Gas turbines have enjoyed a huge surge in popularity in recent years for power generation, notably in the United States, and at the larger end of the market. But at smaller sizes, 50 MW or below, reciprocating engines look like an increasingly competitive alternative, even though the footprint of a recip tends to be larger than for a gas turbine installation of equivalent power.

Reciprocating engine technology has also made significant advances in the area of emissions, which has tended to be a big handicap in the past. With a modern lean burn engine, one can achieve NOx emissions of 250 mg/Nm3 (120 ppm) at an oxygen level of 5 per cent, which is half the requirements laid down in the current German clean air legislation.

For the burgeoning distributed power sector, plants typically have generating capacities in the 10 to 20 MWe range, they tend to be privately owned and good profit margins are a paramount consideration (which has not always been the case for large power plants in the past).

Reciprocating engines are suited to this sector of the market, increasingly so in today’s conditions, where lead times are getting shorter and the decision-making process faster.

Peaking power plants, in particular, must be built rapidly, with a typical project time measured in months, and there must be flexibility in plant size. This latter flexibility can be achieved with a multi-unit reciprocating engine power plant, such as the recently launched Wärtsilä Power Module. This can have an installed capacity ranging from 2 to 50 MW, with the incremental addition of power generation units designed to be easy when energy requirements grow. Time scales are also very short. A typical factory lead time for a 10 MW three-engine Power Module configuration is only about 80 days, with an installation time of 12 days.

Fuel and other costs

Another inherent advantage of reciprocating engines is high efficiency relative to gas turbines, with implications for fuel costs. Fuel costs, as, for example, reflected in the price of natural gas, are variable, fluctuating according to market conditions – radically in some cases, as we have seen lately.

The profitability of a gas turbine power plant is markedly more sensitive to fuel price fluctuation than a reciprocating engine plant. And recently we have seen the issue of fuel economy receiving more and more attention, even for peaking plants operated around 1500 hours per year.

The electrical efficiency of a typical simple cycle gas turbine in the 20 MW size range varies between 30 per cent and 35 per cent. However, a reciprocating engine with lean burn technology can reach an efficiency of 44 per cent. This means an additional fuel bill of 9 to 14 per cent for an owner of a gas turbine plant for the production of the same amount of kWh as the equivalent reciprocating engine plant – which amounts to a lot of money, straight off the bottom line.

As well as fuel cost, the production price of electricity in the power generation industry depends on investment cost, financing cost, and operations and maintenance cost. But seldom are factors like the effect of site altitude, of site ambient temperature and of operation on partial load properly compared when the decision on prime mover is made. Again, such considerations can often favour reciprocating engines.

The reference conditions according to ISO for gas turbines and reciprocating engines are different. Gas turbine nominal performance is calculated at 0 masl (meters above sea level) and that of a reciprocating engine at 100 masl. The reference ambient temperature for a gas turbine is 15 °C whereas the reciprocating engine reference ambient temperature is 35 °C.

Altitude and performance

Modern, lean burn reciprocating engines (eg the Wärtsilä 220SG gas engine) do not undergo any de-rating due to site altitude up to a height of 1500 meters. However, gas turbines de-rate power by 1.2 per cent for each 100 meters above 0 masl. This can constitute a big disadvantage for stationary gas turbine technology.

For mining power projects in South America it is not exceptional to have power plant sites at altitudes above 1500 meters, not to mention installations in a country such Mexico where the majority of the industrial sites are above 2000 masl. Another example is a mine in Argentina at an altitude of 4000 masl, where Wärtsilä has a successful gas engine power plant project.

The de-rating of gas turbines with altitude means that, for example, a 10 MWe gas turbine plant loses 18 per cent of power at an altitude of 1500 masl. In contrast, as already noted, the reciprocating engine need not be de-rated at all.

The de-rating has an immediate effect on the investment cost. An equivalent gas turbine plant at 1500 meters will cost 18 per cent more than at sea level, so reciprocating engines clearly have a big advantage when it comes to high altitude power projects.

Effect of temperature

During hot summer periods the gas bill can come as a big surprise for gas turbine power plant operators. At temperatures of 15°C or above gas turbine efficiency decreases. In contrast, the heat rate of a modern lean burn reciprocating engine does not start to de-rate until temperatures reach about 40°C.

For example, during a period averaging 30°C a gas turbine will lose 0.2 percentage points in efficiency per one degree centigrade of temperature above 15°C.

At 30°C this means a decrease of 3 percentage points in efficiency for gas turbines and zero for reciprocating engines. The 3 per cent may not sound like a big figure but cumulated over a long period it takes a respectable amount of money straight from the bottom line.

The Red Bluff installation in California (with 16 Wärtsilä 220SG gas engines of 2.8 MW each) runs during the hot summer months. The lower heat rate of the engines installed here has a big effect on the profitability of the installation.

The effect of load

Power plants are traditionally dimensioned for 100 per cent electrical load and prime movers generally achieve their best performance at full load. This means lowest fuel consumption per unit of electricity produced. However, in distributed power applications it is not unusual to operate with partial load.

The partial load performance characteristics of gas turbines and reciprocating engines are different, with reciprocating engines having the advantage. This is shown in the diagram below. At 80 per cent load, for example, the Wärtsilä 220SG lean burn gas engine has an efficiency of 41 per cent. In contrast, industrial gas turbine “2” can reach 32 per cent and gas turbine “1” reaches only 27 per cent efficiency.

The bottom line

When making the decision on choice of prime mover for a power plant, feasibility calculations should take actual site conditions – in particular ambient temperatures and height above sea level – into consideration, as these have a direct bearing on power plant profitability and the bottom line.

Reciprocating engines can prove to be a better bet than gas turbines at high altitudes and in warm climates. They are also less sensitive to fuel price variations and operation under partial load conditions.