Energy and power ➾
Rademacher, senior seismologist with Güralp Systems. “We effectivelymeasure this current as its value is proportional to the strength of themovement,” he adds. The seismometers also contain a
feedback systemthat reduces the movement of themass, so enabling the sensor to capture a wide range of strength of vibrations, “known as a broad band of frequencies,” says Rademacher. Each unit contains a transducer which,
in response tomovement of themass, feeds a current into the coil to force themass back towards its starting position. As the strength of the current is related to the force it is counteracting, readings ofmuch higher amplitude are possible before the mass is pushed to its limit, or “starts clipping,” as Rademacher puts it. Some 25miles away, at the Eden Project
site, a similar testing procedure will begin, it is hoped, in spring or early summer. The EGS teamwill be using between and eight and 10 seismometers deployed in boreholes 100mdeep, backed up by another six on the surface. As well as Güralp products, it will be using ceramic piezoelectric initially developed by EGS technical director, and a veteran of the Camborne School ofMines where research into geothermal reserves was pioneered from the 1970s onwards, Roy Baria. “With a piezoelectric
material, when it is compressed or deformed, it produces a voltage output. These have a tube or cone with amass sitting on top,” explains Baria. Any vibrations affecting themass will create greater pressure on the support structure, so generating an electrical signal. The lack ofmoving parts in these sensors is an advantage, says Baria of the sensors,made by British firmDJB Instruments, formerly known as D J Birchall. Both projects will also be
carrying out borehole imaging, putting a rotating ultrasonic scanner down the length of the well to examine the structure of the rock surrounding the hole. This Ultrasonic Borehole Imager (UBI),made by the likes of Schlumbergermainly for the oil and gas extraction industry, gives a highly accurate measurement of the hole radius allowing for plotting of its shape throughout its length. It also gathers the response of
an ultrasonic wave projected on to the well wall which shows up
The lessons being learned will be valuable in the development of
CO2 sequestration and nuclear waste burial
any changes in rock resistivity. Through analysing this data at a range of resolutions, the changes, which relate to fractures in the rock, can be displayed as images, even showing up the differences between types of fractures: open or closed, deep or shallow and natural or hydraulically induced. Having discerned temperature and
information about the rock’s fracture network, stress is the third factor to be assessed. This can bemeasured through the use of a straddle packer which is used to isolate a desired section of rock by inflating rubber bungs at the top and bottomof the desired space. “We then inject water into this space and pump it in until the rock breaks,” says Baria. By analysing how high and how quickly the pressure of pumped water rose, until it dropped with the rock’s failure, gives an idea of the stress. Again, both teams are likely to use this technique. Once completed and in use, conditions in
Ambitious plans: Ryan Law of Geothermal Engineering plans 25-30 power stations by 2035 of the kind he’s building near Redruth
the well such as pressure, temperature and flow,measured via an impeller, will be recorded during the severalmonths of seismicmonitoring required to accurately site the second well. For EGS, this is expected to be about 600maway fromthe first hole. As well as the technology and
testing regimes being shared by both projects, they also have much greater ambitions in common. Baria says the Eden Project will be an excellent demonstrator but the long term plan is to buildmuch bigger plants, up to 300MW, in Cornwall. Law hopes to have 25-30
geothermal power stations running by 2035. This is one of the reasons both companies are being partnered by one of the giant oil and gas companies, each of which is expected to make their involvement public before the summer. Geothermal Engineering has also been talking to an engineering group about a similar partnering arrangement. While this augurs well for the
future of geothermal power, and the companies supplying the test equipment it requires, there is cause for further optimism. The lessons being learned here will be valuable in further development of CO2 sequestration and deep nuclear waste burial, two technologies thatmay also finally leave drawing boards in the near future. ■
February 2011 ◆ Environmental Engineering ◆ 41
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