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Photograph showing building work at Dinorwig Power Station. Dinorwig Power Station was built on the site of the former Dinorwig slate quarry. The photograph shows how the quarry landscape changed to become a hydro electric power station. The former ‘ponciau’ (galleries) of Dinorwig Quarry can be seen clearly in the photograph.

Following the closure of Dinorwig Quarry in August 1969 the quarry itself was sold at auction on the 23 June 1970 by local agent John Pritchard a Co. The quarry was purchased by Penrhyn Quarries for £19,000. During the same time the Central Electricity Generating Board (CEGB) was investigating possible sites for a new pumped hydro power station. Dinorwig was the firm favourite, and the land was purchased from Penrhyn Quarries for £940,000. Work began on building the new hydro electric power station in 1974. The scheme was built at a time when responsibility for electricity generation in England and Wales was in the hands of the government's Central Electricity Generating Board (CEGB); with the purpose of providing peak capacity, very rapid response, energy storage and frequency control.

The project – begun in 1974 and taking ten years to complete at a cost of £425 million– was the largest civil engineering contract ever awarded by the UK government at the time. The work was undertaken by an Alfred McAlpine / Brand / Zschokke consortium. When it was fully commissioned in 1984, Dinorwig Power Station was regarded as one of the world's most imaginative engineering and environmental project. Today, Dinorwig's operational characteristics and dynamic response capability are still acknowledged the world over. Dinorwig is the largest scheme of its kind in Europe. Dinorwig is comprised of 16km of underground tunnels, deep below Elidir mountain. Its construction required 1 million tonnes of concrete, 200,000 tonnes of cement and 4,500 tonnes of steel. The station's six powerful generating units stand in Europe's largest man-made cavern. Adjacent to this lies the main inlet valve chamber housing the plant that regulates the flow of water through the turbines. Dinorwig's reversible pump/turbines are capable of reaching maximum generation in less than 16 seconds. Using off-peak electricity the six units are reversed as pumps to transport water from the lower reservoir, back to Marchlyn Mawr.