SolarPACES 2012 – Commercial thermal storage. Molten salts vs Steam accumulators

8/9/2019 SolarPACES 2012 Commercial thermal storage. Molten salts vs Steam accumulators

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SolarPACES 2012 Commercial thermal storage. Molten salts vs

Steam accumulators

C. Prieto1, A. Jove

2F. Ruiz

3, A. Rodriguez

4, E. Gonzalez

5

1

Master of Chemical Engineering, Abengoa, [email protected] of Mechanical Engineering, Abengoa, [email protected]

3Physical degree, Abengoa, [email protected]

4Master of Mechanical Engineering, Abengoa, [email protected]

5Master of Chemical Engineering, Abengoa, [email protected]

Abstract

From some years ago there is a very big increase of thermosolar power generation industry and with it its

associated thermal storage systems. They are crucial to ensure the success of the technology allowing

dispatchability enough to supply energy when its demanded. Two different technologies are currently

implemented commercially regarding thermal energy storage: the steam accumulator for direct steamgeneration plants and the double tank of molten salts either for parabolic trough with thermal oil or themolten salt tower technology.

Due to diversified demand profiles (with respect to type, amount, and power of needed energy) each

energy storage (electrical, thermal, mechanical or chemical storage) requires a specific, optimal solution

regarding efficiency and economics.

Keywords: Storage, thermal storage, molten salt, steam accumulator, double tank, phase change material

1. Introduction

For thermal energy storage systems it can be derived, that there is more than one storage technology

needed to meet different applications. Consequently, a broad spectrum of storage technologies, materials

and methods are needed. The overall target in designing TES systems is the reduction of investment cost

and the enhancement of efficiency and reliability. To achieve these objectives, material, design and

system integration aspects have to be considered in equal measure.

The assessment of identification and selection of the optimal TES system only is not focus on the storage

material, further important components of the power plant also have to be included in this study: the

containment, and mainly the heat exchanger and structural parts for charging and discharging, and

furthermore devices and sub-components, which are needed for operation and integration such as pumps,

valves, control devices etc.

A key issue in the design of a thermal energy storage system is its thermal capacity. However, selection

of the appropriate system depends on many cost-benefit considerations, technical criteria and

environmental criteria.

Cost: the storage material itself, the heat exchanger for charging and discharging the system and the costof the space and/or enclosure for the TES.

Technical point of view: high energy density in the storage material (storage capacity); good heattransfer between heat transfer fluid (HTF) and the storage medium (efficiency); mechanical and chemical

stability of storage material (must support several charging/discharging cycles); compatibility between

HTF, heat exchanger and/or storage medium (safety); complete reversibility of a number ofcharging/discharging cycles (lifetime); low thermal losses; ease of control.


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Technology: operation strategy; maximum load; nominal temperature and specific enthalpy drop inload; integration into the power plant.

Power of the cycle and hours number of TES requested

Efficiency of the power plant, thus this study should include the assessment of the solar field efficiencyand cycle efficiency.

Investment cost of the solar field and of the storage system and as well as an estimation of O&M cost ofthe overall plant.

Assessment of the trend of the HTF cost and of the material storage cost in the market.

Abengoa is the only company whose commercial portfolio uses different storage concept. Based in this

experience, the aim of this study is to confirm the need of having different storage technologies available in themarket. The study compares both technologies explaining which are the main advantages, disadvantages, challengesand particularities of each one. It is directed to analyse different aspects related with its plant configuration,operational issues, performances and costs associated.

2. Storage with steam accumulator

2.1. General overview

Direct Steam Generation (DSG) in parabolic trough power plants is a technological option with great

development potential, as it eliminates the need for intermediate heat transfer liquids while increasingoverall plant efficiency as well as reducing cost, increasing performance and becoming a more

environmentally friendly technology.

This is due, in part, to the fact that the water inside the receiver tubes absorbs the energy reflected, and

changes from liquid state into saturated steam and, subsequently, into superheated steam. The steam

produced in the solar field is fed directly to the turbine without the need for a heat exchanger. This

eliminates the oil/water heat exchanger train and incorporates water/steam separators. In addition, the

limitations on the maximum solar field temperature imposed by the degradation of the thermal oil (400

C) disappear and, therefore, the technology allows access to more efficient high temperature power

cycles.

DSG technology also improves solar field performance since the average operating temperature of the

solar field is slightly lower than that of the oil system, which reduces the thermal losses. Moreover, the

jump of temperature required in the oil/water exchanger is eliminated in the DSG plant. Furthermore,

investment costs are reduced due to the elimination of the exchange system, expansion tank, the thermal

oil itself and other systems related to the utilization of oil.

Thus, DSG technology allows an increase in production while reducing capital investment compared to

plants that utilize thermal oil as the heat transfer fluid. The savings in the cost of energy could be in the

region of ten percent.

A major design consideration for parabolic trough power plants is thermal energy storage (TES) options.

Currently the best option for DSG is the use of a steam accumulator storage system. A steam accumulator

is a direct storage system eliminating intermediate equipment. It is based on the Ruth accumulator system

where the steam is directly stored at high pressure in accumulator tanks.

The accumulator system is charged or filled with the saturated steam produced at maximum pressure from

the evaporator solar field and this steam transfers the heat from the solar field to the fluid contained insidethe tank. During the discharge process, the pressure inside the tank drop generating a flash evaporation of

steam and this steam is sent to the turbine. Two accumulators are discharged, one of them will produce


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saturated steam which will be superheated in a heat exchanger using the higher pressure saturated steam

leaving the second accumulator. This system allows for a very quick response of the storage medium, and

will have a good performance during transients.

The excess steam produced in the saturated solar field will be used to charge the steam accumulator, this

allows higher mass flow of dry steam to enter the superheater during the charging phase as well as a

higher temperature at the turbine inlet.

During the start-up process in a DSG plant the evaporator solar field has to be preheated and producing a

minimum steam mass flow rate in order to focus the superheater solar field progressively, while

maintaining a part of the solar field defocused. However, this negative impact could be reduced by using

saturated steam from the storage system. Furthermore, the energy remaining in the steam accumulators

after the end of the discharge process could be used to preheat lines and auxiliary systems reducing thestart up time for the next operating day.

2.2. Advantages and disadvantages

The accumulator system will be composed of several numbers of tanks depending on the desired storage

capacity and a heat exchanger system where the saturated steam will be superheated during thedischarging process. This concept is very simple from the operational point of view.

One of the main advantages of this storage system is that the storage fluid is water, which eliminates thenegative environmental impacts and reduces the uncertainty in the price of the storage fluid. This

advantage is seen when comparing with motel comparing with molten salts that are based on nitrates and

the oil systems. Also since the storage fluid is water, all of the equipment used in these storage systems is

entirely conventional and therefore the performance of the equipment compared with a molten salt system

is higher.

Another of the advantages is the O&M cost what is significantly reduced by (1) the energy consumption

of pumping the storage fluid, which is almost negligible compared to a molten salt pump, and (2) the heat

tracing system, which is not required neither in pipes nor tanks.

The disadvantages of the accumulator design include the cost and complexity of manufacturing the tanks

and the relationship of the volume to the energy stored. The conventional steam accumulators need a huge

amount of stainless steel in the manufacturing of the tanks due to a high dependence on the steel cost.

This making the scaling up complicated due to the high pressure discharge required. Also the large

specific volume in relation to amount of the thermal energy stored is another disadvantage to consider.

On the other hand, the possibility of using new materials in the storage tanks gives these systems a great

potential to reduce current costs based on carbon steel tanks.

The next picture shows a basic diagram of the plant with storage system.

Fig. 1. Scheme of a DSG plant with steam accumulator


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Another main disadvantage of the steam accumulators in comparison to molten salt double tank systems

is, that the discharge will occur at a lower pressure than the nominal pressure of the cycle. For this reason,

the energy stored by the steam accumulators has to be greater than the two-tank system for the same

amount of energy production.

In any case, the net benefit of the DSG technology and the simplicity of the steam storage system justify

and advise this solar technology, in comparison with the two-tank system for oil, make this an interestingsolar technology to pursue further.

3. Storage with Molten salts


Molten salts are the most widespread fluid for thermal storage in CSP commercial applications due to

good thermal properties and reasonable cost. Nowadays, molten salts provides a thermal storage solutionfor most of the technologies available on the market due to this fluid could be used as direct and indirect

storage depending of the selected plant philosophy.

Both, trough and tower technologies, use the double tank system as thermal storage configuration. Moltensalts are used as indirect storage in parabolic trough facilities which works with oil as heat transfer fluids

and as direct storage for tower concepts which molten salts are also used as circulating heat transfer fluid.Other concepts under development like the parabolic trough with molten salts as heat transfer fluid which

it could be comparable to the tower with molten salts regarding thermal storage point of view.

In general, molten salts storage system offers the possibility to provide electrical production at constant

conditions thanks to maintain the storage material in different tanks when it is charged or discharged. In

addition it becomes an interesting solution due it has very high energy density per specific volume and

very high thermal inertia due to its characteristic thermal properties of high heat capacity and low thermalconductivity. Due those thermal properties the system can be designed with minimum thermal losses

which represent higher global effectiveness.

The double tank of molten salts requires less specific volume for the same energy stored thanks to the

higher thermal capacity of the salts, specifically when it is used as direct storage medium where inventory

is minimized due to temperature gradients between hot and cold focus are bigger. On the other hand,

double tank storage concept involves intermediate equipments in the system configuration as heatexchangers. In this way, two extra heat exchangers (thermal oil to salts and thermal oil to steam) in the

case of indirect storage and one (salts-steam) in the case of direct storage are needed for these

configurations.

The most common fluids for double tank storage system are sodium and potassium nitrates mixtures with

a weight composition which optimizes cost and thermal properties. These mixtures, which have pricessignificantly stable in the market, are well known from decades ago with wide bibliographic information

and proven feasibility at pilot scale. Regarding materials compatibility, corrosion phenomena should be

taken into account due to impurity content of these mixtures but it can be assured the good performancewith the most common materials used in the industry.

Molten salts as storage material has inherent risks due to high freezing point of these fluids. Electric heat

tracing systems and tank heaters are installed to minimize freezing risks. These equipments involvehigh parasitic consumption in terms of maintain the salts hot enough to avoid freezing or plugs even when

the system is completely discharged.

Going in deep to each different technology there are some different particularities depending on which it

is related. Below there are described the most significant ones for each solution.

3.2 Parabolic trough with thermal oil as heat transfer fluid technology


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The double tank of molten salts represents an optimum system for this technology due it matches

perfectly the thermal sensible behaviour of the thermal oil used currently (see figures). Thermal oil

operation temperatures are from 300C to 400C approximately and molten salts are efficient and

operable enough at those temperatures.

250

270

290

310

330

350

370

390

410

Temperature(C)

Length of the HX

HTF

Molten Salts

Fig. 2. Profile of temperature in the heat exchanger

The power cycle to be used with this system could be with preheater, evaporator, superheater and

reheater. Depending on the cycle design common efficiencies reached with this technology are around

37%.

Thanks to the utilisation of efficient heat exchangers the hysteresis between charge and discharge it couldbe reduce to a few degrees (around 10C) thus the system is able to generate higher than 90% of the

nominal conditions and as commented before it is also able to maintain constant conditions during the

whole discharge.

From different experiences with this system several assumptions can be confirmed: the system is able tosupply energy at constant conditions; there are no big concerns about corrosion, always related with

chlorides content on salts; degradation of the salts or other components related with the total impurities in

the salts; it is a system with high thermal inertia with the benefit it could represent regarding thermallosses and there are no major toxicity problems than the NOx control within the tanks (strongly related

with the magnesium content on the salts).

On another hand it has been proven that the system needs of significant time to change from charge to

discharge conditions and in relation to that and with the heat exchanger design, it is difficult to produce or

to design a system to produce at partial loads in order to produce jointly with the regular solar field

production.


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Finally it has to be mentioned that it is one of the most cost-effective systems for the differenttechnologies apart of the well-known particularities it has thus it becomes the desired thermal storage

system for parabolic trough technology with thermal oil as heat transfer fluid.

3.3 Tower technology with molten salts as heat transfer fluid

For tower technology with molten salts as heat transfer fluid it seems compulsory to install a double tank

of molten salts thermal storage system. Due the temperature gradient on the salts side it is higher than

with thermal oil (300C - 565C instead of 300C - 400C) the system is able to store more thermal energy

with the same volume making the system more cost-effective. It has to be mentioned that the upper limit

of 565C it is given by the molten salts which are not stable at higher temperatures.

One distinguishing characteristic of the system is that it is able to work with Rankine cycles with

preheater, evaporator, superheater but not with reheater with pressures lower that 65bar thus the salts

would freeze in the process of reheating. All those equipment are common from both the power block

steam cycle and the thermal storage system. The common efficiencies that can be achieved with those

cycles are around 40%.

Another particularity of that system is that it works only in one direction: always exchanging heat with

the water-steam in discharge mode due the charge it is done directly in the receiver, because of that this

system does not suffer any hysteresis between the charge and the discharge thus the discharge is able to

generate power at nominal conditions.

As commented before the system usually needs of a preheater, an evaporator and a superheater, and

depending on the unit the thermal behavior between the steam and the salts is different (see figures). Forthe preheater and the superheater the thermal exchange occurs in good conditions in a counter flow heat

exchanger but for the evaporator the heat exchange occurs from a lowering temperature heat source to a

constant temperature sink (the evaporating steam) with the loss of affective heat transfer area that this

represents.

Fig. 3. Profile of temperature in a steam generator with molten salt


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The possible problems associated are mainly the same as with the parabolic trough technology with

thermal oil but taking into account bigger temperatures which it is more restrictive from the point of view

of corrosion and degradation temperatures.

Regarding the time response of the system and the possibility to work in parallel with receiver production

it has to be mentioned that depending on the design of the preheater, evaporator and superheater it will be

more or less feasible to work at part loads but when those equipment allow to do it the system is able to

supply extra flow to the receiver in order to maintain the nominal conditions in a relatively short time.

Finally about the economics it is true that the system is the one with lowest cost even it is needed one

stainless steel tank. This is because it is avoided one heat exchanger and because the smaller size of the

system needed for the same capacities due to the bigger temperature gradient.

4. Other promising options for the near future1


At first of this paper, direct steam generation technology was presented as one of the most promisingoptions for renewable power plants improving the convention plants based on oil system. This is due to

the fact that with water as the heat transfer fluid first of all the thermal oil it is avoided with the cost and

toxicity that it represents and it is possible to reach higher temperatures and pressures at the BOP with the

increment on the efficiency associated.

In order to achieve those high temperatures, pressures and efficiencies from storage system at the BOP itis used to design those plants with a superheated steam Rankine cycle having two differentiated zones or

modules depending on the conditions of the steam: the saturated module and the superheated module.

The thermal storage system currently commercially available is the steam accumulator described before

and only for the production of saturated steam but there are other options under study and developmentwhich are focused the production of the steam in the two different modules: the saturated section and the

superheated section.

As commented before one of the big problems of the direct steam generation storage is the electricitygeneration at constant and nominal conditions, although it can be achieved with the solutions presented

above it represents a big volume utilization due to the low energy density of the systems.

With the objective of improving this performance, one interesting technology to solve it is the one based

on phase change materials. A system with one phase change material would be able to produce saturated

steam at constant conditions while the storage material is crystallising.

From more than 10 years ago few pilots have been constructed, operated and tested attempting to solvemost of the problems associated to this technology as the low thermal conductivity of the phase change

material, the volume expansion management, the thermal degradation, corrosion issues and so on but

most of them have resulted in material failures or excessive costs.

Related to the problem of producing steam at constant and nominal conditions it has to be considered thatit is also needed the superheated module in order to produce electricity at the same nominal conditions

during the charge.

4.2. Next generation of storage technology

In order to be able to produce electricity at nominal conditions during the whole discharge of the directsteam storage systems some alternatives have been considered:


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Phase change material system for the saturated block and solid sensible heat accumulator tosuperheat the steam

Phase change material system for the saturated block and phase change material cascade tosuperheat the steam

Phase change material system for the saturated block and double tank of molten salts tosuperheat steam

Steam accumulators for the saturated block and solid sensible heat accumulator to superheat thesteam

Steam accumulators for the saturated block and double tank of molten salts to superheat steam

5. Conclusion

Thermal energy storage is the tool that the concentrated solar plant has to be dispatched. The thermal

energy storage technology has to be defined depending on some factors as the heat transfer fluid, the

condition of the power block and other parameter described in the paper.

It is very important to have knowledge of the different technologies existing in order to select the

optimum storage in each plant. Steam accumulator and molten salt storage are the two commercial option

that the market offer and each one has a market share,

New developments are been carried out in order to optimize the existing design but a good portfolio ofstorage technology allow to build the optimum plant per project.


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References

[1] IEA ECES Annex 19, final report 2010

SolarPACES 2012 – Commercial thermal storage. Molten salts vs Steam accumulators

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