Large amounts of excess heat are currently wasted in various industrial processes, remaining unexploited. The Organic Rankine Cycle (ORC) is the most efficient technology for waste heat recovery, while its usual operating range of the units that utilize the rejecteded energy is greater than 200 oC, covering a wide range of applications in various sectors of the industry. However, there are limited products for low temperature applications (below 100 oC), mainly due to low efficiency and high cost.
The new IE-E product, which is being developed as part of the research project, aims to mitigate the existing gap of this market and to offer a new efficient and financially viable product for the conversion of low-temperature heat into power.
The market evaluation, which was carried out in a previous research project, showed that the most suitable and economically efficient market for importing the product is that of biogas. More specifically, the company will develop, design, test and evaluate an improved version of an ORC approaching the isothermal expansion, which will recover heat from the Internal Combustion Engine’s (ICE) cooling system of biogas plants, to produce additional electricity. The ORC system uses the heat of the exhaust gases, while the condensing thermal energy of the ORC is used for heating, as it can provide sufficient heat load for this purpose. A fraction of the heat from the cooling circuit of the machine is driven to the crucible to maintain fermentation, as shown in Figure 1.
Figure 1. Representation of a typical biogas production system with the incorporation of conventional ORC for the exploitation of exhaust gases
The main innovation of the IE-E unit is that the expansion takes place in a process that tends to approach isothermal, in contrast to conventional technologies where the process of expansion is ideally an isentropic process. As is well known from thermodynamics, perfect isothermal expansion produces more work than isentropic, between the same operating limits. Based on this principle, it is estimated that a fairly improved thermal efficiency rate (of 20%) can be achieved compared to conventional ORC, at a comparable cost as technological interventions are limited to modify a single component of the conventional ORC system, the expander.
Figure 2. Quasi-isothermal expansion with heat supply, in pressure-enthalpy diagram
Schematically, the differentiation of a typical biogas system from that of the IE-E system is shown in Figure 3. In a conventional biogas power plant, the heat dissipated by ICE cooling water at temperatures up to 90 oC can be used for heating (co-production of electricity and heat) or in the case that there is no demand for it to be completely rejected to the environment (eg cooling tower). In the proposed configuration, the IE-E unit recovers the rejected heat by converting a percentage of it into electrical power, while the rest is available either to meet thermal needs or to improve its efficiency. Compared to a conventional ORC system, IE-E presents the following advantages: (i) increased thermal efficiency (ii) ability to utilize condensing heat. In the corresponding conventional ORC the condensing temperature ranges at low values which are unsuitable for utilization of the condensing heat.
Figure 3. Conventional layout (left) and proposed layout of IE-E (right) for the heat exploitation of biogas power plant’s ICE cooling system