Thermal energy storage (TES) for industrial waste heat (IWH) recovery: A review

Highlights

• Industrial activities have a huge potential for waste heat recovery.

• TES systems overcome the intermittence and distance of the IWH source.

• More than 35 IWH case studies of on-site and off-site TES systems are reviewed.

• On-site TES systems in the basic metals manufacturing are the most recurrent option.

• Water, erythritol and zeolite are the TES materials more used in IWH recovery.

Abstract

Industrial activities have a huge potential for waste heat recovery. In spite of its high potential, industrial waste heat (IWH) is currently underutilized. This may be due, on one hand, to the technical and economic difficulties in applying conventional heat recovery methods and, on the other, the temporary or geographical mismatch between the energy released and its heat demand. Thermal energy storage (TES) is a technology which can solve the existing mismatch by recovering the IWH and storing it for a later use. Moreover, the use of recovered IWH leads to a decrease of CO₂ emissions and to economic and energy savings. Depending on the distance between the IWH source and the heat demand, TES systems can be placed on-site or the IWH can be transported by means of mobile TES systems, to an off-site heat demand. Around 50 industry case studies, in which both on-site and off-site recovery systems are considered are here reviewed and discussed taking into account the characteristics of the heat source, the heat, the TES system, and the economic, environmental and energy savings. Besides, the trends and the maturity of the cases reviewed have been considered. On-site TES systems in the basic metals manufacturing are the technology and industrial sector which has focused the most attention among the researchers, respectively. Moreover, water (or steam), erythritol and zeolite are the TES materials used in most industries and space comfort and electricity generation are the most recurrent applications.

Introduction

Energy consumption is an important parameter which reflects the influence of a certain sector on the economic growth and environmental pollution of a region [1]. Existing reports from different energy statistics agencies [2], [3], [4] show that both industrial activities and energy sectors (power stations, oil refineries, coke ovens, etc.) are the most energy consuming sectors worldwide and, as a consequence, the responsible for the release of large quantities of industrial waste heat (IWH) to the environment by means of hot exhaust gases, cooling media and heat lost from hot equipment surfaces and heated products [5]. Recovering and reusing IWH would provide an attractive opportunity for a low-carbon and less costly energy resource [6]. Moreover, reducing the environmental impact and costs could, at the same time, improve the competitiveness of the sector.

Despite the fact that some work has been done to review and categorize the existing methods to estimate IWH potential as well as to highlight the importance of well explained and transferable IWH recovery estimations [5], [7], the biggest amount of the IWH is underutilized. The technological, production process, financial and administrative as well as information barriers of IWH utilization was discussed and weighted in an expert meeting in 2010 [8]. They conclude that the five most relevant barriers to develop IWH recovery and reuse are:

• Technological barriers:

  • No nearby heat sink

  • No information about heat sinks nearby

• Production process barriers:

  • Disturbance of the operation

• Financial and administrative barriers:

  • Too high rate of return expectations

  • Uncertainty of the economic future for the investing company

Once the IWH source is identified, the following step is to choose the most suitable technology to recover it. Accordingly, Brueckner et al. [9] categorized the available technologies to recover IWH as passive technologies, whether the heat is being used directly at the same or at lower temperature level, or as active technologies, whether it is transformed to another form of energy or to a higher temperature level (Fig. 1).

Among the available technologies, this review focuses only on thermal energy storage (TES), which strengths are the possibility of solving the problem of matching the discontinuous IWH supply with the heat demand and achieving a better capacity factor, allowing the process components to be designed for a lower maximum output, for avoiding start-up and partial load losses, and for reducing investment cost in combination with cost intensive components (such as refrigerators or Organic Rankine cycle engines) [10], [11]. Moreover, already in 2014, the IEA [4] highlighted the use of thermal energy storage for waste heat utilization as a key application to achieve a low-carbon future due to the temporal and geographic decoupling of heat supply and demand. Depending on the temporal range of this decoupling (hourly, daily, weekly or seasonally [12]), the storage capacity and estimated storage costs, sensible, latent and thermochemical (TCM) TES storages need to be considered and studied in order to choose the most suitable technology. Therefore, the storage period as well as the heat capacity and cost of different TES candidates are presented in Table 1.

Thus, the scope of this review focuses only on industrial and energy production activities (commercial, domestic and service-related activities are excluded) as they are the most energy consuming sectors; therefore, their energetic and economic savings are expected to be significant. The exhaust gases or streams that scape from the processes of these activities are considered without taking into account recovering technology which is already implanted, such as recuperators or regenerators. As the main drawback of those activities are their intermittence and the temporal and geographical mismatch between their heat release and the later use heat demand, TES systems have been proposed to overcome them.

In this article, the case studies in which TES systems were proposed to reuse and recover IWH are reviewed. As search terms in scientific databases, the different nomenclatures of waste heat (waste heat, surplus heat, and excess heat) are considered. Moreover, scientific communications in conferences and other dissemination sources are reviewed. Patents are not included in the search. Based on this analysis, the most representative characteristics of the TES systems and their applications have been identified and their economic, environmental and energy savings are discussed. In addition, the trends and the maturity of the cases reviewed are considered. Last but not least, this review is aimed to identify research niches in the topic.

In order to present the industrial cases reviewed, the structure of this article is divided with regard to the location of the heat demand (Fig. 2): on-site or off-site. On-site is considered, when both the IWH source and the heat demand are in the same site. This section is then divided regarding to the type of industrial activity which generates the IWH. Off-site is considered, when the IWH source and heat demand are not on the same location but separated a certain distance. Therefore, mobile TES (M-TES) systems are required to transport the IWH from the location in which the heat is generated to the heat demand. In this section, the case studies are listed according to their maturity level.