Closing the gap13 April 1999
At Bonneville, the US Army Corps of Engineers will combine environmental benefits with efficiency. Suzanne Moxon reports
Bonneville lock and dam, spanning the Columbia river, is one of 21 dams the US Army Corps of Engineers operates in the Columbia river basin. Since 1938 the facility has played an important role in the region, but it is the dam’s two powerhouses which have always been the focal point. Although power generation is only one of Bonneville’s functions, it produces 5B kWh of electricity annually.
In the early 1990s the Portland District of the US Army Corps of Engineers, which built and operates Bonneville dam, assessed the condition of the first units, which date back to the 1930s. The turbine runners were wearing out and in need of replacement. ‘We decided then that it was time to modernise the plant and replace equipment,’ said Rod Wittinger, turbine expert at the Corps’ Hydroelectric Design Centre.
To appreciate the Army Corps’ actions a problem facing dams in the Pacific Northwest must be understood. Survival rates of juvenile salmon passing through turbines have been a growing cause for concern. ‘Fish can go downstream through the spillway, through bypass systems, or be transported around the dam, but still about one-third will pass through turbines,’ Tom Murphy from Bonneville Power Administration (BPA) explains. ‘You cannot steer them away completely.’ BPA sells the power produced at Bonneville and it also aims to improve fish passage. ‘Improved turbine passage is obviously more compatible with our power system and generation,’ Murphy explains. ‘With the Army Corps we are putting a lot of effort into turbine passage and making it a viable alternative to other passages.’
A difficult journey
With mortality rates for fish travelling downstream reaching 10%, the US Army Corps has embarked on what it calls ‘a difficult and contentious journey to protect fish stocks’. Along with other programmes the Army Corps decided to systematically assess the hazards facing fish passing through turbines. From these and future biological tests, a redesigned turbine could be developed. This would then be installed at an operational powerhouse and its effectiveness assessed during normal operating conditions.
Other targets for the Corps included increasing turbine efficiency to near 95%. Greater efficiency reduces cavitation, turbulence and shear flow which are haz-ardous to fish, making them disorientated and easy prey for predators downstream.
‘It also has been known for a while that a basic change in the design of turbines may improve fish passage and efficiency,’ Wittinger added. ‘Gaps exist between the turbine blades, hub and the discharge ring allowing leakage. Water can sneak out without producing energy and by closing the gap an increase in efficiency can be achieved.’ Closing the gap also means that fish are less likely to be trapped in this new turbine, designated the minimum gap runner (MGR). ‘If we can get the best of both worlds,’ Wittinger adds, ‘we will have a win-win situation.’ ‘The hypothesis,’ says Rock Peters, fisheries biologist at Portland District Environmental Resources, ‘is that by increasing the efficiency of turbines we will also increase the fish survival rates.’ The concept of an MGR is not a new one and similar turbines have been installed at Harry S Truman dam in Kansas. Cost, however, has always been an over-riding factor.
The Portland District of the Army Corps decided to use the refurbishment work at Bonneville dam as an opportunity to install and test the MGR. It was decided that MGRs will be installed on operational units and their performance assessed. Voith Hydro of the US was awarded the contract to produce replacement turbines for Bonneville’s first powerhouse. Working with Voith, the contract was modified to MGR turbines.
All Kaplan turbines have gaps between the moveable blades and the hub, and between the tips and the discharge ring which surrounds the outer perimeter of the spinning turbine blades. The MGR minimises the gaps at the hub and blade by matching the spherical shape of the hub and the interior curvature of the blade. The gaps at the top of the hub are eliminated by machining a notch in which the re-shaped blades can rotate. The gaps at the tip of the blade and spherical ring are minimised by similar matching of the shape of the blade to the outer spherical discharge ring. (If some gaps did not exist the edge of the blade would collide with the hub and the discharge ring when the blade angle changes.) There are reports that the MGR turbine may not provide the power of conventional ones. The Army Corps says that the MGR is handicapped by a factor called the 1% efficiency requirement. Based on field tests and the desire to increase fish survival, the National Marine Fisheries Service has mandated that, except in emergencies, turbines must operate within 1% of their best efficiency potential. This has the effect of reducing the maximum power output of an MGR generating unit.
Conventional Kaplans have a high efficiency over most of the operating range but a characteristic of the MGR is that its efficiency is uneven. So while at peak power an MGR may have the same capacity as a conventional turbine, because of the 1% rule, its operating range will be less. Although the MGR is more efficient, its maximum permitted power output will be less than conventional turbines.
Estimates are that at peak efficiency the MGR will be 1.5% more efficient than other new runners designed today, and 5% more efficient than the turbines presently at Bonneville. As Wittinger says, this will be significant over the plant’s ten units. The extra cost of the MGR— US$200-500,000 — will be offset by the efficiency gains. ‘The MGR does limit our general flexibility,’ Murphy from BPA admits, ‘and evidence does suggest that there will be constraints. But we are working to reduce these constraints while improving fish passage.’ As IWP&DC went to press the first MGR was being installed at Bonneville. The plan is to run the unit under normal operation before biological survival tests are carried out later this year. These will be conducted at an existing unit and the MGR unit in order to compare results. In the long term the Army Corps will replace all of the old turbine runners at Bonneville dam with MGRs, at a rate of one unit each year.