Researching The Ultimate Fireless Steam Locomotive - Part I
The traditional fireless steam locomotive carried a supply of saturated water under high pressure in an accumulator. Thermal energy was stored solely in the heat capacity of the saturated water, which was usually pressurised to 250-psia at 401-degrees F (17.25-bar at 205-deg C) and had an enthalpy of 376.1-Btu/lb. To convert saturated water to saturated steam (with 1201-btu/lb) required an average of 840-Btu of heat per pound (1915-KJ/Kg) be extracted from the remaining saturated water (70% of which had to be devoted to thermal energy storage, 30% for traction). While in service, accumulator pressure and temperature rapidly declined from 250-psia at 401-deg F and 376.1-Btu/lb, to 180-psia at 373-deg F and 346.2-Btu/lb. An accumulator measuring 7-feet (2-m) inner diameter by 20-ft (6-m) inner length initially carried 33,000-lb of saturated water when filled to 80% capacity, with a total thermal energy capacity of 4876-Hp-hr. Of this, up to 1400-Hp-hr could be converted to traction (at 15% cylinder efficiency).
Dr. Gilli of Henschel of Germany designed fireless steam locomotives with accumulators that initially held saturated water at over 1,000-psia at 545-deg F (69-bar at 285-deg C) and 542.6-Btu/lb. This steam fed into a 250-psia running tank that supplied the cylinders. To convert saturated water at this pressure to saturated steam required that an average of 738-Btu/lb (1511-KJ/Kg) of heat be extracted from the remaining liquid in the accumulator. This meant that some 62% of the saturated water was devoted to thermal energy storage while a maximum of 38% could be allocated to traction. An accumulator measuring 6-ft diameter by 20-ft long initially held 22,500-lb of saturated water and 5,000-Hp-hr total thermal energy when filled to 80% capacity, would yield 1,540-Hp-hr available for conversion to traction (at 20% cylinder efficiency). A 30-ft long accumulator would make 2300-Hp-hr available for conversion to traction.
To improve fireless steam locomotive energy density, heat-of-fusion or phase-change fireless steam test locomotives were built in Denmark (for commuter service) and the UK (shunting duties), between 1920 and 1950. Both concepts used molten caustic soda (NaOH with 77.8-Btu/lb latent heat of fusion) as the thermal storage material. It offered fireless steam locomotive the potential of a constant level of power output during their duty cycles. Unforeseen problems that included chemical reactions between the NaOH material and the storage chamber, caused the earlier heat of fusion fireless steam locomotives to operate at well below the expectations. Despite the demise of steam locomotives from mainline service before 1970, fireless steam locomotives continued to be used in industrial shunting service right into the end of the 20th century. The technology still has much potential for improved development.
Advances in Thermal Storage Technology:
1)Ultra High-pressure Accumulators:
One of the new energy storage technologies involves new-generation ultra-high-pressure accumulators able to store steam at the super-critical (SC) and ultra-super-critical (USC) levels. A 1960's 1,000-psia Gilli fireless steam locomotive initially stored 46-lb/cu.ft of saturated water at 545-deg F (285-deg C) and held 25,132-BTU/cu.ft of thermal energy. To convert the saturated water at 1,000-psia to steam (phase change) required that 30,125-BTU/cu.ft be pulled out of the remaining saturated water in the accumulator, reducing accumulator temperature levels, restricting power output and reducing operating duration times.
Ultra-high pressure modern accumulators storing steam at 5,000-psia at 750-degrees F or 400-degrees C, would result in a mass density of 29-lb/cu.ft that could hold 24,790-BTU/cu.ft of thermal energy. Most of it would be available for propulsion due to the elimination of the phase-change energy requirements. Steam from several 5,000-psia accumulators (on the same locomotive) would be fed through pressure-regulated on/off valves and flow into a 250-psia running accumulator (at 500-deg F /260-deg C). The steam in this running tank would supply superheated steam to the steam engine. Fireless steam locomotives operating on USC steam would be able to undertake much longer journeys and venture further afield than their all-saturated-water counterparts.
2) Phase-Change Technology:
As the number of active fireless steam locomotives declined during toward the end of the 20th century, new research got underway in the area of thermal energy storage technology. This research was aimed mainly at storing solar heat and involved both solid-state energy storage as well as high-density phase-change material (PCM) technology. A major breakthrough occurred in this latter field during the mid-1990's, courtesy of researchers D.Y. Goswami and C.K. Jotshi, whose eutectic pcm mixture of equal amounts of hydrated ammonium alum and ammonium nitrate (plus 5% attapulgite clay) yielded a latent heat of fusion of 2185-KJ/Kg (941-BTU/lb) between 48-deg C and 53-deg C (118 to 127-deg F). The heat capacity of this compound exceeded that of water by 67% (see page http://freespace.virgin.net/m.eckert/pcm_solar_energy_storage.htm).
This precedent indicates that similar results of high latent heat of fusion at low temperatures may be possible by combining a nitrate and a sulphate (salts) that share common elements. In some cases, the salt mixture may even be able to absorb water to lower the melting temperature to a level where it may be used with steam power. Based on the precedent, a stabilizer would need to be added to prevent precipitation from occurring, while the phase-change material may also need to be encapsulated inside multiple small packages. Premium PCM mixtures can store up to 600,000-BTU/cu-ft of thermal energy while lower priced varieties will store far less. The range of possible materials that could be used to store thermal energy in steam system may include:
-sulphate (BeSO4) plus oxide (BeO);
-sulphate (BeSO4)plus aluminate (BeAl2O4);
-sulphate (BeSO4)plus nitride (Be3N2);
-sulphate (BeSO4) plus nitrate (Be(NO3)2);
-nitride (Be3N2) plus nitrate (Be(NO3)2);
-nitride (Be3N2) plus oxide (BeO) plus nitrate (Be(NO3)2);
-oxide (Al2O3) plus diaspore (AlO(OH));
-oxide (Al2O3) plus sulphate [(Al2(SO4)3) or (Al2O(SO4)2)];
-oxide (Al2O3) plus nitrate (Al2O(NO3)4 or Al(NO3)3);
-nitrate [(Al2O(NO3)4/Al(NO3)3)] plus sulphate [(Al2(SO4)3)/(Al2O(SO4)2)];
-oxide (Al2O3) plus beryllium aluminate (BeAl2O4);
-oxide (Al2O3) plus alkali-oxide (LiAlO2 or Li3 AlO3 or Na3AlO3);
-oxide (Al2O3) plus tri-calcium (Ca3(AlO3)2);
-sulphate [(Al2(SO4)3) or (Al2O(SO4)2)] plus tri-calcium (Ca3(AlO3)2);
-bauxite (Al2O3.2H2O) plus beryllium aluminate (BeAl2O4);
-bauxite (Al2O3.2H2O) plus alkali-oxide (LiAlO2 or Li3AlO3or Na3AlO3);
-bauxite (Al2O3.2H2O) plus sodium tri-silicate (NaAlSi3O8);
-bauxite (Al2O3.2H2O) plus tri-silicate (Al2(SiO3)3);
-hydrate silicate (Al2O3.2SiO2.2H2O) that melts at 427-deg C;
-sulphate (CuSO4) plus nitrate (Cu(NO3)2);
-sulphate (CuSO4) or nitrate (Cu(NO3)2) plus oxide (CuO or Cu2O);
-sulphate (NiSO4) plus nitrate (Ni(NO3)2);
-sulphate (NiSO4) or nitrate (Ni(NO3)2) plus oxide (NiO);
-sulphate (MnSO4) plus nitrate (Mn(NO3)2);
-sulphate (MnSO4) or nitrate (Mn(NO3)2) plus oxide (MnO2);
-sulphate (MnSO4) plus nitrate (Mg(NO3)2);
-sulphate (MnSO4) or nitrate (Mg(NO3)2) plus silicate (MgSiO3);
-nitrate hydrate (UO2(NO3) 2 .6H2O) plus sulphate (UO2 (SO4)3);
*very scarce, resulting in very high acquisition cost. Needs extensive research to achieve optimal safety when used as thermal energy storage PCM for railway propulsion. May be used on locomotives operating at low to moderate speeds on remote low-density lines.
The optimal alternative PCM materials to uranium mixtures would be beryllium compounds. Beryllium sulphate melts at 362-deg C and mixing it with beryllium oxide could lower the phase-change temperature to below 300-deg C (under 570-deg F). Adding a small amount of clay such as Al2O3.2SiO2.2H2O or comparable agent could prevent precipitation and ensure that the phase-change material (pcm) has a long service-life. The same clay could also be used for nickel or manganese based salt mixtures.
For reasons of mixture stability, the PCM may need to be encapsulated in multiple small packages, perhaps made from a corrosion-resistant high-temperature ceramic such as silicon-carbide. Silicon-carbide (SiC) has high compressive strength and would be able to withstand the pressure imposed on it by saturated water inside an accumulator. Capsules made from this ceramic could contain the PCM. The capsules in turn could be housed in perforated pipes (large perforations or cut-outs in the pipes) located in the lower section of the accumulator. Partial enclosure inside these perforated pipes could provide some protection from the longitudinal jolts that are common in railway locomotives.
Steam lines containing choke valves may be directly attached to these perforated housing pipes. The choke valves would reduce the pressure of superheated steam by 56% and rapidly heat the perforated pipes and the PCM inside the SiC capsules. Externally-supplied superheated steam flowing in the steam lines could drop from 2000-psia to 1130-psia to 638-psia (2-choke valves in series) and temperature drop from 820-deg F to 640-deg F to 486-deg F. Steam at 638-psia and 486-deg F could be injected into the saturated water through perforated pipes (tiny holes) during thermal recharge cycles. The PCM would undergo its phase change at 250 to 260-deg C (480 to 495-deg F). Steam pressures and temperatures could be adjusted to match the PCM material in the accumulator.
Thermal energy storage density levels are expected to exceed 50,000-BTU/cu.ft for a eutectic PCM mixture between 200-deg C and 300-deg C (390-deg F to 570-deg F). The encapsulated PCM mixtures could be submerged under saturated water inside the accumulators, providing the necessary heat required to convert the saturated water to superheated steam. Locomotive power output could be raised along with engine thermal efficiency. The operating range of the PCM-assisted fireless steam locomotive could be increase to include short-line and branch-line services, as well as short-haul intercity services.
Research into and development of eutectic PCM compounds melting at over 100-deg C (212-deg F) is still quite preliminary, a result of most PCM research is aimed at the building and construction industries. An Australian company is developing an engine that can run on stored thermal energy (http://www.teappcm.com). Chemical research begun by aluminium companies is aimed at producing new polymers (super-molecules) based on aluminium, as recently became evident after ads in newspapers invited applications from Phd's in chemistry to undertake such research.
Other metal companies in related areas are doing likewise, raising the potential for the future appearance of new material combinations having higher thermal energy storage densities. These material combinations would mainly be aimed at storing solar thermal energy for the purpose of short-term (overnight) electrical power generation. The same technology could be adapted to mobile operation, including in a future fireless steam locomotive.
While beryllium-based compounds and mixtures may offer optimal thermal energy storage densities after uranium, its scarcity and high price may discourage its widespread use. Future PCM energy storage in the 200-deg C to 500-deg C (390-deg F to 900-deg F) range may be based on aluminium-oxide related compounds and mixtures. In fireless steam railway operation, one type of PCM may be used to maintain temperature levels in accumulators of saturated water, while a higher temperature PCM would provide energy to superheat the steam prior to expansion in a steam engine. In cold climates, it may even be possible to use low-temperature PCM energy storage for non-water Rankin engines in railway propulsion (see http://www.quasiturbine.com/QTPCMLocoValentine0502.doc).
PCM research precedents have opened up new avenues by which to increase thermal energy storage densities over the temperature range that would be most suitable for fireless steam locomotives. In solid-state thermal energy storage, Ranotor of Sweden (http://www.ranotor.se) and Enginion of Germany (http://www.enginion.com) are both working on developing high-density solid-state thermal-energy storage technologies that can be applied to steam power.
Until very recently, saturated water had the highest specific heat (heat capacity per unit mass) of known substances. On a per volume basis, aluminium oxide (Al2O3) had 81% the heat storage capacity as water. More recent developments in ceramic research as well as in eutectic PCM research have seen compounds that have a higher enthalpy than water over a given temperature range. Some solid state compounds have both higher density as well as higher thermal conductivity than water (aluminium oxide has 3.8-times the density and 30-times the conductivity).
High-density, high-heat-capacity solid-state compounds could be used for thermal energy storage in the accumulator of a high-pressure (2,000-psia) saturated-water fireless steam locomotive. Higher-pressure, higher temperature saturated water requires that less heat be added to the liquid to convert it to vapour, compared to the heat requirements at lower temperatures and pressures. Using a solid-state compound such as a ceramic or encapsulated (sealed) PCM to provide the additional heat would serve to increase the power output and extend the operating range of the fireless steam locomotive.
Ongoing developments in materials thermochemistry in areas such as ceramics and PCM research offers the potential of increase thermal energy storage capacity in the future. While these developments would be aimed at stationary applications, some of the new research and developments would be applicable to mobile operation. Thermal energy from solar and geothermal sources may feature prominently in the future of stationary power generation as well as a transportation fuel. Other energy sources that are not suitable for mobile operation could be used to re-energise fireless steam locomotives.
At a future time, fireless steam locomotives could re-appear in short-haul/short-line operations as well as in shunting and passenger excursion railway service. Thermal energy storage PCM technology can be competitive with other technologies in terms of energy storage density, in terms of low long-term operating costs, as well as in terms of service longevity of the storage technology before replacement would be needed. With high-density PCM thermal storage technology, it may be possible for a condensing fireless steam locomotive to be developed for extended operating range.