the opportunity for research-based funding (using as a site for testing) were to arise, the location of Eigg would most likely not be favourable.

 

However, a survey carried out on the island in 2003 when they were building their microgrid initially showed that wave was in fact the most wanted technology on the island. If wave were to become a commercial, affordable and more practice technology then it would be ideal for the Isle of Eigg. (8)

Schematic of nearby Calmac ferry routes (4)

© University of Strathclyde | TEC Eigg | Sustainable Engineering 2016