Marine diesel and gas engines are among the most efficient prime movers. But a substantial part of the energy in the fuel is usually thrown away as waste heat in the funnel gases and heat exchangers.
Recovering this otherwise wasted energy can be an attractive proposition. There are many uses for the recovered low temperature heat, including warming the accommodation, de-icing decks and stairways, tank heating, and others. This source of heat avoids burning fuel to generate electricity, at a cost of about NOK 1 per kWh at today’s fuel prices, for these applications.
However, traditional waste heat recovery systems have been complex and limited in function. This situation has changed with the introduction of the Pyro heat recovery system. Pyro is a long-established Norwegian manufacturer of marine equipment. To create the new system the company has put together some of its existing products, such as exhaust gas economisers and fuel fired heaters, and developed new products plus an advanced and flexible PLC-based control system, to create a waste heat recovery system which is both effective and flexible.
The Pyro system is based on a primary thermal fluid circuit comprising flow and return pipes and a circulating pump module. The thermal fluid is normally a water/glycol mix, and it is pumped around the circuit at a controlled pressure of up to 3 bar, depending on the type of vessel. Pyro systems run at less than fluid boiling point, avoiding the problems of steam waste heat recovery equipment yet providing the heat transfer capacity needed.
This circuit provides the means for connecting the sources of energy, primarily engine exhausts, jacket cooling, turbocharger intercoolers and lube oil coolers, to the systems requiring heat.
The key to the Pyro solution is that suppliers and consumers are all connected as parallel links between the flow and return sides of the primary circuit, and each has its own heat exchanger and control valves. Suppliers of heat, from energy which would otherwise be wasted, can be selected so that the primary circuit is maintained at the desired temperature. Each consumer is separately supplied with the flow it needs to provide the required amount of heat. A PLC-based control system allows the operator to oversee the status of the plant on screen, select suppliers and consumers and set the consumer temperatures. Once this has been done, the control valves work automatically to give the results required by the operator. The waste heat recovery system controls can be interfaced with the vessel’s main control systems and is type approved by the main classification societies for notation (EO).
An advantage of the system is flexibility. Heat can be recovered at different temperature levels. The criterion for the control system is that the temperature of a particular supplier should be above the minimum in the system. This ensures that waste heat is recovered to the greatest possible extent, and that use of primary energy to top up is cut to the minimum.
Equipment that normal supplies waste heat can become a consumer if the operator wishes. A good example is in diesel electric propulsion. Engines will being running to cover the load. One or more additonal engines will be on standby, ready to start if the load exceeds the capability of the running engines. To reduce wear and tear on start up it is important to keep the standby engines well pre-heated. The Pyro system does this automatically once the standby engines have been nominated, keeping them at the preset temperature.
Various safeguards are built in. The Pyro recovery heat exchangers do not replace the engines’ coolers. If there is more heat available than required at the time, the excess is dissipated by the coolers in the normal way. Pyro exhaust gas heaters are shell and tube units which can incorporate independent gas paths for each of the main and auxiliary engines, and also a fired section if specified. Automatic valves bypass unwanted flows if available heat is more than needed, and the control system maintains the exhaust temperature after the heater at above the dew point to avoid corrosion.
It may be that the vessel’s engines are running at too low a load to provide all the energy needed from waste heat. A Pyro oil or gas fired heater is included in the primary circuit, and this is flashed up automatically to make up the shortfall, ensuring that the right total amount of heat is fed to the consumers. However, the system is set up so that ’green’ energy from waste heat is exploited to the full before high grade primary energy in the form of oil or gas is used for accommodation heating and other consumers.
The result of installing the Pyro system is a cut in fuel consumption, and therefore normally in emissions as well. At current fuel prices the payback time is likely to be less than one year. Pyro heat recovery systems are also well suited to upgrading of existing ships.