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¿Heatcrete o sales fundidas?

This article is the first in the series Panel of experts, a space in which two specialists expose your opinion on a topic proposed by CSP Today.

On this occasion, Juan Barragán (NEST AS) and Mark Schmitz (TSK Flagsol Engineering GmbH) debate the following: Heatcrete or molten salts?

Heatcrete - Juan Barragan, NEST AS

The NEST AS Norwegian group believes that its patented technology for storage of thermal energy, Heatcrete, is more competitive than other storage systems, including that based on molten salts. To explain why is Juan Barragán, engineer senior systems of thermal energy in NEST, who has completed a master's degree in engineering specializing in chemical industrial process.

"At NEST AS, we have developed a new, cost-effective thermal energy storage technology. This versatile system of thermal energy in solid state storage can be applied to the concentrated solar energy and other applications. The thermal energy is stored in a medium based on concrete which has been developed together at Heidelberg Cement: Heatcrete. A storage system consists of a large number of elements that are connected.

The (the same used in the solar) thermal transfer fluid flows within elements of the Heatcrete, transferring most of the energy during the period of loading or absorbing it during the download. NEST technology delivers a completely modular, simple and cost-effective scalable system. Moreover, commercial size may range from a few MWhth to several GWhth. Compared with molten salt systems, NEST thermal energy storage technology can be very profitable for the systems on the MWhth scale.

The temperature that could be kept within the solid-state storage range from - 0 ° C to over 600 ° C. And as broad as this temperature range it is beneficial for installations that use other types of fluid heat transfer such as steam or fluids of new generation. In fact, the wide range of temperatures compatible with storage provides many different purposes, besides prolong the generation of turbine, preheating heat transfer fluid, or the unit of energy, as well as prevent freezing without risks for the installation.

Cost-effective solution

The NEST storage system has less capital expenditure than molten salt due to the simplicity and modularity of the design systems. Basically, the idea is to have the same unit repeated as many times as necessary to achieve the expected capacity. It is possible to avoid external circuits with pumps and different fluids to save an important part in the investment. Also, thanks to solid state storage medium, the amount of electric monitoring and necessary heaters in the solution of NEST is also insignificant compared with molten salt systems.

As regards the supply and construction of a storage device, the solution of NEST is also more competitive than other systems because most of the components for storage can be achieved at the local level (especially the conglomerates of concrete and steel) in many regions of the world.

In addition, NEST storage system presents a lower operating costs because the main part of parasitic burdens comes from thermal transfer and do not own storage fluid pumps. Likewise, the rest of the installation presents a very low electricity consumption. For this reason, the loads of parasitic when storage is in standby mode are negligible. It should also be noted that almost half of the components used are similar to those normally used in solar, such as building materials, instruments and valves. This drastically reduces the inventory of spare parts in a plant and the required training period for the r & d team.

Simple and solid design

The Heatcrete storage medium designed so as not to require maintenance. It has shown that standard concrete is a reliable material able to withstand millions of cycles of stress during the so-called proportional loads in the construction sector. In the case of a thermal storage, Heatcrete normally experiences a cycle daily in the course of 30 years. This is simply 10 000 cycles of thermal stress during this period cycles. This is considered low cycle fatigue; Therefore, the expected life of the Heatcrete is far superior to the life cycle of the plant. For his part, welded steel pipes also require little maintenance, so the main part of the maintenance should focus on the interface between the storage and the rest of the solar system. And this is not what happens in other alternatives of thermal energy storage.

How to exploit the system is very similar to the rest of the solar system because all the elements managed during exploitation are quite similar to the rest of components installed in the plant. Therefore, the same team with the same skills working in solar installation can arrange storage system varying simply flow through thermal energy storage.

In addition, design offers a very flexible and solid performance and, as a result, periods of disconnection for NEST thermal energy storage technology will be minimal, while you might expect long-term disconnections occur in systems of molten salt due to the complexity of the fluid (plugs of salt, corrosion, spills, etc.).

The start-up and initial operation of the NEST installation is simpler than the molten salt systems. The initial warming occurs when the thermal transfer fluid flows through the storage previously conditioned with an external drive. This important phase, which is crucial for any storage performance, is more simple control that you for deposits of molten salt Preheat and process of melting salts.

Comparison of performance

To analyze the performance of storage, NEST thermal energy storage solution presents a very different performance compared with the molten salt technology. The main difference is that storage outlet temperature will decrease during the discharge and inlet temperature will increase during the period of load, while in molten salt systems, two temperatures are consistent.

Intrinsic performance of turbine energy production provides a constant for molten salts and decreases in the NEST system. In terms of gross output, the amount of electricity in a molten salt plant could be higher, but if you look at the net value, where the parasitic loads that are consumed during production should be subtracted, it is possible to conclude that NEST thermal energy storage presents a performance enhancement. As a result of the proportional performance of turbine that runs on the storage of NEST, we realized that for a certain period of time, the turbine performance was also superior compared to a molten salt system. This option can be used for projects where the rate agreement for the purchase of energy is higher in the first hours of discharge.

In any case, take a decision on the best thermal energy storage system is a complicated task and it is necessary to analyse and evaluate several factors. For many of these, NEST storage is the most competitive alternative".

Juan Barragan,

Senior for thermal energy systems engineer,

AS NEST.

Molten - Mark Schmitz, Flagsol Engineering TSK salt

Mark Schmitz, head of technology and innovation in TSK Flagsol Engineering GmbH, supports the development of technology for CSP plants in the German company, which has developed the conceptual design for the first commercial system of thermal energy storage in molten salts and now acts mainly as EPC contractor. With experience in linear collectors Fresnel, parabolic trough plants central receiver and for storage and thermal transfer, molten salt systems explains why molten salts offer many advantages to the owner of a CSP plant.

"The main advantage of CSP against wind and photovoltaic energy is the ability to supply that is due to the integrated systems of thermal energy storage. The oldest thermal energy storage system and, therefore, the most widely used in the energy sector uses molten salt. It was developed for Solar Two (1995) and a German consortium adapted it for use in systems with linear approach around Flagsol and was first commercially used in Andasol 1 (2009). In fact, it is currently working on more than one dozen of commercial plants, including a Tower System. The total storage capacity is enough to generate more than 5700 MWh of electricity each night.

In addition, molten-salt thermal energy storage systems can be subdivided into direct and indirect systems. Direct systems, "cold" but still liquid salt is removed from a warehouse, acts as a transfer fluid thermal cooling solar receiver and is stored in the hot tank, from where it is extracted to generate steam when necessary. In indirect systems, a main heat transfer fluid is heated (typically, a type of oil) on the receiver and can be heated in parallel a thermal energy storage system or generate steam. In any case: "Panta rhei, everything flows" and the heat transfer is achieved efficiently through convection. The effects of the transmission of heat, which causes a non-constant thermal transfer, are insignificant.

Salts of nitrate, mostly above 400 ° C, require a thorough consideration of the quality of the material of the container vessels, ducts and pumps. This implies an effect important in economies of scale (i.e., bigger, better) while the storage capacity, proportional to the volume of the reservoir, increases more rapidly than the surface. On the other hand, the Nueva pressure increases stress on the walls of the tank to increase the height and diameter, which produces optimum capabilities, approximately 30 000 tonnes.

Obviously, there is an upper limit, as shown in the floor veranda with six pairs of deposits. Pretty lower than the optimum size mentioned the system sizes have increasingly high specific costs. Another disadvantage is that changing a system like this is not very flexible once it has built since it requires a thorough analysis of the present and future needs of the supply.

Molten Salt-based thermal energy storage systems offer a large number of advantages for the owner of a CSP plant:

-Concrete cost of storage is low. Nitrate salts are a basic product in bulk and even if it increases its use by the sector of the CSP will not affect the large quantities that consumes the fertilizers sector. Andasol power plants, with porous supply chain and the high effort of engineering difficulties, only could be economically viable through the implementation of a system of storage of thermal energy in molten salts.

-In storage systems based on molten salts, nominal power of discharge and charging power are virtually independent of storage capacity, only dependent on pumps that carry the hot salt or salt cold, respectively, and the heat exchangers.

-Well designed systems have a certain redundancy in some components so that, even in the event of a pump fails, the system will operate with a nominal yield. In fact, the levels of reliability depending on the needs of the owners of the plants can be adjusted.

-While the masses of salt in the two deposits have a relatively homogeneous temperature, energy for the loading and unloading of the thermal energy storage system can be easily controlled to control the mass flow. The fact that predominates the convective heat transfer affects the ability to exploit thermal energy storage system since it is full until it empties, and vice versa, to a maximum energy. Furthermore, systems that rely on a conductive heat transfer, such as storage with solids or phase change materials, present a loading and unloading discontinuous. During the download, these systems show a decrease in the temperature of output or only allow one reduction in increasing the mass towards the end of the process flow.

-The systems are very efficient. Single thermal transfer fluid comes in contact with the heat exchange surfaces, there is no flow in the regions of thermal energy storage that are (UN) loaded. This allows the ducts are short and have few curves. Low pressure loss can be seen only then a very low consumption for loading and unloading.

-Due to the rounded shape (thermal energy in molten salt storage depots are cylindrical and have domed roofs), heat losses are low. The efficiency of travel is between 98% and 99%.

-Scheduled a minimum degradation in ability and energy of thermal transfer, for now, has not been observed in the five years of operation.

-The large number of systems that work successfully allows easy access to financing.

In short: storage systems allow operators of CSP plants provide electricity in a predictable manner and use optimally the variable power rates. In addition, molten salt systems are a proven technology and offers excellent financing capability. As r & d in the sector of the CSP and many studies suggest, there is a solid trend for heat directly on receivers molten salts for all types of collectors (either trough or tower, Fresnel) due to thermodynamic advantages offered by higher temperatures".

Mark Schmitz, responsible for technology and innovation, TSK Flagsol Engineering GmbH.


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