CIBSE Case Study: On Course
CIBSE Case Study: On Course
Back to CIBSE Case Studies OverviewArticle from the December 2014 edition of the CIBSE Journal written by Tim Findlay and Olly Paish.
The power of the River Derwent will help to generate 1.1 million kWh of energy as part of the refurbishment of Derby's 1940s council headquarters. Tim Findlay, of Hoare Lea, and Olly Paish, of Derwent Hydro, explain.
The River Derwent has long been a source of energy; it was the force behind the first water-powered cotton mill, at Cromford, in Derbyshire, which gave birth to the Industrial Revolution. Now, nearly 250 years later, Derby City Council boasts its very own hydro-electric power plant, having refurbished its 1940s Council House. The success of the project depended on overcoming a number of technical, legal and financial issues. However, the result is a building that produces enough carbonneutral electricity to gain a 25 and A+ Energy Performance Certificate, as well as a BREEAM Excellent rating. Here's how it was achieved.
The redevelopment of Council House involved the demolition of a 1970s extension, within what was the central courtyard. This was then in-filled, and a new floor built at roof level, increasing the floor area from 5,600m2 to 18,637m2, enabling all city centre council staff to be located on one site. The refurbishment featured a host of sustainable features (see box "The route to BREEAM Excellence"). Although the hydropower project at the adjacent Longbridge weir did not start at the same time, it became clear - from an early stage - that the two projects should be linked. This was not only because of their proximity, but also because of the significant value of feeding the generated power directly into a council-owned building.
However, the hydropower project took much longer than expected. Despite consultant Derwent Hydro conducting the feasibility study in 2006, and the council cabinet granting approval in 2007, the generator only became operational in March 2013. Over the course of several years, £150,000 of the project's £2m cost was spent on fees to get it through planning, largely because of negotiations with the Environment Agency (EA) - over the licence to transfer water from the river - which took until October 2009. Part of the problem was the lack of coordination of the various EA departments (permitting, fisheries, flood defence and ecology), so communication was required with all parties - simultaneously - to make progress. This delayed the project for almost six months.
Satisfying the EA's flood-defence team was the most challenging aspect of the negotiations. It was only achieved after the council employed Black & Veatch to carry out an open-channel, hydraulic flow modelling exercise, to assess the proposed building's probable impact on river levels - both up and downstream - during a flood. The EA required the model to use a flow rate corresponding to that caused by a one-in-100-year flood, plus a further 20%. The model showed that the presence of the hydro building had a minimal effect upon the flood levels, and that only 2m3/s from the 400m3/s flood flow actually crossed the site behind the building.
At the same time as planning approval was being sought, an unexpected legal issue arose. After the Feed-in Tariff (FIT) was launched, the business case for the hydropower plant incorporated payments available under this scheme for exporting power back to the National Grid. However, at a Local Government Association (LGA) workshop in 2009, it became clear that the Local Government (Miscellaneous Provisions) Act 1976 forbade councils from selling power generated by renewable means, unless it was generated in association with heat. Unless the law was changed, Derby council would have to give away or "burn off" all surplus generation. While the value of exported power was only about 8% of the business case, it was, nonetheless, a part that the council did not want to lose. It worked with law firm Eversheds and the New Local Government Network to lobby the government to change the legislation. The campaign was successful and, in August 2010, the law was revised to allow councils to sell electricity generated from renewable sources, such as wind and hydropower.
Run-of-river hydropower generally requires turbines that can operate on low-water heads.Archimedean screw and Kaplan propeller turbines are both able to work in such conditions, with screw turbines suited to flow rates of up to about 6m3/s. Kaplans become the more economical option from around 3m3/s. The turbine selected at Longbridge weir was a two-metre-diameter, double-regulated vertical Kaplan type, operating in a syphon chamber (see "Turbine Design" box). It has a design flow of 13m3/s, falling to a minimum of 2m3/s, and the design output is 230kW, dropping to a minimum of about 40kW.
To establish the turbine design, it was important to understand the considerable variation in the river's flow rate across the year (see Figure 1). While the peak flow briefly exceeded 100m3/s, the average was 18m3/s, and the minimum 5m3/s. There are abstraction points downstream from Longbridge weir, so the flow within the Derwent to these points is artificially maintained at or above 4m3/s by releases from reservoirs. This means hydropower schemes along the river will always have some water from which to generate power. The red line in Figure 1 represents the typical flow over the weir with the turbine operating, while the area between the two lines represents the total energy available to be captured and converted to electricity. The graph in Figure 2 translates the fluctuating river flows into a simpler format, showing the percentage of a year that any given flow rate is exceeded. It also shows how the head across the weir drops with increasing river flow. The two characteristics have been combined to form the anticipated turbine flow line. This shows it tracks the river flow - from a minimum turbine flow of 2m3/s, at a head approaching 2.8m; up to a practical design maximum of 13m3/s, at about 2.5m head; and back to zero when the head, in fl ood conditions, drops below 1m.
The area under the green line represents the available energy captured by the hydro. The original annual output forecast for the plant was 1.25 million kWh, but this had to be reduced to 1.1 million kWh when the EA insisted on a 12.5mm intakescreen gap, as well as a 40mm bar-spacing tailrace screen, both of which increased the parasitic head loss and so reduced the potential output. Total generation from March to the end of November 2013 exceeded 570,000kWh. Given that, during this period, there were the usual teething problems, an enforced two-week shutdown, and unusually low river fl ows, the forecast annual output of 1.1 million kWh does appear to be achievable.
Hydropower Project Team
- Building services engineer/architect/client: Derby City Council
- Project manager/cost consultant: Faithful and Gould
- Hydro consultant: Derwent Hydro
- Project continuity: Hoare Lea
- Main contractor: Balfour Beatty
- Hydro manufacturer/installer: Hydreo
Council House Project Team
- Client: Derby City Council
- Project manager: Mace
- Architect: Corstorphine and Wright
- MEP/BREEAM/Acoustic consultant/fi re engineering: Hoare Lea
- Main contractor: BAM
- Building services contractor: Emcor
The route to BREEAM Excellent
The Council House design team followed the usual carbon-reduction process of using less - and using it more efficiently - and employing renewables as much as possible. However, maximising the potential of the River Derwent allowed them to go beyond what many other projects can achieve. Using less involved: reinsulating the roof and walls; replacing windows with high-performance glazing; improving airtightness; adding solar shading; exposing thermal mass, using stack ventilation; and incorporating natural lighting via three atriums. A number of approaches were adopted to use energy and water efficiently. These include reduction of mechanical cooling; Turbocor compressors in the chillers; use of adiabatic cooling via the exhaust airstream and heat wheels; energy-efficient comfort cooling from displacement ventilation and ECDC fan-coil units; intelligent lighting - controlled by occupancy sensors - and daylight-linked dimming; power-factor correction; rainwater harvesting; and low-water sanitary ware. In addition to hydropower from Longbridge weir, the renewable technologies in the Council House project include: solar thermal panels for hot water; solar photovoltaic panels for additional electricity generation; river water for cooling the fresh supply air; and air source heat pumps.
The selection of a two-metre-diameter, doubleregulated, vertical, Kaplan syphonic turbine has a number of benefi ts. It can be started and stopped, easily - using a vacuum pump and air inlet valve - and does not need expensive, slow sluice gates, which are prone to damage and can be difficult to maintain. The double regulation of the turbine also means that the inlet guide vanes and the turbine runner blade pitch are adjustable. The control system modulates both, to hold the turbine speed at 1,000rpm. The gearbox steps up the rotational speed to 2,500rpm for the generator. Once it is started and synchronised to the grid, holding the runner at 1,000rpm maintains the synchronisation. An upstream water-level sensor in the intake canal is used to control the turbine water throughput, to hold a minimum 50mm water depth over the weir crest. This represents a minimum fl ow over the weir of about 2m3/s. The lead time for the turbine was a year, so the order was not placed until all the main permissions were in place, and the scheme viability was assured. There are very few turbine manufacturers in Europe producing turbines of the necessary type and size. The chosen supplier was the French manufacturer Hydreo.