Crakehall Mill feasibility study.
Author: Dr. Bob Cattley
Summary
It is calculated that there is mean of about 7.5 kW of electrical power available at Crakehall Mill making 65.7 MWh / year.
This is worth about £21,000.00, with applicable subsidies.
The site would save approximately 28¼ tonnes of oil per year, calculated from: 0.43 tonnes of oil per MWh. Or 28¼ tonnes of CO2, calculated from: 0.43 tonnes of CO2 per MWh.
Site description.
The site is on Bedale Beck in North Yorkshire. It has been used as water mill for at least three hundred years.
Figure 1 shows a view of the mill.
Survey details
The difference of elevation between the intake (abstraction point) and the outlet from the works is 2.5 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 for design is 2.13 metres.
Water resource
This is discussed in detail in a later section. It is calculated that an annual mean flow rate of 0.639 m3/s is available. A maximum flow rate of 1.0 m3/s is envisaged.
Power generation capacity
At this preliminary stage it appears that 7.5 kW of electrical energy is available.
Figure 1, Crakehall Mill during winter snow
Figure 2, map of the site and surrounding area, grid letters SE, Ordinance Survey License 0000895956.
Abstraction point
This is an existing weir located at grid reference: SE 242 901 this is shown in figure 2.
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 (residual flow) is usually that which has a 95% probability of occurring (Q95).
It is anticipated that the Q95 flow will be maintained by a passive device (for reliability) in the weir.
Point of return
Water will be returned to Bedale Beck about 250 m from the abstraction point this is shown in figure 2.
Fish passing facilities
The weir is an existing structure that appears to present no difficulty to migrating species. Some repair is urgently needed to the weir and it may be possible to assist fish passing during these works.
Fish exclusion measures
Fish will be excluded from the main intake by screening with the maintenance of a suitable approach velocity and the use of pipe (as a passive fish deterrent) to convey the water to the wheel.
Fish will be excluded from the wheel pit by screening and, possibly, the use of pipes (as a passive fish deterrent) to convey the water from the wheel.
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. Due to annual weather fluctuations there will always be some uncertainty associated with flow predictions.
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.
Figure 3, catchment map used for LowFlows modelling
LowFlows Modelling results
Table 1, results from LowFlows modelling of the catchment (figure 3)
Figure 4, annual and monthly flow probabilities at Crakehall Mill
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 Upper Swale Drainage Board to work in the river 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).
Table 2 shows the design flow and power calculations for Crakehall Mill. The design flow is the difference between the mean flow (Qmean) and the residual flow (Q95). The power in to the system is calculated from the product of: fluid density, head, acceleration due to gravity and volumetric flow rate. The power supplied by the system to an external load is calculated from the product of the power input and the system overall efficiency.
Table 2, annual and monthly design flow and power calculations
System efficiency is the product of the efficiencies of each item in the system. At this preliminary stage it is not possible to allocate efficiencies to any individual item and an overall efficiency of 67% has been assumed.
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. 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 £0.199 per kWh on power outputs less than 15 kW.
Power exported to the grid is also subject to a £0.03 per kWh subsidy in addition to the market price.
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
Table 4, possible earnings and applicable subsidies
I can help with researching and obtaining any grants that may be available and the necessary work to claim the feed-in-tariff and other subsidies if you wish, but this will be an extra cost.
Suitable machines
In this case there are four possible hydraulic machines that could be utilised for energy recovery:
- Waterwheel.
- Propeller Turbine.
- Crossflow Turbine.
- Screw.
There are five designs of waterwheel the undershot, pitch-shot, low-breast-shot, high-breast-shot and overshot. With the head available, the undershot is not suitable as it would require a machine of about 10 metres diameter. Low-breast-shot is not practical because the wheel would be of the order of eight diameter. Most suitable for this site would be a high-breast-shot waterwheel. This shows high efficiency and like all other waterwheels is simple and easy to maintain. Fish can pass safely. Guarding of moving parts is important. The other designs of waterwheels are not as efficient as breast-shot designs but have special niche applications. A waterwheel positioned close to the present overflow could be made to look visually attractive as well as being an effective source of revenue.
Propeller or Kaplan turbines are small efficient and reliable machines with a long history (close to 100 years) of service. They fully utilise all the available head and can be designed to utilise most of the available flow range. They are, unfortunately, expensive and would not add to the visual attractiveness of the site as the fluid is constrained by the casing. A propeller turbine could be buried in an underground chamber and be invisible which may be an advantage depending on environmental considerations. Large fish cannot pass this machine safely small fish usually pass without harm. A propeller or Kaplan turbine would perform well in this situation but would be very expensive.
Crossflow turbines, also known as Michel or Banki turbines and incorrectly called after a German manufacturer, are compact simple machines. The efficiency is low and maintenance costs can be high. The crossflow turbine must discharge into the atmosphere and so must be above maximum river level, causing some loss of head. Fish mortality is of the order of 100%. Due to the low head on this site it is unlikely that a crossflow turbine would perform well.
Screw machines also called Archimedean screws cannot be recommended because they are theoretically flawed and to the writer’s knowledge none have reached the manufacturer’s performance claims. Fish mortality is thought to be low.
Conclusion
The existing waterwheel represents good practice in the 19th century. Due to its design and the dilapidated state of the existing mill gears (figure 5) power generation with the existing plant is not practical. The best plan for the mill is that it should be restored to produce small quantities of artisan flour for specialist bakers. The unusual product should attract a high price and the operating mill will be a tourist attraction.
Figure 5, close up view of the worn mill gear teeth
From tables 2 and 3 it is clear that the mean power output from the plant should be limited to 10 kW to maximise earnings.
I suggest that the optimum power generating installation would be a new breast shot waterwheel designed and made by Northern Millwrights installed close to the present mill pond overflow. This machine would have a diameter of about 3.5 metres and be wide enough to take 1.0 m3/s of water (2.0 metres). Figure 6 shows a CAD model of the proposed waterwheel.
Figure 6, proposed breastshot waterwheel
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.