Raven’s Nest hydropower feasibility study.
Author: Dr. Bob Cattley
Site description.
The proposed site is on Greenhow Sike. It is a remote hill farm in the Yorkshire Dales on the banks of the River Nidd, figure 1. An ancient weir is located in the sike close to the farm. The weir was probably built by 17th century prospectors looking for lead ore in the area. An abandoned and overgrown leat leads off to the northern side. The weir and leat was probably built to provide water for a method of prospecting using water to wash away strips of the top soil to reveal the underlying strata (hushing).
Greenhow Sike runs from the weir down a steep and wooded cliff a height of some 80 metres to the valley floor.
It is intended to run a pipe from the weir to a lower point at valley floor level where a small power house will be situated. The location of the outflow is indicated in figure 1.
Three phase electricity is available locally but it will probably be necessary to reinforce the local grid in view of the large output from this proposed power station.
Survey details
A survey to establish the location of the weir and possible power house location was undertaken on 15/9/2008 by RC and JRJ. The terrain is exceptionally difficult to survey because of its heavily wooded nature. Thick tree cover in Skrikes Wood makes GPS techniques impossible and the steep rocky cliff is very difficult to survey conventionally. The difference of elevation between the intake (abstraction point) and the proposed outlet from the works is 79.9 metres (gross head). There will be some head loss between the intake and outlet this is usually limited to less than 10% of the gross head at the maximum flow rate. The effective head is of the order of 73 metres. Careful design of the pipework will minimise head loss.
Figure 1, map of Raven’s Nest Farm and surrounding area (OS License number 0000895956)
Water resource
This is discussed in detail in a later section. The site owner has made a measurement of the head at the weir and calculated the flow rate, 0.4 m3/s. The water flow was measured in a way that underestimates the true flow. Modelling by LowFlows software would be the most appropriate way forward.
Power generation capacity
At this preliminary stage it appears that about 150 kW are available.
Figure 2, Raven’s Nest Farm from Yorke’s Folly, note the hidden valley in the foreground where the weir is situated
Abstraction point
This is an existing weir located at grid reference: SD 15171 63774. It is intended to repair and strengthen this weir for the proposed power station.
Depleted reach
This is the region between the abstraction point to the point where the water is replaced. Sufficient water must be maintained in this reach to maintain the various biological systems that have colonised it. The minimum required flow rate is usually that which has a 95% probability of occurring (Q95). The depleted reach flow will be maintained by the use of a passive device for reliability.
Outflow
Water will be returned to Greenhow Sike at a point close to SD 4151 4637, about 500 m from the abstraction point. The precise location of the power house has not yet been determined.
Fish passing facilities
It is thought that it is very unlikely that any fish will pass the steep falls on this site. Consequently, no additional fish pass arrangements are expected to be required.
Fish exclusion measures
Fish will be excluded from the intake works by suitable screening and the use of pipes to convey water.
The outlet works will be screened against the ingress of fish in line with Environment Agency best practice guidelines.
Flow estimation in ungauged catchments
The only 100% reliable method of ascertaining the flow in any river is to install a weir at the point of interest and to monitor the flow over it over a period of years. This is a very expensive and time consuming procedure.
The flow in a river is affected by many external influences besides the seasonality of the weather. For instance the surface vegetation of the catchment stores some of the precipitated rainfall and releases it over a period of time to the river. The porosity of the underlying rock strata affects the flow of surface water into a river. In limestone country the presence of sinks and resurgences affect the flow in local rivers. A lake or reservoir upstream of the abstraction point has a profound effect on the river flow. These and other factors make prediction of river flow from rainfall data difficult. A statistical method of flow estimation in ungauged catchments has been developed by the Centre for Ecology and Hydrology. A computer program called ‘LowFlows’ has been developed from this work. The program is used by the Environment Agency to estimate the flow in an ungauged section river for abstraction licensing purposes. LowFlows analysis of the site is contained in the following section.
LowFlows Modelling
Figure 3, Greenhow Sike catchment plan
Figure 4, annual, maximum and minimum flow exceedance probability curves
Table 1, Low Flows calculations summary.
Figure 5, annual and monthly flow exceedance probability curves
From figure 5 it can be seen that the annual flow rate is equalled or exceeded for seven months of the year.
Table 2, detailed LowFlows calculations for Greenhow Sike
Environment Agency Licensing
An abstraction license is required from the Environment Agency to run the hydropower installation. I can help with this if you wish, but this will be an extra cost.
A license is required from the Environment Agency to work in the river again I can help with this at an extra cost.
An impoundment license may be required from the Environment Agency for work on the weir again I can help with this at an extra cost.
Income and subsidies on the generated energy
Earnings
Any small hydropower venture is very sensitive to market forces and regulatory interference.
Power generated may be used on site; this is ‘a permitted use’ of the power for subsidy purposes. Generated power may be fed into the ‘grid’ or may be sold privately. The most profitable use of generated power is own use on site as it replaces energy supplied at retail price, about £100.00 per MWh (1000 units). Power sold to the ‘grid’ can only realise wholesale price, about £45.00 per MWh. Subsidies are payable on any permitted use of the power generated.
If the plant is to be connected to ‘the grid’ it is essential that the control facilities meet the requirements of the electricity distribution company and metering arrangements are approved by OFGEM (the government’s renewable generation subsidy administrator).
Renewable Obligation Certificates (ROCs)
The UK Government subsidises renewable electricity generation through the issue of Renewable Obligation Certificates two of these are issued for each megawatt hour of energy produced by small (less than 50kW) generators. The scheme is operated by OFGEM. OFGEM set a base price for a Renewable Obligation Certificate currently £37.10 each. There is an active market in these certificates and there are several dealers offering prices over £52.00 each. ROCs currently are traded at an average of about £45.00 each. Most electrical supply companies guarantee to purchase any ROCs at the OFGEM price i.e. at the bottom of the market. This is not an advantageous price. The subsidy is payable for any permitted use of the power (including own use). The electricity generated is most profitably used on site as it replaces electricity bought on the retail market (about10p/kWh) excess generation could be sold to the grid but would only realise about 4.5p/kWh. The scheme will change in April 2010 when a feed in tariff will be introduced that is intended to reflect the actual cost of installation and ownership of a specific renewable generation technology. ROCs are payable until 2027. With the introduction of feed-in-tariffs the value of a ROC will be £45.00.
Feed-in-tariff
The British Government have announced that from April 2010 it is intended to subsidise power generation from renewable sources by paying a feed-in-tariff in line with table 3. These will be paid until 2030.
Table 3, proposed feed-in-tariff
For the technology suitable for this site the feed-in-tariff is set at £0.11 per kWh on power outputs in excess of 100 kW and less than 2000 kW.
There is an additional £0.03 paid per kWh of electricity exported.
Renewables Levy Exemption Certificates (Renewables LECs
“On 1 April 2001, the Government introduced the Climate Change Levy (CCL) under the Finance Act 2000. It is a charge on non-domestic supply of electricity in the United Kingdom. This is the industrial and commercial supply of taxable commodities for lighting, heating and power by consumers in industry, commerce, agriculture, public administration and other services.
Electricity is currently (with effect from 1 April 2009) subject to the Levy at a rate of £4.70/MWh (subject to certain exclusions, exemptions, reduced-rate and half-rate supplies).
Renewables Levy Exemption Certificates (Renewables LECs) are electronic certificates. Ofgem issue them, monthly, to accredited generating stations, for each Megawatt/hour (MWh) of renewable source electricity generated. LECs identify renewable source electricity produced by accredited renewable generating stations.
Before it can receive LECs, a generating station must apply to Ofgem for accredited status.
Renewables LECs are part of the evidence required by HM Revenue & Customs to demonstrate the amount of renewable source electricity supplied to non-domestic customers in the United Kingdom. They are used by electricity suppliers to claim the CCL Exemption on non-domestic supply. Suppliers allocate Renewables LECs to a supply pursuant to a renewable source contract.” Taken from HERE on 15/11/2009.
Possible earnings
If the previously calculated flow rate is available for the full twelve months, then the site will produce 790 MWh of electricity per annum. This has a wholesale value of about £60,000.00. The subsidies will increase this to the region £180,000.00 as shown in table 4 assuming that the mean flow is available all year. The monthly totals at the bottom indicate a slightly different story; 738 MWh energy worth £55,000.00 to the grid with a total income of £169,790.00. This is due to seasonal variation and the statistical data derivation.
Table 4, possible earnings if the plant output is used to the maximum
Table 3 indicates that the feed in tariff could be higher for a plant limited to 100 kW output and the difference in earnings is indicated in table 5. It will be seen that the annual generation would be similar but the earnings would rise to £235,280.62.
Table 5, possible earnings if the plant output is limited to 100kW
However, it is unlikely that the annual flow rate could be maintained without some sort of water storage facility at the weir. This could cause serious planning difficulties. If the plant output is considered in monthly stages then the generation 655 MWH earning £49,000.00 for exported power and receiving subsidies that would increase this to £195,000.00 per year.
I can help with researching and obtaining any grants that may be available and the necessary work to claim ROCs and other subsidies if you wish, but this will be an extra cost.
Suitable machines
Determining a suitable machine is by two stages. The first is to use a plot based on experience of a hydropower machinery manufacturer. The head and flow rate or turbine shaft power must be known and these are entered in to plot such as figure 2.
In this example the head is 73 m and the turbine power is 150 kW and it will be seen from figure 2 that a Turgo turbine is the most suitable machine from the Gilkes range. A Francis turbine or a Pelton machine are also possible contenders as the duty point lies close to both envelopes. The ‘Hydec’ is a low cost Turgo derivative and the same comments apply.
Determining a suitable machine now requires some assumptions to be made. These assumptions are then refined when a suitable machine has been identified. Efficiency for the machine and a shaft speed must be assumed in order to refine the selection of a suitable hydraulic machine. This stage should be left to the next stage in the investigation.
The most suitable turbines are:
- 1. A Turgo turbine (supplied by Gilkes) installed at the power house, returning its water to the river close by.
- 2. A Pelton turbine (supplied by a third party or one designed and made by Northern Millwrights) installed at the power house.
- 3. A Francis turbine (supplied by a third party) installed at the power house.
Figure 6, turbine experience plot for Gilkes turbines
Preliminary conclusion
This site is one of the most promising undeveloped hydropower sites in the country.
It is the writer’s opinion that the effect of installing two twin jet Turgo turbines or a multi-jet Pelton turbine should be investigated to better utilise the available resource. The possibility of raising the weir two or three metres to store some water for later release should also be investigated.
Limitations on liability
Data in this report is believed to be accurate; it would be wise to cross check from reliable independent sources and to be satisfied of its accuracy before committing financial resources to the project. My entire liability for any breach of my duties, whether or not attributable to negligence, is limited to the fee that you have paid for this report. In no event will I be liable to you for any damages, including lost profits, lost savings or other incidental or consequential damages arising from your use of this report. Using this report for any purpose indicates acceptance of these conditions.
Dr. Bob Cattley, PhD. MSc.