The waste heat recovery technology has been tested by these five different prototypes with different variations in power electronics, thermal collector contact surfaces, cooling system and the thermoelectric generators to configure the best option for each type of heat source.
Efficiency is one of the main parameters analysed in each project. This parameter shows how much energy is extracted from waste heat.
The efficiency of the prototype depends on the heat source, the heat collector type and the cooling system applied.
In the paper industry tissue paper dryers are well known. They are equipped with a hood that blows hot air at high speed against the tissue paper and a rotating drying cylinder called Yankee that is partially covered by the hood. The tissue paper is dried by combining the drying cylinder that transfers heat by contact from the superheated steam circulating inside and the hood that dries by heat and mass transfer.
The energy recovery system (Heat R-System) installed in the tissue paper manufacturing plant that Gomà-Camps has in La Riba (Tarragona) makes use of the waste heat generated in a tissue paper dryer.
The operating cycle of the dryer is 24 hours a day, 7 days a week, and the gas temperature varies from 200 to 240 ºC. The device must withstand ambient temperatures up to 40 ºC and sun exposure.
The recovery system that is installed is intrusive: the fins of the WHRU thermal collectors are installed inside the chimney to capture the maximum amount of heat from the gases. Although intrusive, this system does not affect the operation of the dryer, as it does not cause any pressure drop or condensation.
WHRU modules uses water heat-exchanger (water-block) for their cooling to obtain the maximum temperature difference between the two sides of the Peltier cells. The water is pumped from a tank. The pump used to pump the cooling water to the water blocks works so long as the Yankee dryer does. The water-cooling temperature is 14-20 ºC.
In this project, the energy recovery system is based on a new type of WHRU:
WHRU -WBCV100
- Cooling system principle: Forced water convention
- Cooling components Cold plate or water heat exchanger (waterblock).
- Heat capture system principle Forced air convection – gas exhaust.
- Thermal collector Intrusive convectional heat sink
- Electrical energy generator Thermoelectric generator TEG – Peltier cell
- Peltier cell number 6 cells 40 *40 mm
| Gas temperature | 200-240 ºC |
| Generation area two WHRU | 531 cm2 |
| Mean power generated | 17.5 W |
| Maximum power generated | 25.92 W |
| Energy generated | 153.30 kWh/year |
| Power density | 329.40 W/m2 |
| Heat flow in the exhaust gas | 2.31 MW |
| Heat flow through Heat R-System | 813.20 W |
| Efficiency | 3.19 % |
Results
– In terms of power generation, this device produces more electrical energy per surface area than the prototype installed at Bodegas Torres. Consequently, its efficiency is higher. However, unlike a solar panel that has a system efficiency of approximately 20 %, the heat recovery system installed in Goma-Camps has a system efficiency of approximately 3%.
– To generate more energy, it is necessary to capture the maximum amount of heat flow. Therefore, the contact surface between the heat collectors and the hot gases should be as large as possible. Therefore, the installation of the fins in contact with the gas increases the energy generation. However, the length of the fins is limited because a longer length could lead to a pressure drop in the gas flow which could cause emission problems in the installation.
-From the standpoint of the installation, the water-cooling system requires a hydraulic loop with a pump. It is essential to ensure a continuous supply of cooling water to avoid damaging the prototype by exceeding the maximum working temperatures of the components. Therefore, installing two pumps instead of one or automating the start-up of the pump when the process starts are options to ensure the water supply.
On the other hand, the quality of the water supply is important. The water must be free of suspended solids to avoid possible clogging that could hinder or prevent the water supply and thus damage the equipment. Therefore, Goma-Camps linked the pump operation to the dryer. In this way, the water pump works while the dryer is working.
-The Yankee cylinder is used in all paper manufacturing factory. Therefore, Goma – Camps is completely scalable to install waste heat recovery systems in other paper manufacturing companies.
