It is important to note that the cost of laying any cable depends greatly on terrain of the location of interest. While our analysis of each generation location and route to the grid is site specific there are a number of points to make that can be applies across the board.

Stringing cables overhead is the easiest and cheapest method of running power cables to the grid. It has also been noted during the course of the project that planning considerations for power lines from renewable power in the current governmental approach is to not hinder such endeavors. This in combination with existing poles at Glen Almond make overhead cables a very attractive proposition.

In comparison, putting cables underground is a much more expensive solution. Prices, in fact, increase to 3 times that of overhead cables (using prices as of April 2010). For the second of the proposed routes from the site to the grid, there is a steep hill to contend with. If this route were to be taken by either method, the costs would increase significantly due to challenges of making the area accessible for the plant associated with construction.

We have compiled an initial spreadsheet of costing for underground and overhead transmission routes here for both 11kV & 33kV.

There are also fixed costs of connecting to the grid; for example circuit breakers and SCADA to the grid control room, which will be incurred regardless of the chosen method to make the connection. The supply voltage and space at the grid connection point will dictate the costs that are incurred. Higher voltages will incur higher prices. The trade off is that power can be distributed and transmitted more efficiently to the grid and over a greater distance as the cable losses are reduced by lower current.

Going underground

If there are non-pressured supply tunnels that offer a route to the grid and under-grounding cables is being considered, then the cost of cable required to go through the tunnel is similar to the full under-grounding costs.

However the logistics of delivery of the marine cable and installation appear to be a major barrier and would need much consideration. For example, the cable for the supply tunnel in Glen Almond would weigh approximately 60 tonnes. Abnormal loads across Scottish roads are governed by Transport Scotland [1]. Height is the main factor and within the Scottish road system, a nominal height limit of 5m exists. If we built a single drum that was 4m high (allowing 1m for the low loader trailer), the diameter of the drum would be approximately 5.5m. The cable could be delivered to the nearest deep water port, which for Glen Almond would be Rosyth, as delivery would come directly from the cable manufacture as all the main European facilities for sub-sea cable have deep water port access (located in Italy, Norway & Sweden).

Though this may seem to initially be, literally, a large obstacle to overcome, when it is compared with the costs of associated ground works of placing a 33kV cable underground (in the region of £65000 per km over accessible terrain) it becomes more realistic. This sets the benchmark for whether the logistics of sub-sea cable delivery are economically viable.

Conclusions and discussions about areas of further investigation can be found here.

_{Ref [1] - http://www.transportscotland.gov.uk/files/documents/roads/Guide-to-Regulations.pdf}