Geothermal powerplant in Ribeira Grande

Overview

Production well

4 MW turbine

Opcenter

Geothermia: the deeper you get into the interior of the earth the warmer you will find it. Average in Central Europe being 3°C per 100 m. In volcanic regions, tectonically active borders the crust of the earth is thinner and the distance to the hot magma less. That is where people started using earth`s own energy. They are using so called aquifers containing huge amounts of hot steam and water (hydrothermal geothermia).Ideally these are found between two impermeable geological strata. There should be feeding reservoirs of water on the surface, in this case a crater lake and the Atlantic Ocean. The temperature of the aquifer`s content should be above 150 °C (high enthalpy).
If you have enough steam at sufficient temperature and pressure (around 5 bar) you can send it directly into a backpressure turbine driving an alternator. In 1980 they installed the first trial plant working as described above. Plant at Pico Vermelho with 1 MW output (desired output 3 MW).After realizing that the technical problems were manageable and the output economically effective they started the originally planned installation in Ribeira Grande This is a so called binary plant which doesn`t use the steam directly. The hot steam goes into a heat exchanger where it warms up an organic fluid (in this case pentane). This because the reactions of this fluid are exactly known and manageable. In its gaseous state they send it to a backpressure turbine whereafter it is condensed, again pressurized and sent back to the heat exchanger. In the beginning they drilled 3 wells whereof one prooved to be ineffective and is now being used as a reinjection well. In the 2. phase they drilled an additional 2 wells (depths between 1500 and 2000m). Each of the four wells feeds one turbine (output 2x2.5 MW, 2x4 MW).The steam enters the plant at a temperature of 153°C and at a pressure of 5.1 bar. The reinjection temperature is still around 95°C because they haven`t found enough consumers for the rest of the heat so far. At the moment there are only a few greenhouses and a dairy making use of this cheap energy resource.
The 16 MW they produce in both plants are about 40% of the island`s electricity consumption. The plants availability is about 98% averaged over the years (compare that to a nuclear power plant).The problem of scaling in the wells was overcome by injection of a special inhibitor solution.
Further geothermal plants on the other Azore islands are under consideration and the capacity of Sao Miguel will be enlarged. 32% of Azorian electricity will be produced geothermal by the year 2005. The goal is to make this small and poor autonomous region a little more independent of world market vagaries.
Geothermia is a clean never ending source of energy. A power plant doesn`t use up much space, security problems are few, additional use of rest heat could be the heating of swimming pools, greenhouses, dairies and whole urbanisations in the neighbourhood.
Rebeira Grande is run by SOGEO SA an enterprise of suppliers of electricity of the Azores EDA.