Solar-thermal driven drying technologies for large-scale industrial applications: State of the art, gaps, and opportunities

1 INTRODUCTION

Solar-thermal driven drying technologies, herein referred to as Solar Dryers (SD) for brevity, includes both large-scale SDs for industry, termed Large Industrial SDs (LISDs) herein, and small-scale SDs for small-scale industrial and non-industrial applications, termed small-scale SDs herein. LISDs are a critical part of the rapidly emerging field of Solar Heat for Industrial Processes (SHIP) that encompasses a wide range of industrial processes. Currently, global SHIP markets are small but have large growth potentials. Growing these SHIP markets has several important societal benefits including (1) increasing energy security for countries with large energy imports; (2) contributing to sustainable economic growth; and (3) contributing to the transition to a low-carbon energy future. To capitalize on these potentials, 28 research institutes in 10 countries are working to develop a European Common Research and Innovation Agenda (ECRIA) on SHIP through the INSHIP project (www.inship.eu). One of the objectives for this ECRIA on SHIP is to benefit society by catalyzing and growing European and global LISDs capacities and markets. Within Europe and across other industrialized countries, a large fraction of industrial heat is used for large-scale industrial drying processes that span a wide and diverse range of sectors. Specifically, while industrial drying consumes 20%-25% of national energy consumption for Denmark and Germany, and 10%-15% of national energy consumption for France, the UK, the US, and Canada,¹ the market uptake of solar thermal driven industrial dryer technologies is negligible. This article is an outcome of an exercise to assess the current state of the art for LISDs, and identify important technological gaps and opportunities that can be targeted through this ECRIA.

The remainder of this article is organized as follows. A clear distinction is made between Original Research Articles that present new knowledge, and Review Articles that compile and synthesize content from a large number of Original Research Articles. The current state of the art for LISDs is defined and associated technological gaps are identified based on a comprehensive review of 45 SD review articles published in journals over the past 25 years (Section 2) and a targeted review of 34 original solar drying research articles with content relevant to LISDs published over the past 5 years (Section 3). LISDs are just one small subset of industrial dryers, and a goal for this article is to facilitate the identification of new opportunities to drive industrial drying processes using solar thermal energy. To this end, a very brief introduction to the vast industrial dryer literature is provided in Section 4 and a very high-level synthesis of ideas found in the existing SD and Industrial Drying literature are presented in Section 5. One of the main technological barriers to market uptake of LISDs is the proper integration of solar thermal into the drying process to yield reliable and cost-effective operation, and integration issues are addressed in Section 6. In Section 7 the technical and economic characteristics for 19 existing LISDs from throughout the world are summarized, and a sample set of industrial opportunities that could potentially benefit from novel LISDs are presented. Conclusions are presented in Section 8.

2 REVIEW OF EXISTING SD REVIEW ARTICLES

The primary exception to this gap is²⁹ published in 2014, which addresses many of these issues. Therefore, herein²⁹ is assumed to accurately reflect the state of the art in LISDs in 2014, and the objective of the next section is to build-on²⁹ by reviewing advances in this area since 2014.

3 REVIEW OF RECENTLY PUBLISHED ORIGINAL SD RESEARCH PAPERS RELEVANT TO LISDS

As described above, the solar dryer review paper²⁹ published in 2014 was identified as the best review paper relevant to LISDs. The objective of this section is to build-on²⁹ by reviewing original SD research articles published since 2014 targeting one or more of the following gaps: (1) applications outside the agro-food sector; (2) higher temperatures; (3) large SDs; (4) more sophisticated energy supply systems such as concentrating collectors and hybridization with other thermal energy sources, PV, and TES; and/or (5) more sophisticated dryer technologies such as heat pump, fluidized bed, and infrared dryer technologies. A literature review was conducted using keywords relevant to these five objectives, which then defined the scope of this literature review; that is, original SD research papers that did not strongly address one of these five objectives were not considered. This literature review was performed in an iterative manner that spanned several months during which time the keywords used for searching were refined, and the content of the articles analyzed to identify major themes, gaps and opportunities. The distilled results from this literature review are summarized in Table 2, where the columns are defined to support the five objectives above with the following supporting information. The temperature listed is a characteristic temperature related to the collector, such as average or outlet, or a temperature inside the dryer, and allows for a course comparison of research based on temperature. The collector area is used to allow a course comparison of research based on size. The Additional Keywords column contains important words or phrases in the title, list of keywords, or similar that are not reflected in the other columns.

A summary of the main findings are as follows. Consistent with Section 2, most of the SD research captured in this literature review is targeting the agro-food sector and uses non-concentrating collectors with air as the HTF for lower temperature drying applications. In contrast, SD research for industrial applications that are outside the agro-food sector, that are large in scale, or that use more sophisticated collector, integration and dryer technologies for higher value-added and more sophisticated drying processes still appears to be limited, including the use of concentrating collectors to drive higher-temperature drying processes. Furthermore, most of the SD research is still occurring in countries that are often classified as developing, emerging, or similar. Even though European countries have some of the most aggressive policies to support the development and uptake of renewable energy technologies in general and SHIP specifically as evidenced by funding the INSHIP project to create a SHIP ECRIA, the relative number of papers with authors from European countries is small. Even,⁷¹ which includes authors from France, is specifically focused on applications in Central Africa. However, it is noted that the research with the largest collector areas^{58, 68} did occur in Europe, and the more limited research that utilized more sophisticated technologies such as concentrating collectors was performed in more developed economies. Taken as a whole, this suggests that gaps still exist in developing LISD technologies that (1) leverage the more sophisticated technologies often found in the most developed countries and (2) can target opportunities in these most developed countries that require more sophisticated drying technologies. In theory, in locations with good solar resources any drying process that uses a thermal energy source (eg, combustion and not electricity) can be driven using solar thermal technologies if the collector and underlying drying technologies are thermally matched. The objectives for the following three sections are to more fully elaborate some of the required conditions, gaps, and opportunities to thermally match collector and dryer technologies.

4 INDUSTRIAL DRYING LITERATURE

The objective for this section is to provide a gateway to the industrial drying literature independent of SDs to frame how solar-thermal technologies may be used to drive existing industrial drying technologies. Industrial SDs are just one small area within the broader field of industrial dryers, and the industrial dryer literature is too broad for any concise summary herein to be meaningful. Rather, readers interested in entering the industrial drying literature are referred to the excellent Handbook of Industrial Drying, Fourth Edition (2014), edited by Mujumdar.⁷⁷ This dense 1301 page handbook assembles65 chapters written by experts covering a wide range of industrial drying topics, including five chapters on fundamental aspects, 18 chapters describing various dryer types, 28 chapters on drying in various industrial sectors, and 14 chapters on miscellaneous topics. Each chapter contains an extensive list of references, and therefore this handbook provides an easy entry point into a vast collection of literature on industrial dryers. Two gaps were identified in this handbook relevant to LISDs. First, while this handbook does contain a dedicated chapter on SDs, and the application of SDs to different sectors is discussed in several chapters, this SD content is in general parallel to the literature reviewed in Sections 2 and 3 in that it generally focuses on small-scale SDs that use relatively simple technologies and are appropriate for less-developed communities, and not LISDs for industrialized countries. Second, as an edited book, each chapter is generally authored by different experts and written to stand independent of the other chapters. Therefore the chapters often contain overlapping content presented from different perspectives, and there is little high-level synthesis of the content from different chapters. In particular, while the handbook does contain 18 chapters on different dryer types, the method used to define each dryer type is not consistent and includes dryer types defined based on one of the following: (1) flow or handling of the product to be dried (fluidized bed, spray, spouted bed, pneumatic and flash, and conveyor); (2) dryer geometry (rotary and drum); (3) flow of the drying gas (impingement); (4) alternatives to air or combustion gases as the drying gas (superheated steam); (5) alternative to the evaporation of “liquid” water (freeze-drying); (6) alternatives to convective heating of the drying surface (indirect, infrared, and microwave and dielectric); (7) alternative energy source (solar); and (8) alternatives to steady-state operating (intermittent, and pulse combustion). Consequently, the classification system used to define each chapter is not mutually exclusive and a dryer technology defined in one chapter is often treated in a different chapter from a different perspective. This gap in dryer classification is addressed in the next section where a fundamental industrial dryer classification is presented that is specifically relevant to SDs.

5 SYNTHESIS OF EXISTING LITERATURE

This section is the result of a high-level synthesis of ideas found in the existing SD and Industrial Drying literature, and as a result much of this content builds-on rather than reflects the current literature. The objective is to provide a fundamental framework to guide and characterize R&D on indirect LISDs for readers with a background in solar thermal engineering but not drying. Specifically, the focus in this section is to define and introduce fundamental concepts for drying technologies. In contrast solar collector technologies and coupling technologies (TES, hybridization, heat exchangers, and so on) are only addressed within the context of drying technologies. Readers interested in recent reviews of solar collector technologies are referred to an excellent review paper on solar collector technologies for SHIP applications published in 2018⁴⁴ and a review paper on flat plate air collectors published in 2019.⁴⁶

In the following subsections, drying definitions and conceptual models are introduced that are specifically focused on (1) bridging the solar thermal and industrial drying literature, and 2) developing a conceptual framework that facilitates the identification of technological gaps and opportunities, and the intuition to formulate appropriate responses. Therefore, these definitions and models are intentionally kept as simple as possible and are often not sufficiently robust for detailed drying studies, and readers are referred to the relevant literature cited above for more detailed models. In particular, the three chapters^{1, 78, 79}in Section 1. Fundamental Aspects in the Handbook of Industrial Drying, Fourth Edition⁷⁷ provide excellent content on drying fundamentals.

5.1 Drying characteristics

In this section, the physical, thermal, and material handling characteristics of dryers most relevant to SDs are introduced.

5.1.1 Physical characteristics

Drying is a thermal process that results in the removal of moisture from a moist material by vaporization to form a dry product. Drying is defined as a thermal process and is distinct from other methods to remove moisture such as mechanical extraction via centrifugation. Key terms for physically describing the drying process are graphically presented in Figure 1 and defined below. While Figure 1 is for one specific and straightforward dryer geometry, the resulting terms and definitions are easily adaptable to a wide range of dryer geometries.

As shown on the left side of Figure 1, the inside of the drying chamber is divided into two macroscopic volumes: (a) the moist material (MM) (1) on the bottom; and b) the bulk gas (BG) (2) on the top. The moist material-bulk gas interface (MMBGI) (3) is the horizontal plane separating the moist material and the bulk gas. The moist material is the material being dried inside the dryer, and consists of two parts: (a) moisture (4), which is volatile; and (b) substrate (5), which is not volatile. The bulk gas contains at least vapor (6), which is vaporized moisture and therefore can be condensed, and often non-condensable gases. Drying processes vaporize the moisture and pass the resulting vapor (7) to the bulk gas via the MMBGI. 99% of all drying applications involve the removal of water,⁸⁰ and therefore, the associated vapor is water vapor. The moist material may contain vapor/gas channels (8) that connect the interior of the moist material to the bulk gas. In addition, the moist material may contain isolated vapor bubbles (vaporized moisture) (9) that do not connect to the bulk gas. In most dryers a drying gas (10) is introduced into the dryer to at least transport the vaporized moisture away from the moist material and in some cases to also provide convective heating to the moist material. The exception are vacuum dryers, which do not have a drying gas. Almost 85% of industrial dryers use air or combustion gases as the drying gas,¹ but other drying gases such as superheated steam are also used.⁸¹ The bulk gas leaving the dryer (11) always contains vaporized moisture and, except for vacuum dryers, also contains the drying gas. The cover (12) is used to contain the bulk gas. When the drying gas is cooled, such as through contact with a cold cover, it may condense to form a moisture droplet (13). The base (14) is used to support, contain, and possibly transport the moist material and is often either well-insulated or heated.

5.1.2 Thermal characteristics

Drying is inherently a thermal process as it involves a vaporization process in which heat is absorbed to transform moisture into vapor. Ignoring the particular case of freeze drying, the simplest model for the state of moisture being a liquid is assumed herein, in which case the heat of vaporization is equal to the heat of evaporation. Both surface vaporization and bulk vaporization are possible. Surface vaporization is evaporation of moisture from a free surface and occurs when the saturation pressure of the moisture is greater than the vapor pressure of the gas at the free surface. Surface vaporization can occur at the MMBGI, and at the interface between moisture and gas in the interior of the moist material, such as at a gas channel or vapor bubble. In contrast, bulk vaporization is boiling of moisture, occurs when the moisture away from a free surface is heated to the saturation temperature and results in the creation of a new vapor bubble. Note the expansion associated with the production of a vapor bubble in a closed or restricted pore of the substrate can damage the substrate's structure and reduce the quality of the dried product, and therefore bulk vaporization is often not desired for high-quality dried products. The type(s) of vaporization present in a drying process will vary with the drying technology, characteristics of the moisture, substrate, and drying gas, and will often evolve as the drying process proceeds. For example, the drying of a moist material with large pores, high initial moisture content, and with moist material temperatures uniformly below the saturation temperature may begin with surface vaporization at the MMBGI and then evolve to surface vaporization from interior channels as the moist material dries.

While the above discussion assumes that the moisture is in the liquid state, in reality, the moisture can exist over a continuum of states ranging from a tightly absorbed state on the surface of the substrate, termed bound moisture, to a liquid state, termed unbound moisture.¹ The degree to which moisture is bound varies with the properties of the moisture and substrate, and decreases as the distance between a moisture molecule and the substrate increases. Bound moisture has a higher heat of vaporization and lower saturation pressure than unbound moisture. As a result, drying processes tend to vaporize unbound moisture first and strongly bound moisture last, which often results in drying rates decreasing as a drying process evolves. The complete removal of strongly bound moisture is difficult and may require high temperatures and low vapor pressures, and these conditions can be challenging to achieve and reduce the quality of the dried product.

Several key conclusions related to the thermal characteristics of dryers are as follows. Due to the onset of bulk vaporization, the temperature of the moist material will not exceed the moisture's saturation temperature at any location where moisture is present. Furthermore, the saturation temperature increases as the moisture become increasingly bound, so the temperature limits due to bulk vaporization will often increase as a drying process evolves. However, the temperature of a dry region (ie, a region that contains substrate but no moisture) is not limited by bulk vaporization, and directly heating a dry region can result in a hot spot. In parallel, due to the onset of condensation, the temperature of the bulk gas will not fall below the bulk gas's dew point temperature (the temperature at which the gas's vapor pressure equals the saturation pressure). Finally, the rate of vaporization, and therefore the rate of drying, will typically vary both spatially and temporally due to spatial and temporal variations in temperature, vapor pressure, total pressure, and the degree to which the moisture is bound.

5.1.3 Material handling characteristics

Although material handling characteristics are not as central to this presentation as physical and thermal characteristics, as elaborated in Section 4 industrial dryers are often described and differentiated based on their material handling characteristics, and therefore a short overview of key material handling terms is appropriate. Dryers are classified as batch, continuous, or semi-continuous based on material handling characteristics as follows. In all cases, the moist material enters the dryer as the moist feed and exits as the dried product. In a batch dryer, a discrete batch of moist feed enters the dryer and is heated and dried through a transient process. During this batch drying process, no moist feed enters or dried product leaves the dryer. At the end of the drying process, the resulting batch of dried product is removed. In a continuous dryer, the moist feed continuously enters the dryer, the moist material continuously moves through the dryer, dry product continuously leaves the dryer, and the drying process evolves as the moist substance moves through the dryer. Continuous dryers are often modeled as steady-state steady-flow systems. In a semi-continuous dryer, the dryer is split into several sequential zones corresponding to different stages of drying. Each zone dries a discrete batch of moist material through a transient process like a batch dryer, but the evolution of the drying process by moving moist material from one zone to another mimics a continuous dryer. To promote uniform drying, the moist material may be mixed, such as through stirring or tumbling in a rotating cylinder. Scraping is used to remove moist material and dried product from the base and may be required for effective mixing.

5.2 Four fundamental classes of dryers based on the heating mechanism

Solar thermal systems can be used to drive the drying vaporization process provided the thermal characteristics of the solar thermal system are properly matched to the drying technology. To conceptually facilitate this thermal matching, four distinct classes of dryers are defined herein based on the heating mechanism as follows: (a) convective dryers; (b) conductive dryers; (c) surface radiative dryers; and (d) bulk radiative dryers. While hybrid dryers combining two or more dryer types are possible, these cases are ignored herein for simplicity. These four classes of dryers are illustrated in Figure 2, with the left side of each of (A)-(D) illustrating the heating mechanism and location, and the right side presenting a sample temperature profile. These temperature profiles are specifically referred to as sample and not representative or typical due to their exact shape typically evolving with the drying process and varying with many factors including but not limited to (1) whether surface and/or bulk vaporization is occurring, (2) whether the moist material's temperature is increasing, constant, or decreasing with time, (3) whether the base and cover are well insulated, and (4) whether the moist material contains dry spots. These sample profiles are specifically for the case where all vaporization is occurring as surface vaporization at the MMBGI, and the moist material does not contain any dry spots. Specifically, bulk vaporization is not occurring and therefore, the temperature of the moist material must be less than the moisture's saturation temperature. Similarly, no condensation occurs from the bulk gas, and therefore the bulk gas must be at a temperature higher than the bulk gas's saturation temperature. While typically an insulated base (IB) or heated base (HB) is used, the uninsulated base (UB) case is shown in Figure 2A,C, and D to highlight where the moist material's maximum temperature typically occurs and how heat flows through the moist material. Similarly, while typically an insulated cover (IC) is used, an uninsulated cover (UC) is shown in Figure 2C to illustrate a solar dryer with a poorly insulated transparent cover used to admit solar radiation. While the details for these temperature profiles are unique to a large set of assumptions, for each dryer class the relative temperatures of the bulk gas and moist material, the location of the maximum temperature in the moist material, and the directions of the heat flows (hot to cold) within the system are representative for a large set of typical operating conditions, and therefore are conceptually significant to compare and contrast these four classes of dryers.

Vapor pressure and saturation pressure profiles of the bulk gas in a direction parallel to the MMBGI are also effective at comparing and contrasting these four classes of dryers. Sample profiles are presented in Figure 3 and illustrate basic trends for common operating conditions. In all cases, the vapor pressure of the bulk gas increases in the flow direction (left to right) due to the uptake of vapor from the moist material. The saturation pressure scales with the bulk gas temperature, and decreases in the flow direction if the bulk gas is being cooled (which is the case for convective dryers) and increases if the bulk gas is being heated (which is the case for conductive, surface radiative, and bulk radiative dryers). Note if the saturation and vapor pressure lines meet, the drying gas is saturated and can no longer absorb more vapor, further drying is impossible, and any cooling of the drying gas will initiate condensation.

Further details for each dryer type are as follows, including options to thermally drive the dryer using different classes of solar collectors.

5.2.1 Convective dryers

In convective dryers, an externally heated, hot drying gas is continuously introduced into the dryer and heats the relatively cooler moist material via convection, as shown in Figure 2A. Among the four classes of dryers, convective dryers are unique in that under normal operating conditions (1) the maximum temperature in the dryer occurs in the bulk gas; (2) heat flows from the bulk gas to the moist material; and (3) heat transfer from the bulk gas to the moist material causes the bulk gas to cool in the flow direction, and therefore for its saturation pressure to decrease as shown in Figure 3A. As a result, the bulk gas's vapor and saturation pressures tend to converge in the bulk gas flow direction. Approximately 85% of all industrial dryers are convective.¹ Solar thermal systems can be used to heat the drying gas directly by using air collectors or indirectly using a non-air collector (eg, liquid collector) coupled to a liquid-gas heat exchanger. In the industrial dryer literature convective dryers are often termed direct dryers due to the hot bulk gas directly heating the moist material, while in the SD literature convective dryers are often termed indirect dryers due to the use of a HTF to thermally couple the collector to the dryer.

5.2.2 Conductive dryers

In conductive dryers, a hot base is used to heat the moist material as shown in Figure 2B. While the name conductive dryer implies that the moist material is quiescent and therefore heat transfer occurs by conduction from the base to the moist material, convective heating from a hot base into a non-quiescent moist material such as a sludge with high moisture content is also possible. In contrast to a convective dryer, the moist material is hotter than the bulk gas and heat transfer occurs by convection from the moist material to the bulk gas. The resultant heating of the bulk gas tends to cause the bulk gas's saturation pressure to increase in the bulk gas flow direction as shown in Figure 3B. Depending on the exact conditions inside the dryer, the saturation and vapor pressures may converge or diverge. Among the four dryer types, conductive dryers are unique in that under normal operating conditions the maximum temperature in the dryer occurs at the base as shown in Figure 2B. Solar thermal systems can be used to heat the base using any solar thermal technology, including both air and liquid collectors. In the industrial dryer literature, conductive dryers are often referred to as indirect or contact dryers.

5.2.3 Surface radiative dryers

Surface radiative dryers use thermal (relatively shortwave) radiation to heat the free surface of the moist material at the MMBGI. Thermal radiation includes both the solar (visible) and infrared (IR) spectrums. Among the four drying technologies, surface radiative dryers are unique in that under normal operating conditions, the maximum temperature in the dryer occurs at the MMBGI as shown in Figure 2C. The thermal radiation can be supplied by direct exposure to solar radiation through the use of a transparent cover (termed a direct SDs in the SD literature), or by infrared (IR) heaters that use electricity or a hot heat transfer fluid. The uninsulated cover (UC) temperature profile in Figure 2C illustrates a direct SD with a poorly insulated transparent cover to admit solar radiation. The bulk gas's saturation and vapor pressure profiles are similar to conductive dryers as shown in Figure 3B. Often the cover of a direct SD is not a good insulator and may be “cold” relative to the bulk gas, and thus susceptible to condensation if the bulk gas temperature in contact with this cover reaches its saturation temperature. One important design consideration for direct SDs is that the maximum flux for unconcentrated solar radiation is approximately 1000 W m⁻²; that is, on a clear sunny day a surface facing the sun will receive approximately 1000 W m⁻² of solar radiation. Equivalently, an IR heater behaving as a blackbody at a temperature of 90°C also emits a radiative flux of approximately 1000 W m⁻². Since the emission from a blackbody scales with the temperature of the blackbody to the fourth power, IR heaters at temperatures greater than 90°C can, at least in theory, achieve much higher radiative fluxes than unconcentrated solar radiation. Typically concentrating rather than non-concentrating solar collectors are appropriate for collector temperatures >~100°C, and based on the literature review above the potential to use concentrating collector technologies to drive IR heaters has not been explored. Specifically, Direct Steam Generation (DSG) technologies may be used where steam is generated directly in the absorber and then condensed in a pipe creating an isothermal IR heater, or oil or molten salts may be used as the HTF to thermally couple the concentrating collector to a IR heater that is non-isothermal due to the HTF undergoing sensible cooling in the IR heating pipe. In the industrial dryer literature, surface radiative dryers are typically called infrared dryers.

5.2.4 Volumetric radiative dryers

Volumetric radiative dryers use relatively longwave microwave (MW) or radio frequency (RF) radiation that are absorbed volumetrically rather than at the MMBGI, resulting in volumetric heating. Among the four dryer classes, volumetric radiative dryers are unique in that the maximum temperature in the moist material does not necessarily occur at a surface, but rather can occur anywhere inside the moist material as illustrated in Figure 2D. The bulk gas's saturation and vapor pressure profiles are similar to conductive dryers, as shown in Figure 3B. In the industrial literature, volumetric radiative dryers are typically called MW dryers. MW and RF radiation are generated using electricity, and unlike the Convective, Conductive, and Surface Radiative dryer classes, there is no opportunity to directly drive volumetric radiative dryers using solar thermal technologies.

To reiterate common differences between the industrial drying and SD literature, direct dryers refers to convective dryers in the industrial dryer literature and surface radiative dryers heated directly using solar radiation in the SD literature. In contrast, indirect dryers refers to conductive dryers in the industrial dryer literature and convective dryers in the SD literature.

6 INTEGRATION

Most of the integration issues related to thermally coupling solar collectors with dryers in indirect LISDs are not unique to indirect LISDs but rather are common to a wide range of SHIP technologies and applications. Therefore, much of this section is approached at the more general SHIP level rather than the more narrow LISD level. The way solar process heat is integrated in the industrial supply system significantly influences its technical and economic feasibility. Integration issues are of central importance in SHIP as the industrial supply and distribution systems are much more complicated than most other solar thermal applications, which is due in part to both solar resources and thermal loads that may vary over multiple timescales ranging from minutes to seasons. Several research projects have addressed integration issues by developing methods to assist in the design and planning of solar process heat with an emphasis on feasibility. One of the most important projects is the International Energy Agency Solar Heating and Cooling Programme Task 49/IV (IEA SHC Task 49/IV; http://task49.iea-shc.org/) that contains three Sub-Tasks (A-C). In particular, Sub-Task B Process integration and Process Intensification combined with solar process heat has produced many resources to assist with integration issues.

6.1 Integration approaches

As illustrated in Figure 4, solar thermal is in general integrated into industrial processes at the (1) Supply Level (A) or (2) Process Level (B). Supply level integration means integrating solar thermal within the central heat distribution lines or central heat storages, while process level means integrating solar heat to one specific process step or process heat storages.

The SHIP integration guideline⁸² developed within IEA SHC Task 49/IV presents the overall necessary steps of an assessment methodology for solar process heat projects and leads the planner in detail through the following steps: (1) consideration of process integration for solar process heat; (2) specification of integration concepts; (3) planning of solar process heat system concepts; and (4) criteria for selecting the most promising integration points. The starting point of the methodology is always a detailed assessment of the processes (load profile, energy demand, temperature, and so on) and the energy supply system to serve as a baseline against which to evaluate the concept. This also includes possible excess heat recovery potentials (thermal energy efficiency), which is an important topic in industrial drying and depends on the drying temperature and system. Based on this assessment, the most efficient industrial process can be designed, followed by the solar thermal system. In Figure 5, an example scheme of a heat integration concept is presented based on Direct Steam Generation (DSG) using concentrating collectors. There are also several other tools available on the IEA SHC Task 49 website supporting the planner in system design (http://task49.iea-shc.org/).

6.2 Integration concepts

Overall, a SHIP system consists of up to five subsections as shown in Figure 6: (1) collector loop; (2) charge; (3) storage; (4) discharge; and (5) integration point. Most drying processes are low-temperature systems, and in these low temperature SHIP systems that include storage, typically all five sections can be distinguished. However, depending on the envisaged system design, specific components can be dropped (eg, the storage) and/or a backup system may be included to ensure a trouble-free and continuous production irrespective of variations in solar resources.

Following the methodology developed within the IEA SHC Task49/IV and consistent with standards on energy audits and energy efficiency (eg, EN 16247), the first step in the design of a SHIP system is the identification and characterization of the industrial energy demand. Based on the assessment of the existing industrial system and its energy supply, specific load profiles (energy demand, process temperature) are developed and integrated into energy efficiency measures such as excess heat recovery. Especially for drying processes, this is mandatory, as usable excess heat (air or other media) should be re-integrated into the system before investing in solar thermal systems.

Depending on the supply system of the drying process and the drying technology used (type, media, temperature, duration, and so on), a suitable integration concept for solar thermal has to be identified, which can be facilitated through a standard classification of heat consumers. This enables the choice of the most suitable integration concept among a large number of possible concepts. Details on the classes of heat consumers and respective integration concepts are documented within the Task 49 Integration Guideline.⁸²

As for all industrial processes and their possible solar thermal supply, integration depends strongly on the process technology used with respect to the chosen/existing supply system based on the demand of the moist material.

• Convective dryers—Drying by drying gas (hot air) in the dryer (drying chamber): In case the drying is done via externally (pre)heated air (or other gas) that is introduced continuously into the dryer and regularly refreshed, either the drying gas can be (pre)heated in a collector directly or this is done via an external heat exchanger transferring the heat from a collector using a heat transfer fluid (eg, water, thermal oil) to the gas. The targeted temperature strongly depends on the moist material and the envisaged product quality.
• Conductive dryers—Drying by a hot base (contact dryer): Using this technology the base has to be heated up to a certain temperature, designated based on the requirements of the moist material (the product) and its behavior on the conductive heat and moisture transfer. The heat transfer from the supply to the base can be done using a heat transfer medium such as water or thermal oil.
• Surface radiative dryers—drying based on thermal radiation mechanism: The supply of the heat to the dryer is similar to conductive dryers and has to be based on the behavior of the moist material and its inner heat and moisture transfer respectively and the required product quality. Again, the heat transfer to the radiation area can be done using a heat transfer fluid such as water or thermal oil.

The key for reliable continuous energy supply to the dryer is the design of the overall system including the collector, the storage, the integration point, the charging and discharging strategy and if needed a backup system.

• Collector: The selection of the most efficient and appropriate collector technology and the design of the collector field strongly depend on the process requirements (temperature, load profiles, capacity, production breaks, holiday breaks, and so on). Depending on the temperature levels needed, different collector technologies can be implemented including flat plate collector, evacuated tube collector, and concentrating technologies such as parabolic trough collector. An optimized system design increases the efficiency and yield of the collector by the interaction with charging and discharging strategies and components, and the orientation and placement of the field. Finally, the collector field design has a large impact on overall costs of the system.
• Storage and back-up system: Many industrial processes require an interruption-free thermal energy supply. In cases when the heating demand and the solar availability do not match in matters of time and load profiles, thermal energy storage and/or a back-up thermal energy supply is commonly used. By the optimized integration and design of the storage (size, insulation, and so on), the economic feasibility can be significantly increased. The use of existing systems and its optimum integration in the solar system (eg, via the storage) is part of the overall design methodology.
• Integration point: As noted above, the integration point can be at the supply or the process level. Supply level integration results in the solar thermal system supplying heat to all industrial consumers, and not just the dryer(s). In contrast, process level integration results in the solar thermal system only supplying heat to the dryer(s). Process level integration in most cases requires a more complex hydraulic design than supply level integration, and as the number of processes being supplied increases, the number of heat exchangers and thermal energy storage systems required also increases. On the other hand, the required temperature to be delivered by the solar system is in most cases higher for system level integration than for process level integration, as in most cases for system level integration the solar thermal system must be designed to supply energy to the whole system and therefore its operating temperature is dictated by the thermal energy consumer with the highest temperature requirements. These higher temperatures at a minimum typically result in lower solar collector efficiencies and larger distribution losses, and may require more expensive collector technologies to be used. Due to these reasons, while more complex, process integration is often more efficient (ie, has a higher specific solar yield) than supply integration. Ultimately, the whole energy system has to be designed in the most technical and economic optimum manner, including possible excess heat recovery as well as a backup system (if necessary).

Industrial processes and their associated technologies are steadily being developed. New process technologies aim to enhance heat and mass transfer to “intensify” reactions, mixing or heating/cooling of process media, including those associated with drying. These intensification strategies can also increase the potential for solar thermal process heat by decreasing the process temperatures (higher system efficiency), increasing the process efficiency, or smoothing the energy demand (reduction of peak loads). Strategies used in new technologies with impacts on solar heat supply are:

• Heat transfer enhancement;
• Conversion of batch processes to continuous processes;
• Increasing selectivity in separation processes;
• Intensification of electrohydrodynamics.

7 LISD APPLICATIONS

8 CONCLUSIONS

Most of the existing SD research is focused on non-industrial or small industrial agro-food applications in less developed regions, and in many cases is focused on preserving food to alleviate poverty. In contrast, relatively little research was found that focused on SDs for (1) the large industrial drying sectors found in heavily industrialized regions; (2) the drying processes for higher-value industrial processes outside the agro-food industry; and (3) higher-temperature drying applications. In particular, a large gap was identified for indirect LISDs that use existing or slightly modified solar collector technologies to drive existing or slightly modified industrial drying technologies. This gap includes the potential to use more sophisticated concentrating solar collector technologies to drive higher temperature drying processes and more sophisticated indirect LISDs for the higher-value drying processes often found in highly industrialized regions. A specific opportunity not addressed in the reviewed literature is the potential to use concentrating collectors to generate steam either directly in the collector (Direct Steam Generation/DSG) or indirectly in a heat exchanger, and then using this steam to drive a superheated steam dryer or condensing this steam to drive a surface radiative dryer. The challenges in coupling solar collector and industrial drying technologies are in large part not unique to indirect LISDs, but rather are similar to the solar thermal integration challenges found across almost potential SHIP applications. Especially in higher value-added drying processes where high-availability and reliability of the dryer are required, further research to integrate thermal energy storage and to hybridize solar thermal with other thermal energy sources in a cost effective manner is required. Ultimately developing indirect LISDs for highly industrialized regions can play a critical role in decarbonizing these industrial sectors.