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Rankine cycle

The Rankine cycle is a closed two-phase thermodynamic power generation process that is commonly used in power plants such as coal-fired power plants or nuclear reactors.4 The Rankine cycle is the fundamental operating cycle where you have a fluid, the working fluid, that is continuously evaporated and condensed. The main components in the cycle include a rotating steam turbine, a boiler pump, a condenser and a boiler.

Ideal Rankine cycle

The process for an ideal Rankine cycle where the friction losses, some pumping losses and irreversible properties of the components are neglected is described below and illustrated in the left image in figure 3.6 The right image shows a pressure-enthalpy and a temperature entropy diagram of the thermodynamic cycle.

Isentropic expansion – work output

The high-pressure saturated vapor is expanded through the turbine and produces work. In practice, the expansion in this step is limited by the cooling medium’s temperature (pressure in condenser), turbine design. One design factor to bear in mind is that wet expansion and droplets can cause erosion of the turbine blades and for that reason, a certain super heat may be needed.10 Different working fluids tend to behave quite differently here and for common application of the Rankine cycle with water as working fluid this is a challenge.
Isentropic suggest that the process is ideal. In practice there are losses and they are typically expressed as an isentropic efficiency that can reach up to 80%.

Isobaric heat rejection

The low-pressure fluid is condensed isobarically, usually using cooling water or air (ambient). The pressure in the condenser is determined with the temperature of the heat sink. A suitable working fluid makes sure that the pressure is not below one bar at any point of operation.

Isentropic compression

When the condensate has reached its lower phase boundary the fluid’s pressure is raised again by a feed pump. Due to the low specific volumes of liquids the pump work is relatively small and therefore, the work of the feed pump is often neglected in thermodynamic calculations of the Rankine cycle.12 A real pump is not isentropic but the effect of this is quite small if compared to losses in the turbine.

The Real Rankine Cycle

In reality, where the Rankine cycle is not ideal, parameters such as friction losses, pressure drop, heat transfer and the physical construction of the components need to be considered. The pump and the expander do not operate isentropically, which need to be considered in calculating the cycle’s net work. Pressure drop due to fluid flow in the condenser and boiler cause them to not operate isobarically and pressure losses in piping needs to be considered as well.13 Additional losses can occur from friction in bearings and between moving parts of the system’s components as well as from cavitation that typically form in the pump’s propeller because of fast vaporization and condensation. To avoid cavitation the working fluid is typically sub-cooled which require further energy and decreases efficiency. There is even a risk for leakage of the working fluid from the cycle to the ambient and air that leaks into the condensation chamber that needs to be taken into consideration.

Organic Rankine Cycle

A typical steam Rankine cycle, as described above, is highly effective when utilizing heat sources with high temperatures. However, for low heat source temperatures (500°C or less), the traditional steam Rankine cycle becomes challenging to use due to physical limitations caused by the properties of water. Water has for example a relatively high boiling point (at a given pressure) and therefore requires a high temperature energy input to be able to change phase from liquid to gas. Also, smaller systems with lower power output have difficulties achieving cost efficiency and reliability with the expander. For the type of applications studied here with a low temperature heat source, different fluids are used as a working fluid instead of water. This type of cycle is called an organic Rankine cycle.
While the name might suggest that only organic fluids are used, other inorganic fluids have also been explored by researchers and practitioners. Examples of such fluids include carbon dioxide (CO2) and ammonia (NH3). Table 1 shows some common working fluids in various commercial ORC applications. Much research has been focused on finding an optimum working fluid for all ORC applications. This has, however, led to many studies to recommend different working fluids as optimal choices for ORC applications, due to the varying assumptions made to conduct such studies.

Applications of ORC-technology

The main advantage of ORC-technology as opposed to the traditional Rankine Cycle is its ability to tap into heat sources of lower temperature with higher efficiency and better performance. This opens the possibility of tapping into heat sources previously not utilized with traditional Rankine cycles. The Knowledge Center on Organic Rankine Cycle technology (KCORC) presents several possible applications of ORC technology in Europe in their report where waste heat is underutilized and have large potential of harvesting energy with ORC-technology:
o Natural gas supply infrastructure could be a large source for thermal energy because of the gas recompression stations along the pipelines that produce waste heat from gas turbines. In figure 4, an example can be seen of how an ORC-WHR system can be integrated with a gas compressor station to generate electric power from waste heat.24 Additionally, liquified natural gas (LNG) presents further possible ORC applications since gas turbines are used both at the production sites (to compress and then cool down and then liquified to cryogenic conditions), and also after transportation when the LNG is vaporized at the regasification stations for further usage.
o Industrial combustion processes in the production of steel and metal, cement and glass, refining processes, and other processes mainly use natural gas or other fuels. These fuels will eventually be substituted with hydrogen in accordance with the European green deal policy. Since hydrogen likely will be relatively costly as a fuel, energy efficiency improving technologies (such as ORC) would be more incentivized merely from an economic perspective since the cost benefit is larger from recovering wasted thermal energy with more expensive fuels.
o Propulsive engines such as heavy-duty diesel engines in trucks, ships and trains not powered with electricity produce waste heat that can be recovered with ORC-technology, about two thirds for truck engines through the exhaust gas and the liquid coolant. Also, gas turbines used as engines, like in airplanes, emits about 70%-55% of the fuel’s chemical energy to the environment at temperatures between 400 ºC and 600 ºC.
There are also examples of applications of ORC-technology used today. One such example is in geothermal power production where geothermal heat is converted into electricity.25 ORC-technology allows for the usage of lower temperature heat sources, which consequently allows one to not drill as far down into the earth’s crust to reach sufficient temperature levels, thus reducing installation costs. It also allows for a wider range of possible locations where geothermal power can be harnessed. Figure 5 shows a schematic of how a basic geothermal based ORC power generating system can look like.

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History of ORC-technique Development in Heavy Duty Trucks

Development of ORC technique in heavy duty trucks started as early as the 1970s as a reaction to the energy crisis. Manufacturers began looking into ways to decrease operating costs and reduce fuel consumption by utilizing mini-ORC systems. One of the most notable projects was the Mack Trucks and Thermo Electron Corp27 project. The Thermon Electron Corp project had the goal of equipping a 676 Mack diesel engine with a small mini-ORC system of 10 kW. The concept was to recover exhaust gases with a high temperature regenerative Rankine system using a high-speed turbine expander. The program was in three stages. In the first stage, design studies were carried out for later in the second stage have the system operating on an engine test bench before finally being demonstrated under real life operating conditions in a vehicle on a road. The development project was successful as fuel consumption benefit was proven. Unfortunately, though, the product never made its way to production as there was a drop in oil prices in the beginning of the 1980’s which led to the project being shut down.

Developments in Europe

The next upswing for development of waste heat recovery technologies began in the end of the 2000’s again due to an increase in oil prices. In the late 2000’s truck OEM’s (original equipment manufacturers) started once more to consider mini-ORC systems as a solution to improve fuel economy and reduce emissions. In the 2010’s all major truck manufacturers have reported working on waste heat recovery systems using ORC technology. In Europe, the development projects were mostly supported by either R&D budgets or public funding. The most notable R&D projects were done by Renault Trucks, Mercedes trucks, and CNH Industrial. An example of a public supported project was NoWaste within the FP7 program.29 Little public knowledge can be found regarding the projects done by Mercedes and CNH industrial, however more has been published regarding the project done by Renault Trucks.
The project focused on two promising technologies for waste heat recovery systems in a heavy-duty truck application: a Rankine heat engine and a thermoelectric generator. The main objective was to understand how WHR systems can be implemented in heavy-duty trucks, what their predicted performance would be, and to understand how physical limitations affect the system’s performance. After setting the boundary conditions as the thesis work should define a so-called 0-D modeling using the software Engineering Equation Solver30 (EES) for both systems, a commercial tool was used for further investigation of the two technologies.
Regarding the thermoelectric generator, the commercial tool modeled a thermoelectric generator architecture. Parametric studies were then done on the integration between the thermoelectric generator and the existing gas recirculation cooler. The main studied materials were Mg2Si and MnSi and transient aspects were evaluated to gain a greater understanding of the system and its bottle necks. From these studies, the conclusion was drawn that thermoelectric generators were not yet mature enough for an integration into heavy-duty trucks due to the low performance of thermoelectric materials.

WHR in heavy duty trucks today

In recent years, the development of waste heat recovery in heavy duty trucks has again been up for discussion because of the European Green Deal. The European Green Deal, is the EU’s main new growth strategy, presented in December 2019, for transitioning its economy to a sustainable economic model. The main objective of the strategy is for the EU to become the first climate neutral continent by 2050.
As a part of the strategy, CO2 emission standards for heavy duty trucks were set, because road transportation is a major contributor to climate change. CO2 emissions from heavy duty vehicles accounts for over 25 % of road transport’s CO2 emissions and the emissions have also grown 25 % since 1990. The regulation EU 2019/1242 set the first ever CO2 emission standards for new heavy-duty vehicles. By 2030 heavy duty trucks should decrease their emissions by 30 % (with 2019 levels as a baseline) with an intermediate target of 15 % by 2025.
To meet these requirements several new technologies and powertrain concepts have been proposed “including improvements in combustion and air management efficiency, predictive powertrain control, hybridization, reduction of friction and other losses, renewable fuels, hydrogen, fuel cells, and waste heat recovery (WHR)”.
WHR systems generate power from the engine’s waste heat that would otherwise be lost to the surrounding environment. Common heat sources for WHR systems are the air cooler, the exhaust gas recirculation (EGR) cooler, the engine coolant and the exhaust gases. Performance of such systems has been the topic of many publications (mostly simulation studies).

Economic analysis

The economic analysis was done in two parts. Firstly, a literature study was done to determine which assumptions and what estimates are appropriate to use in an investment analysis. The study aimed to investigate a heavy-duty truck’s average yearly mileage and fuel consumption, the additional maintenance cost that comes with a WHR-system as well as determining the underlying metrics to calculate the weighted average cost of capital and the average economical lifetime of a heavy-duty truck.
To analyze what it will require for a truck manufacturer to invest in and use ORC-technology an investment analysis was done. To calculate what the maximum investment cost would have to be for an implementation of an ORC system to repay itself within its economic lifetime, the net present value method was used.
To calculate the project’s net present value, an appropriate weighted average cost of capital and the annual savings the ORC system will bring, first needed to be determined. The WACC was calculated by using KPI:s from a typical Swedish truck manufacturer, Volvo AB. The annual savings was determined by using the following formula: 𝑆𝑎𝑣𝑖𝑛𝑔𝑠𝑦𝑒𝑎𝑟= 𝐹𝑢𝑒𝑙 𝐶𝑜𝑠𝑡∗𝐹𝑢𝑒𝑙 𝐶𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛∗% 𝐹𝑢𝑒𝑙 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦−𝐴𝑑𝑑𝑒𝑑 𝑚𝑎𝑖𝑛𝑡𝑒𝑛𝑎𝑛𝑐𝑒 𝑐𝑜𝑠𝑡.
The fuel consumption was determined by taking the average yearly mileage of a European truck, which is 130 000 km/year56, times the fuel consumption of Volvo’s heavy-duty trucks that is 30 liters/100km57. The added yearly maintenance cost was assumed to be 100 €/year, which Daccord claims is the average added maintenance for an ORC system in Europe.58 The economic lifetime that was used was 10 years which is the average economic lifetime for a heavy-duty truck.

Table of contents :

2.1.1 Ideal Rankine cycle
2.1.2 The Real Rankine Cycle
2.2.1 Working fluid selection
2.4.1 Early History
2.4.2 Developments in Europe
2.4.3 Developments in the US
3.1.1 Net present value
3.1.2 Calculations
3.1.3 Limitations
5.1.1 Cycle configuration and system design
5.1.2 Heat sources
5.1.3 Cooling limitations
5.1.4 Ambient conditions


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