RANOTOR

Modern High Performance
Small-Scale Steam Power
( SSSP )

1. Background

In applications with a small power output range below 1000 kW as automotive vehicle and stationary electricity generation, power cycles such as the internal combustion engine ( ICE ) and gas turbine ( GT ) is used today. When discussing environmentally friendly power cycles for the future also fuel cells technology is mentioned. However, steam power cycles have many inherent physical qualities that unfold new possibilities to realize cost effective and environmentally friendly power cycle for many different small scale power output applications. Steam power cycles can be utilized as a self-contained ( stand alone ) power system offering several competitive features compared to ICE, GT and fuel cells. However, steam power is not only a competitor to ICE, GT and fuel cells but is also interesting to combine with the other power cycles and form hybrid ( Combi ) system offering high efficiency, flexibility matching electricity and heat demand , and backup facilities .
When the power output is below some 1000 KW the most proper expander type is the reciprocating piston engine and in order to implement the competitive Small Scale Steam Power ( SSSP ) cycles the old steam engine has to be updated. Novel high performance steam engine system has a potential to offer attractive SSSP for mobile as well as stationary Combined Heat and Power (CHP) applications.

2. Why SSSP ( Small-Scale Steam Power ) ?

The following gives a brief description of why SSSP should be expected to be interesting as future prime mover.

2.1 Fuel flexibility

Like the Stirling engine the SSSP has external combustion and offers great fuel flexibility compared to the ICE and GT and the fuel cell. SSSP is also expected to offer advantages when harnessing solar energy in solar thermal power application. The multi fuel possibilities make it possible to use locally available energy (solar, bio fuel) and realise autonomous distributed off-grid power system.

2.2 Exhaust gas and noise emission

The exhaust gas emission has a potential to be very low. Due to the absence of preheat or pre-compression of the combustion air, the combustion temperature will be lower than for other power cycles which imply lower NOx emission. NOx emission will almost be eliminated without any catalytic conversion. By coating the steam generator surfaces with an oxidation catalytic layer other emissions such as CO and HC can be reduced in a cost effective way. The SSSP has also a potential to be very silent and vibration free power source, and hence offer an environmentally friendly power source that can be placed close to people in mobile applications as well as in buildings.

2.3 Energy consumption

The modern steam engine has a potential to offer low energy consumption both as a self contained stand alone system as well as a bottoming cycles in hybrid systems. The main reason for that is the high part load efficiency. In almost all applications ( automotive and stationary ) part load efficiency is more significant than full load efficiency. The advantages of high part load efficiency will be especially pronounced in small scale distributed autonomous off-grid CHP (Combined Heat and Power) and most mobile applications. RANOTOR R&D Corp. is expected to achieve about 32 % at part load in a modern steam engine, which is almost 3 times higher efficiency than a conventional ICE operating at part load.

2.4 Investment costs and power density

The power density for a modern SSSP can be very high (1200 kW/litre displacement volume ). Besides high power density for the steam engine proper also compact heat exchanger approach for the steam generator and the condenser will together imply low weight to power ratio ( 0.5 kg/ kW peak power) for the complete SSSP, and hence a competitive price ( $/kW peak power ) is expected. Further more , by only designing and supplying different burners the same basic SSSP system can be used at many different markets depending on the local fuel supply No after treatment of the exhaust gas is necessary. Silent and vibration free, that is, can be integrated close to the user without mufflers and suspensions.

2.5 Maintenance needs

The modern oil free steam engine power system will be similar to a heat pump and refrigeration system where the working fluid is well defined and operates in a closed loop. The steam engine proper is therefore expected to have a long life time without any maintenance needs. The primary energy input will be conversed to mechanical energy ( electricity ) and almost all of the rest 60 - 65 % will be rejected as heat in the condenser. The heat losses occurring in the steam generator rejected as exhaust gas is less than 5 % and at low temperature and no expensive exhaust gas system is involved which has to be replaced. In CHP applications where heat losses are utilised all heat losses are harnessed in the condenser. In order to collect and recover all heat losses from an ICE- engine several heat exchangers have to be used as for instances an exhaust gas heat exchanger , oil cooler, coolant water and intercooler. Further more , the condenser where the waste heat is collected operates with working fluids (condensing steam and water ) in closed loops which eliminate fouling problem. The only components involved in the SSSP which is expected to require maintenance is the burner and to some extent the steam generator.

2.6 Torque characteristic and momentary respond

The steam engine will provide very high torque already from zero shaft speed and with a proper designed engine, gear box can be eliminated in automotive applications. A steam engine will provide instant response to power demand if steam is available without any lag and delay . Modern compact steam generators and a so called steam buffer will promptly provide the steam engine with steam. The torque characteristic is excellent for automotive applications with high low end torque and an “elastic” torque profile. The prompt response is important also for CHP applications

2.7 Hybrids

SSSP makes it possible to realise hybrids (combicycles ) with high efficiency at significant load. Hybrids also offer APU ( Auxiliary Power Units ) features in automotive applications and backup features in CHP applications. Hybrids offer also higher flexibility to meet mismatch between electricity and heat demand. The SSSP can act as bottoming cycle together with almost any kind of prime mover as ICE and the development of the ceramic gasturbine with higher temperature should justify the micro bottoming cycle concept further. SSSP is also interesting as bottoming cycles together with the emerging high temperature fuel cells.

3. New Steam Power technology - RAN concept

The RAN-concept is developed by RANTOR R&D Corporation. The “RAN” concept involves some components that are crucial when improving performance and economics further for SSSP. Worth mentioning are;

• Oil free reciprocating steam engine
• Compact heat exchanger for steam generator and condenser
• Steam buffer and condenser buffer.
• Fed pump for long life time

3.1 Oilfree reciprocating steam engine

When realising a modern steam engine the sealing issue is of paramount importance The RAN concept includes piston rings without gap which is expected to give both low blow-by and low frictions losses.

3.2 Compact heat exchangers for steam generator and condenser

The compact heat exchanger approach makes it possibly to realise very compact steam generators far away from the power density of common steam boiler in large power plants. The high power to material weight ratio indicates low capital cost. For air cooled applications as automotive applications it is of paramount importance that the condenser is design in a proper way in order to realise an effective SSSP.

3.3 Steam buffer and condenser buffer.

The RAN-concept includes two types of energy storage. The storage is called Steam buffer and Condenser buffer. The steam buffer is a high temperature storage that will offer several attractive features in automotive applications as well as small scale CHP. The steam buffer makes it possible to design the steam generator for only average power out put. High electric and hot water peak power can be obtained by the steam buffer. The steam buffer smoothing the combustion and enable optimizing of the combustion to avoid environmental harmful emissions. The steam buffer can also be used to store low tariff electricity during nights. The steam buffer unfolds also new possibilities for solar thermal energy applications when it can store the solar energy until occasion when the sun is not shining .The Steam Buffer also makes it possible to matching the mismatch between the electricity and heat demand. Further more Steam Buffer will imply reduced risk for outages when there will be steam available to propel the steam engine even if the burner will break down.
The condenser buffer makes is possibly to condense large amount of steam at a high rate without noise which is important in all kind of applications. For more detailed information see separate document.