In March 2000, Inversiones Pasabien S.A. completed construction of a 12 MW high-head hydroelectric project on the Rio Pasabien in the department of Zacapa in the Republic of Guatemala. The site is located approximately 125 km north of Guatemala City. Electricity is sold to INDE (National Electricity Institute) under a 15-year power purchase agreement.
System description
The Pasabien project develops a total gross head of 599.8 m between the upper level of the daily storage pond (Elev. 886.5 m) and the centre line of the pelton turbines (Elev. 286.7 m). The design flow selected was 2.5 m³/s and the total installed capacity is 12 MW.
The 36-m long, 10-m high dam creates a daily storage reservoir with a capacity of 125,000 m³. The full-supply level in the reservoir is 886.5 m. Flow out of the reservoir is controlled by gates in the dam. A sandtrap with a length of 65 m is located just downstream of the dam. The water level elevation in the sandtrap is 877.5 m (allowing 9 m for reservoir regulation). Water is transported from the sandtrap to the pipeline/penstock via a series of above-ground and sub-surface canal (tunnels). In total, there are four section of canal totaling 2,415 m and four tunnel sections totaling 1,685 m. A 1,175-m long, 44-inch (1.12 m) diameter pipeline with a wall thickness of 0.25 inches (6 mm) leads to the penstock which is 1,038 m long and varies in diameter from 44 inches down to 38 inches (0.97 m) over its length. The wall thickness of the penstock varies from 0.25 inches where it joins the pipeline up to 0.625 inches (16 mm) where it enters the powerhouse. The powerhouse encloses two 6 MW, 900 rpm, horizontal-axis pelton turbines directly connected to synchronous generators along with the associated protection and control equipment. Power is generated at 13.4 kV and transformed to 69 kV. The substation is located adjacent to the powerhouse. A 30-m tailrace returns the water to the Rio Pasabien.
Lessons learned
Land Tenure: Obtaining the necessary land rights for a hydroelectric project is fundamental to the success of the project. For the Pasabien project, inadequate land surveying information led to difficulties and added to the cost of obtaining land rights.
Tunnel to Pipeline Transition: Understanding the full extent of the geology at a site can be difficult and extremely costly. During construction of the Pasabien project unforeseen geotechnical problems were encountered at the transition between the end of the last tunnel, which acts as a forebay, and the pipeline. Poor rock conditions were found in the last 60 meters of the tunnel and required that the pipeline be lengthened and realigned.
Flood Flows: Adequately estimating the design flood flow is key to the success of a hydroelectric project. During the construction of the Pasabien project, Hurricane Mitch caused extremely high flows in Rio Hondo, which were much higher that the anticipated (design) flood flow. Luckily, only a small part of the dam and intake works had been completed and the resulting damage was limited primarily to construction equipment that was on-site at the time. Given the intensity of the event, it was decided to redesign the gates in the dam to allow for a significantly higher flood flow.
The big picture
The Pasabien project was developed soon after the Government of Guatemala deregulated the country's electricity market. Guatemala was experiencing an electricity supply shortage and in order to encourage private sector involvement, contracts with favorable terms and conditions were offered and negotiations were streamlined. Recently, however, the government has ceased offering power purchase agreements. Private sector projects can still be built but the power generated must be sold to an electricity distributor or wholesaler. In addition, environmental review and approval of projects has become significantly more complicated, time consuming and costly. As a result, development of new projects has slowed down significantly in Guatemala.
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