Riccardo Vescovo, Emma Spagnoli/ Energy Procedia 00 (2017) 000–000 1
IV International Seminar on ORC Power Systems, ORC2017
13-15 September 2017, Milano, Italy
High Temperature ORC Systems
Riccardo Vescovo, Emma Spagnoli[*]
Turboden S.p.A., Via Cernaia 10, 25124 Brescia (Italy)
Abstract
ORC systems are well known as the best available technology for electricity production from low temperature heat sources -such as geothermal ones - and continuous technical development is done to optimize performances by means of finding new working fluids and improved design criteria and thermodynamic cycles.
With regard to medium and high temperature heat sources exploitation for electric power production, ORC technology and Steam Rankine Cycle solutions are competing in terms of power production, capital expenditures and O&M costs to become the preferred choice on a case-by-case basis.
Commercially available ORC technology is currently limited to a maximum working fluid operating temperature of about 300°C, leading to a strong limitation on performances achievable.Finding new working fluids that could operate at Higher Temperatures and developing the related technical solutions, will enable to improve ORC technology competiveness also in other niches that now are Steam Rankine Cycles territory.Very High Temperature ORC technology will lead to new solutions for both power only production and cogeneration, allowing ORC technology to enter market segments traditionally belonging to other technologies (e.g. Steam Rankine Cycles, Otto Cycles, Bryton Cycles, etc.).
The paper will present the technical and industrial development on High Temperature ORC systems, potential market development and a technology comparison to other generation technologies.
Keywords:Very High Temperature ORC, Cogeneration
© 2017 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems.
- Introduction
This paper describes the progress of the recent Turboden research and development activities aimed at increasing the ORC systems working fluid operating temperature with the results of improved efficiencies.
The first part of the paper provides an assessment of the state of art ORC technology with focus on the working fluid type and the related operating temperatures, performances and fields of application.
Afterwards, working fluids with higher temperature thermal stability are introduced and schemes of power only and high temperature cogeneration (CHP) solutions presented with the related performance indications.
Particular focus is given to the CHP scheme, which opens a brand new market segment for application of ORC systems to high temperature heat carrier cogeneration, medium pressure steam as heat carrier in particular.
Finally, technical and economical comparison with traditional CHP solution (e.g. Reciprocating Engine, Gas Turbine and Counter-pressure Steam Turbine) are presented identifying niches in which the proposed new ORC High Temperature CHP system solution can ensure benefits and advantages.
Nomenclature
MT Heat SourceSource in the temperature range between 200 - 250°C
HT Heat SourceSource with temperature higher than 250 - 500 °C
VHT Heat SourceSource with temperature higher than 500 °C
LT CHPHeat generation through ORC system cooling media ina temperature range 60-100°C
MT CHPHeat generation through ORC system cooling media ina temperature range 100-130°C
HT CHPHeat generation through ORC system cooling media ina temperature range 150-250°C
RE Reciprocating Engine
GTGas Turbine
SRC Steam Rankine Cycle
CP-SRC Counter Pressure Steam Rankine Cycle
ST&P Steam & Power ORC system i.e. Very High Temperature ORC coupled with natural gas fired thermal oil boiler
NHP and NPPNet Heat to Process and New Power to Process
ηel, ηth, ηI, ηIIElectrical efficiency, Thermal efficiency, First Principle Efficiency, Second Principle Efficiency
- State of art of Medium and High Temperature ORC systems
Most of the commercially available ORC systems use as working fluid compounds and/or mixtures coming from the following three different families: hydrocarbons, siloxanes and refrigerants.
In particular, fluids belonging to hydrocarbon and siloxane groups are commonly used to exploit the Medium Temperature (MT) and High Temperature (HT) heat sources, which are of interest for the present paper, while refrigerants are normally used for Low Temperature Heat Source applications [1].
In Table 1, a non-comprehensive list of the most common working fluid used in commercial ORC systems for MT and HT heat sources application and respective temperature range of operation is given.
None of the working fluids listed in Table 1 demonstrated thermal stability at temperatures greater than 300°C. With this maximum operating temperature limit, a related maximum electrical efficiency can be calculated and set as upper limit for today commercially available ORC systems.
In Table 2, a picture of the typical performance of the commercial ORC system is given, depending on the condensation temperature level and on the heat source temperature. Medium and High Temperature ORC systems are commonly applied to the following sectors: Biomass, Heat Recovery (from prime movers or from industrial processes), Waste to Energy and Solar Thermal Power.
Table 1. Common use working fluid for MT and HT heat source ORC application [1]
Working Fluid / Max Operating Temperature [°C] / Evaporation Temperature Range [°C] / Condensation temperature Range [°C]Octamethyltrisiloxane (MDM) / 290 / 250 - 280 / 80 – 150
Hexamethyldisiloxane (MM) / 290 / 180 - 250 / 30 – 60
Cyclopentane / 300 / 200 - 230 / > 0
Table 2. Typical Performances of medium and high temperature ORC cycles. Source: Author’s processing.
Electrical efficiency / Thermal efficiencyHeat Source Temperature / Heat Source Temperature
HT / VHT / MT / HT / VHT / MT
Power only / 25 – 28 % / 20 – 22 % / 0 % / 0 %
LT CHP / 20 – 22 % / 15 – 18 % / 77 – 79 % / 81 – 84 %
MT CHP / 15 – 18 % / 12 – 15 % / 81 – 84 % / 84 – 87 %
- Very High Temperature ORC system
There is an extensive literature with theoretical studies about ORC systems operating at very high temperatures with organic fluids stable over 300°C [2]-[4]. Real exploitation of this technological development would lead to better heat sources utilization, improved system efficiencies and expanded range of application of ORC systems. In the following paragraphs, organic compounds stable at very high temperatures are presented
3.1.Working fluid
The most interesting organic compounds stable at very high temperature and suitable as working fluid of ORC cycles are:toluene, biphenyl, diphenyl oxide, terphenil, quadriphenil, linear hydrocarbons, alkylated aromatic hydrocarbons, phenilcycloesane, bicyclohexyl,perfluoropolyether. Examples of commercial names of these fluids are Therminol® VP1, Dowtherm A, SYLTHERM®, HELISOL® 5A, Therminol® LT, Therminol® VP-3.For reasons not treated in this paper, Turboden developments on Very High Temperature ORC technology have been concentrated considering the diphenyl - diphenyl oxide mixture as working fluid, whose operating conditions are reported in Table 3.
Table 3. Diphenyl and diphenyl oxide mixture working temperature ranges
Working Fluid / Max Operating Temperature [°C] / Evaporation Temperature Range [°C] / Condensation temperature Range [°C]Diphenyl - Diphenyl oxide mixture / 400 / 390-350 / 250 – 160
Comparison between saturation curves of steam and different ORC working fluid suitable for MT, HT and VHT operationis reported in Figure 1, giving reason of the advantages represented by exploitation of VHT-ORC technology respect to traditional ORC and SRC. In fact, given a certain heat source temperature, thanks to higher evaporation temperatures, VHT ORC technology can obtain higher energy conversion efficiency.
3.2.Performance evaluation
The simplest configuration of the Very High Temperature ORC cycle is in CHP mode, as represented in Figure 2(a). In particular, this configuration allows the production of different high temperature media (for example steam,
thermal oil, superheated water or hot air), in the range of temperature between 150-250°C (identified as HT CHP), which are directly heated by the ORC working fluid in its condensation phase.
On the other hand, in power only configuration, because of the high temperature condensation, the Very High Temperature ORC loop needs to be combined to a Medium Temperature ORC cycle, as described in paragraph 2, as bottom cycle,creating the two loops cascade configuration represented in Figure 2(b).Performances of Very High Temperature ORC in CHP mode and in Power Only mode are reported in Table 4.
Table 4. Indicative Performances of Very High temperature ORC cycles. Source: Author’s processing.
Electrical efficiency / Thermal efficiencyVHT heat source / VHT heat source
Power only / 30 – 34 % / 0 %
HT CHP / 15 – 20 % / 79 – 84 %
3.3.Applications
Very High Temperature ORC systems in power only mode may exploit the same VHT heat sources described at paragraph 2, in which ORC systems are in competition with SRC systems. In Power Only mode, the Very High Temperature ORC technology allows an increase of about 15 - 20% of the net electrical performance, when compared to the best available alternative of commercial High Temperature ORC systems. Such an upgrade in electrical performances, can lead to erosion of market share of SCR technology, traditionally considered as more efficient, especially for large size (> 5 MW) power plants.
On the other hand, HT CHP can be utilized not only for the traditional ORC based CHP application sector (e.g. with cooling media at around 100°C, LT and MT CHP), but also for a new market segment for the ORC technology. Indeed, the high temperature cooling media obtained by the cogeneration together with the very high global efficiency, allow the convenient application of Very High Temperature ORC technology as a topping CHP cycle. In this case, as it will be presented later on in the paper, ORC technology will start to compete with other topping CHP technologies like Reciprocating Engines (RE), Gas Turbines (GT) and Counter-pressure SRC (CP-SRC) connected to fired boilers.
The following part of this paper will focus on HT CHP application, with the target to evaluate its profitability and to compare it, on technical and economic standpoints, to the CHP technologies available today in the marketplace.
- Manufacturing Industries Typical Energy Requirements
Many manufacturing facilities require large amount of electricity and valuable heat sources at high temperature, in particular saturated steam, to satisfy internal processes, such as distillation, drying, evaporation, concentration, process heating, atomization, sterilization, pasteurization, vulcanization, moisturization, etc.
Most demanding Manufacturing Industries in terms of steam and electricity are Pulp & Paper, Chemicaland Pharmaceutical, Food & Beverage, Textile, as results from a market study commissioned by Turboden to Poyry Italy Srl.In all these sectors, while there are large variations in the manufacturing facilities sizes and related energy demands (there are small and very large plants), what is relatively constant is the ratio between the Steam demand to the Electricity demand. In Table 5,an estimation of the mean steam and electricity demands of the some manufacturing subsectors of interest is provided.
Table 5. Manufacturing Subsector steam and electricity mean demands. Source: Poyry market study.
Capacityrange / Average Electricity / Average
Steam / Ratio
[ktons/year] / [MWe] / [MWt] / [MWt]/[MWe]
Paper / Specialties / 6-948 / 8,9 / 16,7 / 1,9
Paper / Packaging / 10-1214 / 18,5 / 42 / 2,3
Paper / Tissue / 8-1115 / 10,4 / 18,7 / 1,8
Chemical / Organic Chemical / 1-420 / 5,4 / 55,5 / 10,3
Chemical / Petrochemical / 20-1100 / 8,2 / 44,7 / 5,5
Chemical / Plastic materials and Resins / 20-430 / 2,6 / 13 / 5,0
Food & Bev / Sugar / 42-546 / 7,7 / 46,2 / 6,0
Food & Bev / Diary / 20-720 / 3,3 / 11,8 / 3,6
Food & Bev / Oils / 0,4-150 / 9,9 / 14,3 / 1,4
Normally,these manufacturing facilities can choose whether to buy electricity from grid and produce steam from a separate boiler fed by fuel (Separate Heat and Power - SHP)or to install a CHP system to address both electric and thermal requirements with a single plant.
Depending on the specific geographical region, some fuels are more commonly used for this purposes than others. Especially in Europe and Nord America, very often natural gas is available and can be considered as the best fuel supply solution under economical and environmental points of view. Based on these considerations, natural gas has been selected as the reference fuel for the following analysis.
- Technology benchmark
In what follows, the combination of a natural gas fired thermal oil boiler, with Very High Temperature ORC system will be referred as Steam & Power ORC (ST&P).
In this paragraph, the different CHP systems available on the market are compared to the alternative ST&P system under the technical and economical points of view. The main hypothesis used in this comparison are here summarized:
- Reference SHP with steam only generation at 12 bar(g) and feed water return temperature at 90°C,
- CHP system for internal facility consumption only,
- CHP plant power range between 500 kWe and 3 MWe.
Table 6summarizes performance parameters for the different CHP technologies. It is possible to notice that RE and GT present consistently lower global efficiencies in case only steam is required for the industrial process. For RE, this means that the jacket water heat is considered as a heat loss and only the exhaust gas can be used for heat recovery. Instead, for GT this means that the flue gas can be cooled only until a moderate temperature. On the other hand, RE and GT offer a much higher electrical efficiency and exergetic efficiency compared to CP-SRC and ST&P.
Table 6. CHP system efficiency parameter for a 2 MW unit. Performance based on OEM datasheet and Poyry market study [6]
CHP system / RE / GT / CP-SRC / ST&Pηel1 / % / 44% / 27% / 8%2 / 16%3
Electrical output / MW / 2,0 / 1,85 / 1,8 / 2,1
% captive consumption 4 / % / 3% / 5% / 5% / 8%
ηth / % / 18% / 55% / 84% / 76%
ηI / % / 64% / 82% / 92% / 92%
ηII / % / 52% / 47% / 38% / 43%
In Figure 3, a simplified representation of the Electrical Power Output and Steam Power Output Ranges for the different CHP technologies studied is given.
Reciprocating engine is a technology capillary diffused over a wide range of power output, available from few kW up to many MW per single unit. Other CHP technologies have some limitations in the available power rating. For example, gas turbines are available in the market in the 100 - 200 kWe range (micro gas turbines) and then from 3MWe and above, with only very few cases of commercially available product for intermediate power outputs.
Considering the heat and electricity requirements of a given manufacturing facility, each CHP technology can satisfy in different ways theseenergy needs. As depicted in Figure 2, depending on the specific ratio Electric Power to Steam of the facility, the different CHP technologies will either saturate the electrical plant requirements (e.g. RE), or the thermal power demands (e.g. CP-SRC). The remaining uncovered energy requirement will be respectively satisfied by heat production from a gas fired boiler or purchasing electricity from grid.
1 Gross electrical efficiency
2 Considering steam generation at 50 bar 400°C
3ORC efficiencies as per Table 4 multiplied by thermal oil boiler efficiency equal to 90%
4On gross electric output
- Economics
In this paragraph, the results of a feasibility studycomparing the different CHP technologies, listed above, are presented, varying plant NHP and NPP ratio (Ratio). Mean NPP is set to 3MWe, the maximum power output in the hypothesis of present comparison (par. 5). The sizing of the CHP unit for each given technology is obtained, for each scenario of process heat to power demand, saturating either the electrical demand or the heat requirement, depending on the heat to power ratio characteristic of each specific technology as per rule (1). Plant sizing for the different CHP technologies and process heat to power demand (“Ratio”) is summarized in Table 7, with respective CAPEX and maintenance costs.
(1)
Table 7. CHP electric output in MW for different heat to power ratio
Ratio / RE / GT / CP-SRC / ST&PElectric output [MW]
1 / 3,00 / 1,40 / 0,30 / 0,60
2 / 3,00 / 2,79 / 0,57 / 1,20
3 / 3,00 / 3,00 / 0,86 / 1,80
4 / 3,00 / 3,00 / 1,14 / 2,40
5 / 3,00 / 3,00 / 1,43 / 3,00
CAPEX cost [€]
1 / -1.850.000 / -2.882.000 / -1.326.000 / -1.748.000
2 / -1.850.000 / -3.635.000 / -1.872.000 / -2.555.000
3 / -1.850.000 / -3.724.000 / -2.291000 / -3.191.000
4 / -1.850.000 / -3.724.000 / -2.644.000 / -3.736.000
5 / -1.850.000 / -3.724.000 / -2.955.000 / -4.222.000
Maintenance cost [€/kWh]
0,014 / 0,012 / 0,01 / 0,005
Fuel cost, maintenance costs and electricity purchaseavoided costs have been computed, evaluating annual operational costs in three different wholesale energy prices asreported in Table 8.Feasibility results in terms of Pay Back Timeand Net Present Value at 10 years, considering a discount rate of 3%, are presented in Figure4.
Table 8. Wholesale energy prices scenario
Parameter / Unit / high price scenario (HPS) / mean price scenario (MPS) / low price scenario (LPS)Natural gas price / [€/MWh] / 25 / 25 / 25
Electricity price / [€/MWh] / 125 / 100 / 75
As it can be seen, high wholesale electricity price results to be a strong driver promoting the CHP technologies with high power output instead of steam, particularly considering the PBT index.
On the other side, it is possible to understand that matching the manufacturing process requirements with the most similar CHP technology (in terms of steam to electricity ratio) improves the profitability of the CHP system.Such beneficial effect of choosing CHP system with corresponding Steam to Power Ratio to the served process are evident considering the NPV index. In particular, High Temperature ORC technology (ST&P) can give better economic results in case of process heat to power ratio higher than 3, which is a value representative for many of the application fields listed in Table 5.
- Conclusions
The ORC technology finds one of its limits on the maximum operating temperature of the working fluids, linked to the thermal stability of the same. Currently, the state of the art ORC technology sets the maximum operating temperature at about 300 °C. Using new fluids with thermal stability at higher temperatures would create new frontiers for the ORC technology in terms of the electric efficiency, achievable in power only configurations (above the 30%), and also with regard to new ORC applications like HT CHP. In particular, in this field, the Very High temperature ORC systems show that this technology could be an effective and profitable CHP solution, to be preferred against competing CHP technologies for those manufacturing facilities where the heat to electricity demand is above 3.
Fig. 4. CHP technology features Steam & Power a) RE saturating electrical demand b) CP-SRC saturating thermal energy demand