The TopHat turbine cycle

The TopHat cycle, as reported at VGB’s Düsseldorf conference in October last year, has been developed by Alpha Power Systems. The first commercial installation of the key system needed to implement the cycle, SwirlFlash, is due to come into operation in May 2001 at EPZ’s Amer unit 8, where its performance will be monitored for a period of twelve months.

Speakers from Alpha noted that the TopHat turbine cycle is based on the achievement of quasi-isothermal air compression by the evaporation of water in the compressor. An old idea, but never properly realised because the water must be in the form of extremely fine droplets if it is to evaporate during a residence time of only 1-10 milliseconds. An additional advantage is that the humidified air reduces the flame temperature during combustion, resulting in significantly lower NOx emissions. The power augmentation of existing gas turbines is 10-25 per cent,while the NOx reduction is 25-40 per cent. The retrofit is relatively cheap and costs less than 10 per cent of new installed capacity. The gas turbine can be designed for 57-62 per cent efficiency and a very high power density. The turbine is compact, cheap and does not need cooling water in the co-generation mode. It can be designed in the power range from 100 kWe to 400 MWe.

Kema and TNO in the Netherlands jointly founded the marketing and sales organisation Alpha Power Systems bv to bring SwirlFlash and TopHat technology to the market. The technology was developed by Kema with financial support from the Dutch utility industry.

Kema invented and patented SwirlFlash, which is able to provide an aerosol spray fine enough (about 2 µm drop size) for evaporation and air cooling to take place in the compressor. It is based on the supply of hot pressurised water to nozzles, situated directly in front of (or in) the compressor. As a result of evaporation cooling the compressor discharge temperature is reduced to such an extent that effective regeneration of heat from the turbine exhaust is possible. The Rankine cycle can thus be avoided, creating a very compact and flexible Brayton cycle with high efficiency and low capital cost.

Laboratory tests at Kema and tests at the Royal Naval Institute (in den Helder) with a 400 kWe gas turbine proved that the evaporation cooling took place as predicted. Based on these experimental results, detailed calculations have been made for an optimised TopHat configuration. It is concluded that an efficiency of over 60 per cent (LHV basis) can be achieved, even for gas turbines as small as 1 MWe. A benchmark test with six other cycles, all calculated for the same conditions, indicates that the TopHat cycle outperforms even a modern combined cycle while the capital costs are significantly lower.

The TopHat working principle

As already mentioned the TopHat cycle is a gas turbine cycle. The problem with gas cycles is that adiabatic compression of air requires a lot of energy. In addition to being pressurised, air is heated up, and pressurisation and heating together require approximately 2/3 of the energy generated during expansion. As a result, the efficiency of a gas turbine cycle is low. By using the gas turbine exhaust gases to produce steam in a waste heat boiler, the efficiency of this combined cycle can be increased. The disadvantage, of course, is the capital cost of building a plant which uses two cycles ie the open Brayton cycle and the closed Rankine cycle. TopHat addresses this problem by creating a mixed Brayton cycle.

The chief aim is to reduce the proportion of the generated work that is used for adiabatic compression. In most designs, inter-coolers are used to reduce the temperature, but heat exchangers are expensive and should be avoided whenever possible. By spraying water into the compressor and allowing the droplets to evaporate, a similar effect can be achieved. The challenge is to generate a spray of tiny droplets (typically 1 to 5 micron in diameter) in a quantity sufficient to cool the air.

Experiments showed that this can best be done by the so-called SwirlFlash technology. The water to be sprayed is pressurised and heated up. Then, as it is expelled from a swirl nozzle, explosive flashing takes place. The result is a surprisingly fine hot plume of water droplets, ready to evaporate as they enter the compressor. The amount of heat extracted from the compressor air by evaporation is much greater than the amount added through the hot water spray. As a result, the temperature drops and the compressor discharge temperature is reduced. The cool compressed air can be heated from the gas turbine exhaust.

If the water is recovered after recuperation, cleaned and fed back into the compressor, the TopHat cycle is created (Figure 1). Condensation of the water can take place in a heat exchanger for district heating. A simple representation of the TopHat cycle in co-generation mode is shown in Figure 2.

SwirlFlash technology

It has hitherto proved difficult to generate a flow of very small droplets sufficient for quasi-isothemal compression. The computer model “FLUENT” predicts that droplets of 1 to 2 micron must be produced if impact damage on the metal surfaces and the separation of liquid water are to be avoided. Following twenty-five years of basic research at the Technological University of Delft, an answer to this problem has been found. By using a swirl-spray device and by supplying pressurised hot water, the combination of spraying and flashing results in droplets roughly ten times smaller in diameter than the droplets of a normal swirl spray device. Figure 3 shows the results of spray tests in stagnant air. In the swirl flash mode the nozzle produces between 10 000 billion and 100 000 billion droplets per second.

The average droplet size in the cold condition (measured using laser diffraction techniques) is about 24 micron, in the swirl flash mode 2.2 micron. The droplet size distribution is amazingly steep; all droplets are smaller than 3.5 micron and all are larger than 2.0 micron (Figure 4). Since the droplets are very small and the droplet temperature is high, the evaporation rate is extremely high. Droplets this size and temperature tend to evaporate in only a couple of milliseconds, depending on the conditions. So evaporation takes place almost instantly in the inlet air (when the relative humidity permits) and in the first stages of the compressor. SwirlFlash technology differs therefore from inlet air chilling technology since an overspray of water droplets is created, intended to enter the compressor. This is of great practical value when the technology is used for retrofit of existing gas turbines; SwirlFlash technology produces 3 times more extra power than an inlet air chiller.

In order to test the evaporation rate, laboratory tests have been performed. A flow of hot air (100°C) was created with a velocity of 100 m/sec to simulate compressor conditions. Various types of nozzles were placed in the flow to assess the behaviour of the generated mist. Feed water pressures and temperatures were in the range of 100 to 250 bar and 150 to 250°C respectively. It was found that evaporation of the droplets took place in less than 3 milliseconds, which is approximately the residence time in a compressor. Depending on the amount of sprayed water and the RH, the air cooled down to 60-90°C. This complies with the temperatures calculated from the heat of evaporation minus the supplied feed water heat. The results were sufficiently promising to perform a test in a gas turbine.

A set of spray devices (21 nozzles) was installed in the inlet duct of a 400 kWe Centrax gas turbine at the Royal Naval College in den Helder, in the Netherlands. Over 30 tests were carried out in dry and wet conditions. The compressor discharge temperature decreased instantly by 25°K per per cent of injected water. The measurements show that 1.5 per cent injection of hot water resulted in 10 per cent power augmentation, 2 per cent (relative) efficiency improvement and 25 per cent NOx reduction. All water evaporated in the first stages of the compressor. A patent application for the combination of SwirlFlash spraying and air compression has been awarded in Holland and pending in 50 other countries.

Applications

Various market segments seem to be promising for application of the swirl-flash technology and the TopHat principle.

These segments are:

• the modification of existing gas turbines or compressors to reduce parasitic compressor work and increase output and/or efficiency;

• the design and manufacturing of a dedicated TopHat turbine for the dispersed power and heat market;

• solid fuel-fired gas turbines in TopHat mode, for example a biomass combustion-TopHat cycle.

Retrofit of existing gas turbines

The number of standalone or combined-cycle integrated gas turbines is growing steadily. A minor modification – adding a bank of SwirlFlash spray nozzles – with a major impact would be very attractive in this market segment. Adding approximately 2 per cent water to the mass flow would result in a significant increase in the gas turbine’s power output.

As an example, the impact of water injection in a Siemens V94.2 gas turbine is calculated. In Table 1 the results are summarised for dry operation and for the injection of 1 per cent, 2 per cent and 3 per cent of hot water (250°C) directly in front of the compressor. Inlet air is at 15°C, with RH at 60 per cent.

The second example is based on an ABB GT8 combined cycle (see Table 2). In this calculation the air intake is kept constant and the amount of water is limited to 4.2 kg/sec. Since the water must be heated-up in the waste heat boiler, the efficiency of this configuration will decrease slightly. This results in an overall efficiency decrease, but the output increases significantly.

Despite the efficiency drop the increase in output is still interesting enough to consider this type of modification. The investment costs for this modification are less than for a stand-alone gas turbine since the water supply and a demineralisation installation are generally available.

  The maintenance costs of the gas turbine are reduced owing to lower temperatures of the hot gas parts. The reason for this is that the available cooling air is about 60 degrees colder than the normal cooling air, while approximately 2.5 per cent more air is available. In addition the air contains water vapour, as a result of which the Cp has increased and the cooling effect is more pronounced. This results in an extended life of the hot gas components. On the other hand it makes higher turbine inlet temperatures possible. Such a modification, however, requires extensive discussions with gas turbine manufacturers.

TopHat turbines

Cogeneration of heat and power is a sound principle, but the low-level heat produced cannot economically be transported over long distances. This means that heat must be produced near to the point of consumption and that the power must be transported. New types of gas turbines must be developed, since small units cannot economically be used for combined-cycle generation. A TopHat unit would be suitable for applications such as power and heat production on offshore platforms, propulsion and power generation at sea, for power and heat production in hospitals, shopping malls etc.

Kema modelled and calculated the TopHat cycle to assess its potential and its properties. A benchmark test of six cycles was performed: the simple (Brayton) cycle, the combined cycle, the Cheng cycle, the ICR cycle, the REVAP cycle, the HAT cycle and the TopHat cycle. All cycles were calculated or assessed for the same conditions, ie inlet temperature of 15°C, firing temperature of 1200 °C etc. The pressure ratio of all cycles was varied between 4, 8, 12, 15, 20, 30, 40 and 50. Other important data for the main components were; a recuperator efficiency of 95 per cent, an air side recuperator pressure loss of 3 per cent, a flue gas side pressure loss of 25 mbar, a polytropic compressor efficiency of 88 per cent, a polytropic turbine efficiency of 87 per cent, a generator efficiency of 98.5 per cent and overall losses of 0.5 per cent of the heat input. In Figure 5 an overview of the test is shown. The figure shows a plot of the efficiency versus the specific power in kJ/kg of air. Based on this test it can be concluded that the TopHat turbine outperforms all cycles. The turbine can be constructed for the highest efficiency (57.4 per cent) with a specific power of 430 kJ/kg or for the highest power output (700 kJ/kg) and an efficiency of 55 per cent.

The market segment of larger gas turbines also offers very promising perspectives. As an example a calculation has been done for a hypothetical large gas turbineto assess the properties of a TopHat configuration. Table 3 shows the results, with a standard machine and a combined cycle for comparison.

For the TopHat the amount of injected water is maximised. As a result the power output has increased dramatically. It is obvious that such an amount of water demands a redesign of the gas turbine compressor. The values indicate, however, that the reward of such an effort is high. In addition to a very high power density, the NOx emission will be low without additional dry low NOx technologies. When the stochiometric adiabatic flame temperature is calculated for normal gas turbine operation and for quasi-isothermal compression with recuperation to 500°C, the flame temperature is found to be 300-400°C lower. This results in a much lower NOx formation.

The basic TopHat cycle can further be optimised to achieve efficiencies of over 60 per cent. This can be done by the use of effective recuperators in combination with a gas preheat. Another, completely different, option is to move to a condensing gas turbine. The idea is to maintain the expansion ratio of the turbine, but to expand from (for example) 16 bar to 0.5 bar rather than from 32 bar to 1 bar. In this configuration a water-cooled condenser is needed to condense the water vapour from the exhaust gases. The cold gases must than be re-compressed from 0.5 bar to 1 bar for discharging through the stack. In this configuration a smaller air compressor is used for the gas turbine in combination with an extra compressor after the condenser. Since the (wet) combustion air is now compressed to a lower pressure, the compression process will require less energy. The savings in energy will more than compensate for the energy, required by the flue gas re-compressor for three reasons:

• the quantity of gas to be re-compressed is less due to the much lower water content;

• the temperature of the gas to be re-compressed is about 100°C lower than the combustion air;

• more heat can be extracted from the exhaust gases.

Calculations indicate that an additional 2 percentage points of efficiency can be achieved. In combination with some other modifications the efficiency exceeds 60 per cent (LHV). Figure 6 shows the patented bottoming cycle integrated with the TopHat turbine, also called the condensing turbine.

Biomass-fired cycle

The essential elements of the TopHat biomass cycle are as follows:

• compressor cooling by evaporation of water (T~120 °C)

• air recuperation (T~475 °C)

• pressurised biomass combustion (p~8 bar)

• cyclone for flue gas clean-up at T~900 °C

• atmospheric biomass drying (T~110 °C). The cycle is based on biomass combustion at 8 bar, followed by expansion of the flue gas at 900°C in an uncooled expansion turbine. Proper use of the exhaust heat is made for heat recuperation of the combustion air and biomass drying. The principle of the cycle is illustrated in figure 7.

The calculations were based on a GE frame 3 gas turbine, which more or less fulfils the conditions for this concept. This machine compresses the air to 7 bar and expands the combustion gases at 900°C in a massive wheel. Air can be tapped off after partial compression. This seems to be an appropriate point for the injection of additional water, but the residence time of air in the compressor is sufficient for complete evaporation. The unit can generate 14.5 MW of electricity with an efficiency of about 42 per cent, based on the LHV of dry biomass. These are surprisingly high values for a gas turbine whose output and efficiency are normally much lower, 10.9 MWe and 25 per cent LHV, respectively. A number of points, such as the combustor and the feed systems, still require attention,. It is anticipated that the Torbed combustor, a spheroidal floating bed system, can be used as the biomass combustor. This device has relatively small dimensions and is able to process fairly large and irregularly shaped pieces of biomass. The Torbed combustor produces hot flue gas (950°C), which is the proper temperature for the expander. But it has never been tested in pressurised conditions and therefore needs additional development. Various feeding systems are available, but so far little experience has been gained in their use. Despite these uncertainties, the biomass TopHat is worth pursuing, since the combustion alternatives have a lower efficiency and the gasification alternatives are complicated and expensive.

Market conditions

The TopHat turbine is expected to comply better with the liberalised market conditions than the existing combined cycle. Utilities demand lower investment costs, higher flexibility (power by the hour), increased efficiency under partial load condition, reduced maintenance costs and lower emissions of NOx and CO2. The TopHat cycle fulfils most of these demands since the investment costs are around 20 per cent lower than a combined cycle of similar efficiency. In addition the TopHat turbine has a much faster response than a combined cycle. The TopHat cycle is simpler and cheaper; Table 4 shows a comparison of investment costs; the costs per kWe are 406 euro and 333 euro for the combined cycle and the TopHat cycles respectively.

It is evident that the TopHat cycle has no steam turbine, drums, deaerator, steam pipes, feed water pumps, evaporator and condenser. Instead of a waste heat boiler it has a somewhat smaller recuperator. The footprint of the plant is smaller and in the cogeneration mode the plant needs no cooling water, giving greater freedom in site selection.
Tables

Table 1. Calculations for a Siemens V94.2 gas turbine retrofitted with the swirl flash technology
Table 2. Effect of water injection and evaporation cooling in the compressor of an ABB GT8 combined cycle
Table 3. Calculation for a hypothetical large gas turbine, operated in the stand-alone mode, combined cycle mode and TopHat mode
Table 4. Comparison of investment costs, estimated by Kema for a 350 MWe plant