Large amounts of heat released by modern industrial processes remain largely unexploited although their value is estimated at around 300 TWh / year in European industry. This wasted heat can be used either for simple processes (e.g., water heating) or even for conversion into useful power. The most efficient technology for the conversion of waste heat into electricity is the Organic Rankine Cycle (ORC), which already dominates the market, recording a large number of installations. However, the application of ORC cycles at low temperatures (below 100 oC) and for small power generation (below 50 kW) is commercially limited, mainly due to low efficiency and high specific costs (in € / kWe).
The IE-E product – developed within the frame of project, aims to create a 20kWe nominal power cycle that will be connected to a biogas ICE and operates at an upper temperature limit slightly lower than the temperature of the cooling water of the ICE (~75-80°C) offering the market a new efficient and cost-effective product for converting low-temperature heat into power. The acronym IE-E (Isothermal Expansion Engine) reflects the main activity of the project, which is the development of technology to approach the isothermal expansion of a working fluid, ultimately leading to increased cycle power.
Figure 1. Illustration of a typical biogas production system with integration of conventional ORC for the utilization of exhaust gases
Figure 2. Schematic ORC engine with recuperator (a) and corresponding recording of the thermodynamic states in a temperature-entropy diagram (b)
During the project’s first Deliverable, the simulation of the power cycle of the IE-E unit was performed. Figure 1 schematically shows a typical biogas production process from biomass (produced from livestock waste, energy crops, agricultural residues, etc.). The primary biomass is led to a digester where anaerobic fermentation produces biogas which is burned in ICE to generate electricity in order to supply the distribution network.
The ORC cycle applied to the present system is described by the diagram below (Figure 2), using an additional heat recovery in relation to the layout of a simple ORC and taking advantage of the low-temperature of the ICE cooling water for biogas combustion.
The IE-E unit developed within the frame of the project is practically an ORC engine with recuperator with the fundamental difference that the expansion takes place in an isothermal process in order to maximize the work produced during the expansion. The expected benefit of the produced work and ultimately the improvement of the thermal efficiency of the IE-E in relation to the simple ORC is shown in Figure 3.
In the simple ORC cycle (isentropic expansion), the work produced is represented by the area enclosed between points 1-2-3a-4-1. The work of an ideal isothermal expansion (temperature at the outlet of the expander equal to that at the inlet) would correspond to an area of 1-2-3b-4-1, while that of a quasi-isothermal expansion to an area of 1-2-3c-4-1. The implementation of the IE-E requires the appropriate development and/or modification of the expander, which will have on its outer surface a water chamber in which water will circulate at the temperature of the heat source (in this case, at the temperature of the cooling water circuit). Continuous heat supply to the expander will reduce the temperature drop during expansion, which translates into production of additional work
Figure 3. Thermodynamic cycle of a simple ORC engine with isentropic expansion, isothermal and quasi-isothermal expansion (IE-E)
Figure 4. IE-E system flow diagram
The IE-E project approach focuses on delivering a small amount of available heat to the expander, in order to increase its power production. If this heat supply is high, isothermal expansion is approached. The implementation of the project began with the development of the model, which simulated the operation of the power cycle, in order to determine key technical characteristics. The simulation model was developed with the use of the Engineering Equation Solver – EES. The model is based on the energy balance of its main components, while taking into account the pressure drop in the various parts of the cycle (e.g. piping, heat exchangers).
The flow diagram of IE-E is shown in Figure 4. The main components of the cycle modeled to simulate the operation of the system are the heat exchangers (vaporizer, recuperator, condenser), the pump and the system expander. The heat transfer analysis based on the LMTD method was used to model the heat exchangers, dividing each heat exchanger into zones of liquid, biphasic fluid and gas, while the pressure drop in the heat exchangers and piping was also calculated. Moreover, a minimum temperature difference (pinch point temperature difference – PPTD) between the two flows was considered, equal to 5K.
The pump of the system is of special specifications, in order to increase the pressure of the working medium by several bar and it is usually chosen to be a multi-stage centrifugal pump. The power consumption of the pump is 5-10% of the power produced, while its efficiency (np) is practically constant and equal to about 60%.
The system expander produces the useful power. The most common types of expanders in the power range below 100 kW are scroll and screw. The isentropic efficiency of the expander gets standard values between 50 and 75% for the IE-E power range. The coefficient was estimated based on the performance characteristics of commercially available compressors, given as a function of vaporization and condensation temperatures, assuming that the same efficiency is found in reverse operation as an expander. The methodology is based on the conversion of all quantities into specific input/output volumes. In the case of the IE-E system, heat is given off during expansion. To calculate the power produced, as a function of the heat to the working fluid during expansion, a new calculation methodology was developed. More specifically, the expansion was divided into N steps, where in each step a real expansion takes place and the fluid expanded receives heat from the cooling water of the ICE under constant pressure. The expansion steps are shown in Figure 5.
Figure 5. Expansion steps with the heat transfer from the cooling water of the ICE
An important research element has been the selection of the appropriate working fluid. According to the European Refrigerant Regulation, also known as the F-gas Regulation, the use of organic fluids with high Global Warming Potential (GWP) has begun to be limited in order to reinforce the use of environmentally friendly fluids. In addition, all fluids should have zero Ozone Depletion Potential (ODP). In this context, and in order to ensure the future use of the IE-E unit without any restriction, the R1234ze (E) has been chosen, which is one of the most efficient for heat transfer in the range below 100 oC and also has a GWP around 1.