Portugal turns to decentralized energy


Cogeneration in Portugal - installation at a ceramics factory

More than 80% of decentralized power production is currently produced by cogeneration plants.

From the beginning of 20th century, steam-condensing power systems and small hydro systems, running in island operation, were the main electricity suppliers to the first industrial plants in Portugal, namely in the paper and textile sectors.


Figure 3. Installed electrical capacity, 1990-2010. Source: DGE

CHP plants with steam boilers and reciprocating engines, replaced latterly by back-pressure turbines, were established from the 1930s, in all applications where technically and economically feasible. Cogeneration had reached 530 MW of installed capacity by 1990, dispersed through several sectors - see Table 1.

Table 1. CHP in Portugal in 1990
Industry Proportion of CHP capacity (%)
Pulp and paper 48
Petroleum refineries 23
Petrochemical 16
Chemical 4
Steel 3
Others (textiles, food, wood) 6

Some of these plants are being refurbished to increase capacity with combined-cycle gas turbine schemes.

During the 1990s, due to electricity high costs, some 64 cogeneration plants using diesel engines (with an average output of 5 MW and a total capacity around 350 MW) were built, mainly by companies which needed to compete in foreign markets. The projects were developed in industrial sectors as summarized in Table 2.

... by cycle design, cooling inlet air and cleaning the compressor

The efficiency of gas turbines is crucial to the performance of any power generation plant that uses a turbine. Efficiencies have risen steadily over the years, partly through the adoption of modified working cycles and higher operating temperatures. However, maintaining these efficiencies requires a well designed method of cooling inlet air and a programme of compressor cleaning - writes Carl Hjerpe.

The gas turbine is one of the most remarkable machines that mankind has created. A well engineered turbine can be extremely efficient and powerful while, on the other hand, it does not require many engineering errors to turn a gas turbine into a poorly performing machine with a low efficiency.

When the idea of a gas turbine was first outlined more than a hundred years ago there were strong doubts that it would work at all. The first person to give proof of concept was the Norwegian engineer Aegidius Elling who, in 1903, managed to build and run a gas turbine. All the power produced by the Elling engine was needed to drive the compressor - that is, the engine produced no useful power. However, the key thing to the Elling machine was that it did run. From there, any improvement in machine design would result in useful power being extracted. Moreover, an improvement would also mean that the machine would be able to show some level of efficiency.


A well engineered turbine can be extremely efficient and powerful
(GE Power Systems)

The efficiency of a gas turbine, or more correctly the overall thermal efficiency, is the ratio of mechanical work done to the heat supplied. In the case of the Elling machine, the mechanical work was zero, and the efficiency was therefore also zero.

Developments in gas turbine design and technology since then have led to ever increasing efficiencies of the machines. How far can efficiency go? This delicate question is set by the laws of thermodynamics.

The person to first outline this was the Frenchman Carnot who, in 1824, described the heat engine working cycle and its efficiency. The Carnot efficiency is defined as:

çcarnot = W = Tmax - Tmin
Q Tmax

where:
W = mechanical work
Q = heat supplied
Tmax = maximum temperature
Tmin = minimum temperature


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