Linear active disturbance rejection control of waste heat recovery systems with organic Rankine cycles

In this paper, a linear active disturbance rejection controller is proposed for a waste heat recovery system using an organic Rankine cycle process, whose model is obtained by applying the system identification technique. The disturbances imposed on the waste heat recovery system are estimated through an extended linear state observer and then compensated by a linear feedback control strategy. The proposed control strategy is applied to a 100 kW waste heat recovery system to handle the power demand variations of grid and process disturbances. The effectiveness of this controller is verified via a simulation study, and the results demonstrate that the proposed strategy can provide satisfactory tracking performance and disturbance rejection

Tube shape selection for heat recovery from particle-laden exhaust gas streams

Heat recovery from exhaust gas streams is applicable to a wide variety of industries. Two problems encountered in exhaust gas heat recovery are: the high heat transfer resistance of gases and the presence of entrained particulate matter, which can limit the use of extended surface area. Standard heat exchangers use round tube. This study uses Computational Fluid Dynamics (CFD) to investigate whether round or another shape is the best tube selection for exhaust heat recovery. Tube shape rankings are based on taking into account heat transfer, gas flow resistance and foulability. Foulability is inferred from the average wall shear stress around the front or back of each shape. An estimated asymptotic fouling resistance is used to calculate an equivalent fouled j factor, jf. CFD results suggest the best tube for exhaust heat recovery is an elliptical tube. The ellipse shape produced j/f and jf/f ratios (where f is the tube bank friction factor) over 1.5 times larger than that of standard round tube. A flattened round tube is also promising and may be the practical and economic optimum

Applications of thermal energy storage to waste heat recovery in the food processing industry

A study to assess the potential for waste heat recovery in the food industry and to evaluate prospective waste heat recovery system concepts employing thermal energy storage was conducted. The study found that the recovery of waste heat in canning facilities can be performed in significant quantities using systems involving thermal energy storage that are both practical and economical. A demonstration project is proposed to determine actual waste heat recovery costs and benefits and to encourage system implementation by the food industry

Integrating heat recovery from milk powder spray dryer exhausts in the dairy industry

Heat recovery from milk powder spray dryer exhausts has proven challenging due to both economic and thermodynamic constraints. Integrating the dryer with the rest of the process (e.g. evaporation stages) can increase the viability of exhaust recovery. Several potential integration schemes for a milk powder plant have been investigated. Indirect heat transfer via a coupled loop between the spray dryer exhaust and various heat sinks were modeled and the practical heat recovery potential determined. Hot utility use was reduced by as much as 21% if suitable heat sinks are selected. Due to high particle loading and operating temperatures in the particle sticky regime, powder deposition in the exhaust heat exchanger is perhaps the greatest obstacle for implementing heat recovery schemes on spray dryers. Adequate cleaning systems are needed to ensure continuous dyer operation

Waste Heat Recovery in Food and Drinks Industry (Abstract only)

Most baking processes in the food manufacturing sector involve use of gas-fired ovens. Only about one-third of the total energy used in these ovens adds value to the final product. The remaining two-thirds is discharged with the exhaust gases at 150-250o C and thus represents an opportunity for heat recovery. However, the low temperature range, fouling and presence of corrosive materials in the exhaust streams make heat recovery technically challenging and uneconomical. The existing low grade heat recovery technolgies mostly use gas to liquid heat transfer to produce hot water for use in other areas of the manufacturing plant. The performance of these systems is governed by hot water demand in the factory and is therefore not recommended if there are frequent fluctuations in demand or if a more efficient technology, such as combined heat and power, is already in place. This study involves design, manufacturing and testing of a novel low-temperature gas to gas heat recovery system using an array of heat pipe heat exchangers, for industrial-scale baking ovens at a large confectionary manufacturing plant. Unlike gas to liquid heat transfer, a gas to gas heat transfer system provides direct savings in oven fuel consumption, independent of the hot water and other energy demands elsewhere in the plant. The heat recovery potential of the system is estimated using a thermodynamic model developed based on energy and mass balance for the ovens. The design enables recovery of up to 50% of the energy available through the exhaust stack, increasing the energy efficiency of the overall process to 60% and reducing food manufacturing costs by one third

Feasibility study of a heat recovery system in an office building in Malta

The new Energy Performance of Buildings Directive (EU) 2018/844 has brought about a new drive to renovate existing buildings, especially for heating and cooling systems, whereby heat recovery techniques have become the order of the day. However, the real energy and financial benefits of applying such techniques have not been studied in Malta, which has a temperate Mediterranean climate. Thus, this study has performed a technical and financial analysis of using different heat recovery options for the most common office type, that is a medium-sized flatted office, using EnergyPlus dynamic simulation tool and multiple linear regressions. Results showed that the coefficient of performance of the air-conditioners, the window to wall ratio and the cooling set-point temperatures, have the greatest impact, while heat recovery has an insignificant contribution to energy efficiency, thus making it rank low in the list of energy efficiency priority measures for medium-sized offices in Malta.peer-reviewe

Heat recovery refrigeration in New Zealand dairy sheds : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Agricultural Engineering at Massey University

Increased energy costs initiated an investigation into refrigeration heat recovery as one conservation alternative available for reducing water heating costs on farm dairies. A theoretical energy balance was conducted, from which the potential of recovering refrigeration condenser heat was estimated at up to 60% of the water heating energy requirements. Preliminary tests with heat exchangers lead to the use of a tube-in-tube, counter flow, heat exchanger with fins on the refrigerant side, and cores on the water side, to improve the heat transfer characteristics. The exchanger, designed to provide 300 litres of 60°C water from a 2.25 kw refrigeration system cooling 2000 litres of milk per day, had an area of 0.84 m2, and an overall thermal conductance of 100 W.m-2.°C-1. This heat exchanger was inserted between the compressor and condenser of the refrigeration plant and tested with two condenser systems (air and water), four condenser pressures (6.5 bar, 7.5 bar, 10 bar and 12 bar), two milk inlet temperatures(23°C and l8°C), and two milk final temperatures (4°C and 7°C). In addition, tests on receiver pressure and suction superheat were performed to determine overall system performance. Increasing condenser pressure increased cooling times from 2 hours 32 minutes to 3 hours 17 minutes, after the completion of the 1200 litre morning milking (thus failing to comply with the 3 hour cooling regulation at high condenser pressures.) Also, C.O.P. decreased from 3.05 to 2.35 for the water cooled condenser system (2.70 to 2.00 for the air cooled condenser system due to fan power consumption). Gross heat recovery rose from 4.2 kWh.day-1 .m-3 to 8.l kWh.day-1 .m-3 for the water cooled system, giving water outlet temperatures of 45°C to 64°C as condenser pressure rose. The corresponding ranges for air cooled condensers were 3.8 kWh.day-1 .m-3, to 6.6 kWh .day-1 .m-3, and 38°C to 55°C. Changing milk inlet and final temperatures gave a proportional change in cooling times and total heat recovery, but had no effect on C.O.P. or heat recovery rates. Suction superheating increased total heat recovery by 15%, and water outlet temperatures by 9%. Increases in gross heat recovery with increasing condenser pressure were partially offset by additional compressor power, and yielded nett heat recoveries of 4.0 kWh.day-1 .m-3 to 6.0 kWh.day-1 .m-3 for water cooled, and 3.6 kWh. day-1 .m-3 to 4.3 kWh. day-1 .m-3 for air cooled, condenser systems. The maximum gross and nett heat recoveries (at 12 bar condenser pressure) were applied to the energy requirements of a monitored 220 cow town supply dairy. This analysis showed that the gross heat recovery was 51% of the water heating requirements, but the nett heat recovery dropped to 17% of the total heating and refrigeration demand. Based on current electricity and equipment prices, it is estimated that the payback period for this level of recovery would be 16-17 years. Changing the electricity pricing structure, to reflect up to a 1:3 differential in favour of water heating power costs, results in the 6.5 bar condenser pressure giving optimum results, but the nett returns are significantly lower than those reported. The potential for improved savings is greater from larger capacity systems as the capital investment is not proportionally increased with an increase in scale