The interest in energy consumption reduction, together with its aftermaths, is evolving continuously; among the available options, the recovery of waste heat from production processes is an interesting possibility. It is quite common that industrial activities have to dispose some process heat, operation that could even lead to a further consumption of energy and material. The recovery of this heat allows to cover a fraction of the energy required for the process itself or other uses such as offices heating and domestic hot water.
In order to exploit effectively the recovered heat two condition must be fulfilled: it has to be present a thermal user and the recovery’s temperature must match (i.e. be higher) with the requirements. The PCM thermal storage ensure a great flexibility in heat recovery because is able to store thermal energy at a specified temperature close to the source one, and to release it whenever necessary; the physical concept of phase change with the release of energy at a constant temperature, allows to keep high the useful heat content (exergy) supplied to the user.
The first step for the realization of a heat recovery system is the calculation of available energy: it becomes necessary to evaluate thermal fluxes and their availability over time. Then is carried out a quantification of the fraction of heat that can actually be recovered out from the total and which depends on the thermal level, the process features and the recovery activity. The heat recovery process is outlined once the end use of energy is defined.
When all the previous operation have been completed is possible to move towards the energy storage design, the material which is going to be used and the operation timing.
Proposal and expected results
The present case concerns a factory that produces mechanical components by hot pressing with ambient air cooling of the objects at the end of their production.
Following the steps already presented, an estimation of available energy at the end of the process stands over 2 MWh per day, but only a part of it can be actually exploited. To determine this value we need to outline how this energy will be recovered considering the thermodynamics features.
These glowing components are collected inside metallic tanks and in order not to revise the current operation has been decided to confine them into an insulated metallic box containing air/water plate heat exchangers. Following some experimental tests occurred in the production plant, the heat recovery has turned out promising, allowing the collection of useful information for a definitive and scaled up system.
Internal view of the pilot; both mechanical components and heat exchangers are visible
External view of the pilot; it is visible the series connection of heat exchangers
External temperature of the box during operation.
Initial temperature of mechanical components before heat recovery.
Thanks to this information, a new analysis has led to the definition of technical specifications and the best solution to be proposed. It results in three sections (heat recovery, thermal storage and high temperature water/water heat pump) at the service of office building space heating and domestic hot water, replacing the current natural gas consumption.
This intervention main goal is clearly the recovery of heat which will be otherwise wasted, but it is also important to collect information in sight of an enlargement of thermal use including the warehouse heating.
The following pictures show some technical features and the system scheme.
|WINTER HEAT DEMAND||80 MWht|
|EXPECTED THERMAL ENERGY FROM HEAT RECOVERY||ca. 60 MWht|
|HEAT PUMP SIZE||10 - 15 kWe|
|THERMAL ENERGY STORAGE SIZE||45 - 60 kWh|
Considering that every application has to be evaluated both under technical and economic point of view, it is proposed a preliminary assessment for an industrial activity committed in hot pressing.
|OVERALL INVESTMENT COST||€ 50.000|
|NEW YEARLY EXPENDITURES||€ 2.200|
|YEARLY AVOIDED COSTS||€ 5.200|