Source: MENA CSP KIP
Cédric Philibert, Senior Analyst in the Renewable Energy Division of the International Energy Agency
Using solar heat for industrial processes isn’t a new concept and with advancements in CSP technology, new opportunities are being presented. From aiding in the high temperature oil extraction process to low temperature food processing, the application of CSP in industry is providing unique opportunities and, of course, challenges. In this interview, Cédric Philibert of the IEA talks about CSP in industry and reveals these opportunities and barriers.
L.N. What areas of industrial application are starting to experience an impact?
C.P. So far, non-concentrating solar technologies have dominated industrial applications, with the bulk being in low temperature heat which is basically food and drink.
You can see CSP technology in sectors where you need higher temperature levels. There are several applications, mostly in industries such as food and drink, pharmaceuticals, and textiles.
You also have some in extractive industries. For example, the enhanced-oil-recovery (EOR) project by Glasspoint in Oman. They have adapted CSP to local conditions by using a greenhouse to protect the parabolic troughs from wind and sand. The greenhouse allows for a nightly roof cleaning and the use of lighter troughs. The steam produced is pumped into wells to extract larger quantities of oil.
L.N. In what fields do you think CSP could have the biggest impact in terms of industrial applications?
C.P. There are two big areas: the extractive industries like oil, gas and mining and food processing. Both industries are usually remote, which drives up the cost of obtaining fuel and which usually means that they have available space for the installation of CSP troughs or towers.
However, the higher the temperature, the more difficult it is for solar to compete with fossil fuels. Efficiently reaching higher temperatures using CSP requires a more complex and well-built system, resulting in a higher cost. Furthermore, fuel is burnt at very high temperatures which means that burning fuel to meet low temperature demands is a bit of a waste, giving CSP more of an advantage in the low temperature range than in the high temperature range.
L.N. What are the main barriers for CSP in industrial applications?
C.P. Space is a barrier for sure. The others are cost and uncertainty. CSP has a different financing structure which requires all investments and costs upfront and you need 15-20 years to make a profit. However, mining companies are reluctant to sign a contract beyond five years because they only have visibility for the next few years (except for diamond mining) due to fluctuations in the market value of the commodities they produce. This makes it very difficult for them to get equipped with renewables which are only profitable if the industry or plant lasts 15-20 years.
PV developers have developed an offer of five years for the mining industry which is doable. The material is light enough that it can be reused in different places if the plant is closed, but this is difficult with CSP technologies. So yes, uncertainty over returns in volatile commodity markets is a real barrier.
L.N. How would you say these barriers can be overcome?
C.P. Space is probably the most difficult to overcome. We may see the relocation of industries over time to areas where you have good conditions, such as abundant sunshine and ample space.
Learning, economies of scale and moving to places where the resources and cost of capital are more favorable are very important dimensions in terms of overcoming the barrier of cost. Everything that can help de-risk the investment could lower the cost of capital and therefore reduce the total cost.
Additionally, governments can provide support to jump start the industry and reduce costs. France, for example, offers government funding to invest in process heat.
Lastly, an increase in the cost of burning fuels such as a carbon tax or an emissions trading scheme could help as well.
L.N. You’ve mentioned that parabolic trough is one of the CSP technologies that is being used in industrial processes. What CSP technologies are best suited for industrial applications?
C.P. The best solar heat technology depends on what temperature is needed for a certain process. You have three different categories of temperature: low which is up to 150° C, medium-high which is between 150-500° C and high which is above 500° C. If you look at industry needs, you will have a big chunk of low temperature industrial processes are mostly done by flat plates or evacuated tubes. Troughs work very well for medium-high temperature needs. For temperatures above 500° C, which represents half the total need, you would use towers or ovens. It needs to be point-focus concentration to efficiently collect energy at high temperatures.
L.N In your professional opinion, what does the future hold for advancements of CSP and industrial application?
C.P. I see significant potential. Unlike space heating, which is difficult because of its inter-seasonal liabilities, industry has year-round energy needs and you make significant savings in the summer.
I see a future for CSP especially in medium temperature levels whereas it’s not yet economically viable for high temperature levels.
Along the last decade, the potential use of Solar Heat for Industrial Processes (SHIP) has been studied from different perspectives. However, the scope of these diverse studies has usually been more focused on the technical adequacy dimension considering temperatures, pressure and demand profiles. Despite of the fact of the relevance of either solar irradiation, availability of low cost fossil fuel, or surface availability, these variables have been either usually taken into account by a statistical approach or they have not been considered. The detailed analysis of these variables leads to a case by case approach for each industry, which might be considered as not feasible/practical due to the large amount of different industries in Spain.
This study has reduced the target industrial companies to a sufficient low figure that enables the case by case analysis of their own potential. For enabling this approach, it has been necessary to go deeper in terms of resolution. Previous studies stopped at province detail resolution, so that this study has moved a step further up to municipality resolution.
At municipality resolution, three filters have been applied for identifying those places with higher chances to implement SHIP solutions within 4 strategic sectors (Food & Beverage, Agriculture and Cattle raising, Paper and Textile). These three filters are: solar irradiation, availability of piped natural gas and energy demand profiles. In order to take advantage of synergies with previous studies, these 4 sectors have already been selected as the ones with the higher potential of implementing solar heat for industrial processes (SHIP) projects.
The solar irradiation filter is key as the higher the solar irradiation, the higher the solar heat production will be. There are several available solar irradiation studies in Spain, however none of them, based on author’s knowledge, breaks down the solar irradiance by municipality. The source of information for calculating the solar irradiance per municipality has been the information released by the Project named ADRASE conducted by the CIEMAT. A solar irradiation base line has been set in order to consider the sunniest municipalities in Spain.
The competitor of SHIP projects is the current fuel that the industry is using for heat generation. If the current fuel cost is low, the return on investment period of SHIP projects increases, therefore SHIP projects become less appealing for the industry. Nowadays, the piped natural gas supply is the cheapest energy source for heat production. Besides it’s widespread in Spain. As a working tool for the study, a database showing the current gas infrastructure in all municipalities in Spain, has been built up. Coming back to the study outcomes, those municipalities that have piped natural gas infrastructure online have not been analyzed, as most likely, the industries located in such locations will have a current cheap solution for heat generation. So that, the municipalities that have been the focus of the study are those that do not have piped natural gas infrastructure.
The third filter has been used for selecting the municipalities that have industries of the already chosen sectors (Food & Beverage, Agriculture and Cattle raising, Paper and Textile). For this purpose, the INE (Instituto Nacional de Estadítsica) database and the MINETUR (Ministerio de Energía, Turismo y Agenda Digital) database have been deeply analyzed at municipality resolution.
Once the abovementioned filters have been applied, it’s granted that the non-excluded municipalities meet the following features: solar irradiation is likely to be high, there is not piped natural gas infrastructure, and there are industries of the selected strategic sectors. This procedure leads to a reduced number of municipalities which enables the detailed analysis of each of them. Then, a list of all the companies of each municipality have been created. The next step has been to select those firms with the CNAE code matching with the 4 strategic sectors. Furthermore, for each industrial company the following variables are know: name of the company, activity, location and size (micro, small, medium, big).
Among the whole list, the micro firms have been excluded as most likely these firms are local shops for either selling or distribution purposes, so that they are not manufacturing plants. Based on each firm activity, those that do not have thermal processes (for instance: storage activity) have been also excluded from the list. Eventually, among the non-excluded listed firms, a visual check through google maps has been carried out for evaluating the surface availability on their roofs.
The above-mentioned process has generated a list of 200 industrial companies where most likely the use of solar energy may be a very interesting solution for industrial energy savings using renewables source of energy. The very last step of the study has been to validate the assumptions that have been considered in the methodology. For this purpose, some meetings have been arranged with the selected industrial firms. These meetings have had the goal of sharing the potential benefits of using solar energy in industrial processes between 100ºC up to 400ºC. Additionally, the meetings have been the final cross-check of the methodology carried out in the study with the current real situation of the interviewed industrial companies.
Once the assumptions used for the selected industrial companies list creation have been validated, the outcome of the study is a visual representation of several maps showing Spanish areas with higher density of industries with a high potential of integrating solar energy for medium range temperature processes among one of their existing industrial processes.
En el mes de julio, con la información estimada a 31 de julio, la generación procedente de energías renovables ha representado el 31% de la producción.
Según la fuente de generación de la energía, el 20,6% fue nuclear, el 18,8% del carbón, el 17,8% de ciclo combinado, el 15,5% fue eólica, el 10,8% procedió de la cogeneración, un 5,3% hidráulica, un 4,2% solar fotovoltaica, un 4,1% termosolar, un 1,3% de residuos, y un 1,6% de otras energías renovables.
La demanda peninsular de energía eléctrica en julio se estima en 22.423 GWh, un 0,9 % superior a la registrada en el mismo mes del año anterior. Si se tienen en cuenta los efectos del calendario y las temperaturas, la demanda peninsular de energía eléctrica ha aumentado un 0,9% con respecto a julio del 2016.
En los primeros siete meses del año, la demanda peninsular de energía eléctrica se estima en 147.417 GWh, un 1% más que en el 2016. Una vez corregida la influencia del calendario y las temperaturas, la demanda de energía eléctrica ha aumentado un 1,4% respecto a la registrada en el año anterior.
Fuente: Notas de Prensa REE
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Concentrated solar power systems have until recently focused on bulk electricity production, with the main focus on solar towers and trough type collectors. Recent developments have focused on smaller units to supplement thermal power stations and to provide heat for industrial processes. Collectors based on the linear Fresnel reflector design are leading the pack.
There is a gap in the market between non-concentrating rooftop collectors used for solar heating and large scale systems used for bulk electricity generation. This gap is being filled by systems which use linear instead of circular focusing. Two main systems are the solar trough and the linear Fresnel (LFR) based collector. Although the trough based system has proven itself over many years of operation in different applications, the Fresnel system is emerging as an alternative with several advantages.
LFR plant is highly modular, ranging from a few hundred kW to several MW in size, and offers the lowest land occupancy compared to other CSP technologies. Parabolic troughs represent the optimal solution for achievable concentration ratio and achievable energy yield per aperture area and, hence, the best overall plant efficiency in line-focussing mirror systems. For that reason CSP development efforts have concentrated on parabolic trough geometry.
Nevertheless, in the last decade LFR systems have aroused an increasing interest. The main reason for this is the search for cheaper solar field solutions. The considerable economic advantages of Fresnel collectors are principally related to their constructive simplicity. In addition, Fresnel solar fields permit higher land use efficiency than any other type of solar fields. These advantages can offset the lower solar-to-heat efficiency, and LFR power plants represent an interesting alternative to parabolic trough power plants .
The main advantage of LFR systems is that their simple design of flat or flexibly bent mirrors and fixed receivers requires lower investment costs and offers a wide range of configurations. Originally designed for low and medium power applications, LFRs are now being designed for higher temperatures which facilitate direct steam generation (DSG) which can be used efficiently in the industrial or power generation sector.
Fresnel systems can be configured to operate over a wide range of temperatures, from 200 to 500°C, although systems with temperatures as high as 550°C are under development. Applications range from industrial process heat, distributed power generation using the organic Rankin cycle to steam turbine systems.
Developments in energy efficiency in both industry and power generation have focused attention on medium and high temperature solar thermal systems. LFRs have great potential in southern Africa due to the low cost and high percentage of local manufacture inherent in the technology.
The linear Fresnel reflector technology receives its name from the Fresnel lens, which was developed by the French physicist Augustin-Jean Fresnel for lighthouses in the 18th century. The principle of this lens is the breaking of the continuous surface of a standard lens into a set of surfaces with discontinuities between them. This allows a substantial reduction in thickness (and thus weight and volume) of the lens, at the expense of reducing the imaging quality of the lens. Where the purpose is to focus a source of light this impact on the image quality is not of major importance .
The principle of dividing an optical element into segments which have the same (or a very similar) optical effect as the original optical element can also be applied to mirrors. It is possible to divide a parabolic mirror into annular segments, forming a circular Fresnel mirror which focuses the light that arrives in rays parallel to the optical axis onto the focal point of the paraboloid mirror.
In an analogous way, a linear Fresnel mirror can be constructed substituting a parabolic trough by linear segments that focus the radiation that arrives in a plane parallel to the symmetry plane of the parabolic trough onto the focal line of the parabolic trough. A LFR has a similar effect to a parabolic trough, when considering the concentration of the radiation in a focal line i.e. a LFR behaves like a parabolic trough with the same focal length and the same aperture .
Fresnel mirror system
The mirrors focus the sun onto a receiver which contains the heat transfer medium which could be water, oil or even molten salt in some designs. The heat transfer medium used will depend on the operating temperature of the system. The main difference between the two systems lies in the way that the sun’s rays are tracked, and this is what gives rise to the cheaper cost of Fresnel.
In the trough system the whole structure rotates about an axis coincident with the focal point of the trough. This means that the mirrors and the collector are connected mechanically, requiring bearings through which the collector tube must pass. In the Fresnel system the individual mirrors rotate to track the sun. There is no mechanical connection between the mirrors and the collector.
Separation of the mirrors and the receiver allows high temperature heat transfer mediums to be used, and also a much wider scope in the design of the receiver. The Fresnel system also allows individual control of each mirror, effectively changing the configuration of the reflector to optimise its function.
The mirror aperture is the area of the reflector mirrors in the horizontal position, and defines the amount of solar radiation collected by the LFR. Aperture is usually quoted on a module or single mirror length basis. Increasing the mirror aperture increases the amount of solar radiation reflected to the receiver module. Typical aperture widths for large systems are of the order of 15 m.
The receiver aperture is the area of the receiver per mirror or module length, and varies according to the design of the system. Receiver width is close to the mirror width, but may be less if curved mirrors are used, or wider if other focusing methods are used.
Mirrors may be flat or elastically curved and are generally constructed of glass with a composite/metal backing, although other materials are finding their way into the sector. Adding curvature to the mirror increases the concentration ratio and makes the design of the receiver simpler. Mirror width and length will depend on the design. The mirrors are mounted in the tracking system in several different ways. A typical mounting would be a circular loop driven by a tracking motor (Fig. 4).
The receiver generally consists of a secondary reflector mounted above the receiver tube which contains the heat transfer fluid. The receiver may consist of a single tube or several tubes, which may be contained in a vacuum glass tube enclosure. Typical designs are shown in Fig. 5.
The LFR system alone cannot reach the same radiation concentration as a parabolic trough. The sun changes its position in relation to the optical axis plane of the system. It is theoretically impossible to design the curvature of the individual mirror strips in such a way that there is always a sharp focal line for parallel radiation, and it is necessary to mitigate the unavoidable optical inaccuracy of the Fresnel collector. This can be done by a secondary concentrator that is located above the receiver tubes.
The collector concentration ratio is the ratio of mirror aperture to receiver aperture. The low-profile setting of LFR collectors maximises the concentration ratio, which enables high temperature output.
Fundamentally, increasing the mirror aperture allows more sunlight to be reflected onto the receiver. Because the low-profile architecture provides for great flexibility in the selection of length and concentration ratio, linear Fresnel collectors can be readily tailored for different target temperatures to meet varying application needs, thus providing practical versatility of usage. As an example, the Fresdemo system has a primary mirror field width of 21 m and a receiver width of 0,5 m, giving a concentration ratio of 42 .
Heat transfer fluids
The fixed design of the receiver gives a wide choice of heat transfer fluids. Water is used as the transfer fluid at low temperatures, oil as temperatures increase, steam for much higher temperatures and molten salt for the highest ranges. In addition, pressurised CO2 or air are being considered for higher temperature operation .
LFR exhibit optical conversion efficiency in the region of 65%. Conversion efficiency depends on the angle of incidence of the sun, and thus average efficiency will vary with latitude. Thermal peak output of 562 W/m2 in terms of primary reflector aperture area, and 375 W/m2 in terms of installation area usage, is claimed for a typical commercially available system .
Medium heat generation
Medium heat systems for industrial applications or supplementary power systems operate in the temperature range 100 to 250°C and may use water or oil as the heat transfer medium. There are numerous systems of this type in operation.
Direct steam generation
Thanks to the fixed absorber tubes, direct steam generation (DSG) is easier in LFR power plants than in parabolic trough power plants. DSG has several advantages over oil heat transfer:
- Steam as heat transfer fluid allows higher temperatures because there is no danger of thermo oil cracking. Novatec Solar is planning a new Fresnel power plant generation that operates at 450°C .
- The number of construction components can be reduced because no heat exchange has to be realised between a solar field heat transfer fluid/thermo oil) and Rankine cycle working fluid (water/steam).
- The thermo oil itself is an expensive component of CSP plants, so the lack of thermo oil is a direct advantage.
- As there is no heat transfer between two heat transfer fluids, there is one thermal loss factor less.
- The use of steam as a heat transfer fluid may reduce the mean heat transfer fluid temperature in the absorber tube (even at higher final temperatures) and reduce thermal losses. This reduction is possible because in a large part of the receivers the boiling process is realised, which takes place at a reduced temperature. Only in the small part, where the superheating of the steam is realised (if there is superheating), high temperatures are reached.
- Water has further advantages in comparison to other HTFs: It is environmentally friendlier than thermo oil so that leakages in a steam generating plant do not produce environmental dangers.
Water is less corrosive than salt. Its freezing temperature is much lower than the freezing temperature of salt and even slightly lower than of thermo oil. The effort required to ensure adequate anti-freeze protection is reduced significantly.
There are several systems on the market that use DSG. In this system water is fed through pipes in the receiver where it is converted to steam and then to superheated steam. Steam is used to drive turbines directly or as supplementary heating for thermal power plants. Steam is also used for industrial purposes.
Molten salt systems
High temperature systems (≈550°C) using molten salt as the heat transfer medium have been developed. Such systems incorporate heat storage systems for prolonged operation. Fig. 6 shows a high temperature plant using salt as the heat transfer medium .
Because of the simplicity of design and the high operating temperatures now possible, LFR are being considered as alternatives to tower and heliostat based CSP systems. In addition, the short optical path between mirror and receiver eliminates many of the tracking problems as well as environmental objections associated with tower/heliostat based systems. In addition LFR salt based systems are able to recover from “salt freezing”.
Supplementary heat or steam generation for power plants
Fresnel mirrors are being considered as a means to supplement heat in thermal power stations. Parts of the cycle such as preheating of the boiler feed water use low to medium temperature heat sources, and solar thermal systems can provide this heat. Typical applications are to substitute the bleed steam used for preheating boiler feedwater as shown in Fig. 7.
The solar heat source is connected in parallel with the bled steam source and when available provides the heat normally used from the steam. The increased steam output then enables additional power generation from the turbine (solar boosting mode) or fuel consumption can be reduced (fuel saver mode).
More complex systems use several stages of regeneration with heat substitution at each stage. Heat may be stored for periods when the solar resource is low, or the plant may be operated in a two shift mode. Using solar power may have two effects:
- Increase in the power generation capacity of the plant during periods when solar power is available.
- Decrease in coal consumption with no increase in power generation.
The integration of solar thermal collectors into conventional fossil plants, or solar aided power generation (SAPG), has proven a viable solution to address the intermittency of power generation and combines the environmental benefits of solar power plants with the efficiency and reliability of fossil power plants. In SAPG technology, thermal oil can be used as solar heat carrier and no solar steam needs to be generated, therefore the pressure of solar system can be much lower than that of the solar collector using water/steam as the heat carrier. Newer developments make use of the higher heat content of solar steam directly, provided from DSG plants.
The temperature of the heat source is one of the major defining factors of a power plant – higher temperature results in higher overall power plant efficiency. With SAPG, the heat source temperature is not limited by the solar input temperature and is therefore an effective means of utilising low or medium solar heat (250°C) for power generation. However, internationally the adoption of the technology has been slow, despite it being a viable and quick means of CO2 emission reduction.
A study was conducted on a simulated SAPG power plant at Lephalale, which was based on a generic 600 MW subcritical fossil power plant with a reheater and regenerative Rankine cycle with two low pressure feedwater heaters (FWH) downstream of the deaerator and three high-pressure FWHs upstream .
This study showed that integrating solar thermal with steam plant is approximately 1,5 times as efficient at converting solar energy to electricity as a CSP stand-alone generating plant. Therefore, a solar assisted high pressure feedwater heater system at an existing coal-fired power station is 1,8 times more cost-effective than a stand-alone CSP plant. The conversion efficiency is claimed to be of the same order as solar PV .
The Fresnel mirror solution is gaining popularity because of cost and simplicity advantages over the parabolic trough solution for SAPG systems. Costs of the Fresnel based system are estimated to be approximately 70% of the equivalent parabolic trough system .
 M Gunter: “Linear Fresnel Technology”, DLR, www.energy-science.org/bibliotheque/cours/1361468614Chapter%2006%20Fresnel.pdf M J Montes, et al: “A comparative analysis of configurations of linear Fresnel collectors for concentrating solar power”, Energy, 2014. Novatec: “Concentrated solar power by Novatec solar”, Brochure. E Hu, et al: “Solar Aided Power Generation: Generating Green Power from Conventional Fossil Fuelled Power Stations”, Intech open science. W Pierce, et al: “A comparison of solar aided power generation (SAPG) and stand-alone concentrating solar power (CSP): A South African case study”, Applied Thermal Engineering, 2013. Areva: “Areva integrates energy storage in solar CFLR design at Sandia national labs”, http://us.areva.com/EN/home-1977/areva-solar-power-salt-storage.html S Benmarraz: “Linear Fresnel Reflectors Concentrated Solar Power: cost reduction and performance improvement trends”, IRENA Workshop, March 2015. Industrial solar: “Industrial Solar linear Fresnel collector LF-11”, Technical brochure.
Mike Rycroft, features editor, EE Publishers
More than 500 industrial manufacturers trust solar heat worldwide. SHIP is the acronym for Solar Heat for Industrial Processes and describes systems which provide solar heat in a factory. A collector field heats a process fluid by means of solar radiation and a heat exchanger transfers this heat to a supply system or production process in the factory as hot water, air flow or steam. Storage units make it possible to use the generated heat at night-time