Energy comparison analysis between direct and indirect dry saturated steam generation, thermally powered by EFPCs’ solar fields

Steam is a key energy vector in the industrial sector and each application requires it at a specific pressure and temperature. In this paper the production of low pressure dry saturated steam for industrial use through high-vacuum flat plate solar collectors (HVFPCs) is discussed. This technology can produce steam from solar energy, hybridizing it with existing fossil powered steam generators to obtain significant energy savings and reduce CO 2 emissions. An energy comparison using the 0-D TRNSYS® software between numerical results of different plant configurations is made, which differ in the type of dry saturated steam production device. These devices are necessary as it is not possible to produce steam directly inside collectors. Two possible steam generation methods were analysed: direct steam production, using a Flash vessel, and indirect steam production, using a Kettle reboiler. Finally, each configuration was simulated by imposing a solar field ΔT of 10 °C and 20 °C. Dynamic results show that flash vessel configurations are generally the most efficient, with the same operating parameters, compared to the configurations with Kettle reboiler. Furthermore, configurations with certain ΔT, such as to determine lower operational solar field temperatures, lead to the best results due to the higher HVFPCs’ efficiency.


1.
Introduction Dry saturated steam is widely used within global industry, the latter made up of various production sectors, each one requiring steam at a specific pressure level.Generally dry saturated steam at low and medium pressure is used in the construction sector, and in chemical, medical and food industries.This is generally produced on site using firetube steam generators, which allow to produce dry saturated steam up to pressures of 20 bar (upper limit of medium pressure dry saturated steam).Dry saturated steam at high pressure, up to 60 bar, is used, instead, in the metal and petrochemical industries, and is generally produced by direct flame generators, i.e., industrial water-tube boilers [1].All these traditional steam production systems exploit the fossil fuel source (natural gas, coke, etc.).It is estimated that around 30% of the fossil fuel consumed in the world's industrial sector is burned to produce process steam [2].This practice is the cause of significant  2 emissions into the environment and so far, has benefitted of limited decarbonization efforts.However, these could be partially reduced by resorting to innovative steam production devices that use renewable solar energy as a thermal input source, and which can well be included in the global industrial decarbonization discourse.
In recent years, an extremely efficient thermal production technology using solar energy is being developed.These are MT-Power ultra-high vacuum solar collectors, i.e., high efficiency solar collectors, operating in the temperature range of 80-180°C, manufactured by TVP Solar company.These collectors belong to the family of non-concentrating solar panels and are direct descendants of the much less efficient flat plate solar collectors [3].The distinctive feature of these collectors is that they have an evacuated air gap between the absorber and the protective glass, in which an air-pressure of 10 −4 mbar is reached [4].This air evacuation is much deeper than in the traditional evacuated collectors [5] and allows convective heat exchanges between absorber and glass to be almost eliminated, resulting in a net reduction of the device's heat losses, and, consequently, in high heat absorption efficiencies even at relatively high operating temperatures [6].
Currently these collectors can only absorb thermal energy in the form of an increase in the absorbing fluid's sensible heat up to 180 °C, for which an absorption efficiency of no less than 50% is estimated, as some studies conducted in 2018 [7] and 2020 [8] have shown.This is since steam cannot be produced directly inside the collectors.A recent study [9] has analysed the possibility to directly generate steam, but the practical activities results have shown that under the effective working conditions it is not possible to reproduce turbulence conditions inside the copper pipes of the collectors such as to determine an efficient mixing between liquid and vapor phases produced, necessary to obtain an acceptable thermal absorption capacity, since the operating temperatures are too low to determine it.Therefore, because of the boundary conditions and the technological limits mentioned, the production of steam directly in the collectors would lead to a too low heat absorption efficiency.
In this paper alternative steam production systems which do not contemplate steam production directly inside collectors are analysed.The dry saturated steam production technique proposed involves the use of thermo-fluid dynamic devices which carry out the so-called sensible-to-latent transformation.Two possible steam generation methods were analysed: direct steam production, using a Flash vessel (FV), and indirect steam production, using a Kettle reboiler (KR).A comparison between numerical results of different plant configurations is made, which differ from each other in the type of dry saturated steam production device, and in the level of solar field operating temperature.

2.
Case studies A dynamic study is conducted on some existing TVP Solar plants, using the 0-D dynamic simulation software TRNSYS®.Within this software, the logical functioning of the solar field and steam production device is reproduced with appropriate "Types", and the behaviour of the entire plant is dynamically analysed as the boundary conditions, such as the hourly solar radiation and ambient temperature, and secondary climatic conditions such as wind, humidity, etc., vary.
The case studies analysed concern two TVP Solar plants: one installed at the operational company headquarters in Avellino, Italy, in which only a part of the available single side solar field is usable and equal to approximately 140  2 , corresponding to 70 collectors arranged in 10 parallel strings of 7 panels each; the other located in Qurayyah, Saudi Arabia, having a double-side type solar field with a net receiving area of approximately 1000  2 and made up of 510 collectors.
In addition to evaluating the dynamic operational differences of the two devices for the two case studies, in this study, the plant production difference that occurs by modifying the design temperature difference between the fluid entering the solar field and the fluid leaving it is also evaluated.Two possible design ΔT are simulated for both configurations, equal to 10 °C and 20 °C.

3.
Steam generation devices' main features The analysed thermodynamic devices, i.e., Flash vessel (FV) and Kettle reboiler (KR), use the sensible heat of the fluid coming from the solar field as thermal input to produce steam.In the first case, steam is produced inside the flash vessel unit through a flash-type expansion [10] of the same solar field fluid (primary fluid), while in the second case, steam is produced thanks to the heat transferred by the primary fluid to a secondary evaporating fluid (which does not circulate in the solar field), using the Kettle reboiler.In both cases, the exclusive circulation of liquid fluid inside collectors is ensured.
However, both devices impose a certain reduction in the temperature necessary to produce steam with respect to the maximum temperature reachable in the solar field (currently 180 °C).For this reason, an upper limit of dry saturated steam production temperature of around 150 °C can be defined.
However, only low-pressure steam can be produced, as it is below 6 bar.This is the highest pressure achievable, and it is possible only under specific conditions of irradiance, i.e., in the Middle East and North Africa.Instead, under average Italian climatic conditions, an upper production temperature limit of 160 °C is assumed, which allows the production of dry saturated steam up to 2 bar and 130 °C.

3.1.
Flash vessel Flash vessel is a direct generation device, in which the primary flow entering-it is the same evaporating fluid coming out of it.This flow, which is necessarily hot pressurized osmotic water, undergoes a flashtype expansion thanks to a throttling valve, which imposes a designed pressure reducing.During the expansion, the amount of heat in the fluid is conserved (isenthalpic transformation), but the saturated liquid cannot hold this amount of heat, so it flashes.Therefore, saturated steam, composed of a mixture of steam and largely of water, is produced at pressure lower than the input one, and is sent to the flash separator, which allows to separate the dry saturated steam from the saturated condensate, thanks to the different densities of the two phases.

3.2.
Kettle reboiler This device is like a shell-and-tube heat exchanger, in which a primary fluid circulates in U-bundles and transfers its sensible heat to the secondary fluid.The generation of steam is made possible because the secondary fluid sits in the boiling device during a residence time sufficient to ensure the transfer of energy from the primary to the secondary fluid.The first difference with respect to the flash vessel is that two distinct fluids are required (heat transfer fluid & osmotic water to be evaporated).As is the case for the flash vessel, the steam generated is sent to the application of the customer plant.Once it has transferred to the user its latent heat (and eventually some of its sensible heat) it returns to the kettle reboiler in the form of pressurized condensate.In order to maximize the generation of steam by the kettle, the returning condensate is beforehand preheated in a heat exchanger so as to bring it as close as possible to the saturated temperature at the desired pressure.An important difference to point out is that kettle reboiler needs to impose a pinch point  between primary and secondary fluid.This entails an increase in the maximum temperature required at the primary, with the same steam production temperature, compared to the configuration with flash vessel.In this paper a pinch point  equal to 10 °C is considered.

3.3.
Dry saturated steam generation devices' installation issues Some industrial activities require the use of both low and medium pressure steam, so the thermal output from the boiler is often medium pressure dry saturated steam, while the low pressure dry saturated steam demand is satisfied using pressure reducers [11].Therefore, steam supply lines connecting generator to utility/reducer are mostly designed to manage medium pressure steam, and, since these cannot be decommissioned, because of the immovability of traditional generators, necessary to carry out thermal integration in the hours of absence of solar availability, direct or stored, these would not be able to manage low pressure steam produced by collectors; so it is necessary to install new supply lines, parallel to the existing ones, the cost of which is to be considered in a technical-economic feasibility analysis.

4.
TRNSYS® implementation logic In the Figure 1 and Figure 2 the TRNSYS® plant's logic schemes respectively with flash vessel and kettle reboiler are shown.The solar field is highlighted in blue, while the application side of the flash vessel and kettle reboiler units is highlighted respectively in red and yellow.The main "Types" used in this logic schemes are shown in Table 1 e Table 2.In Table 2, the kettle reboiler appears under the wording ISG (indirect steam generator) and modeled using the Equa-Type, as a customized type that simulates its behavior is not available in TRNSYS®.
Equation (1) shows the logical calculation of the steam produced instant by instant by kettle reboiler: where  is the thermal energy available as input to the steam production device, ℎ  % are the kettle reboiler percentage thermal losses towards the environment (imposed equal to 1% of the available thermal energy), and   and  ℎ are the design enthalpy variations inside the kettle reboiler and preheater.

Results
The annual results of plant steam production, total production hours and electricity consumption due to the circulation and pressurization pumps, are presented.In the Figure 3 and Figure 4 the Avellino and Qurayyah TVP Solar plant simulation results, respectively, are reported.The comparison analysis between all the different plant configurations is unique for both case studies.Plant steam production is a function of a net balance between the energy collected in the solar field and the thermal losses inside the pipes and devices.The configuration that determines a higher annual production of steam (111 ton/year for Avellino plant, and 1101 ton/year for Qurayyah plant) is the one with a flash vessel and primary flow ΔT equal to 10 °C (FV & ΔT=10 °C).That is since the flash vessel configuration allows for a lower maximum temperature at the primary side than the kettle reboiler one.Therefore, collectors tend to work with greater efficiency, determining greater productivity.This result is magnified by the fact that this configuration presents the lowest average solar field temperature, since, at the same solar field inlet fluid temperature, a  of 10 °C is considered.Furthermore, with a lower average working temperature, less heat losses in the pipes are detected.Moreover, thermal dissipation from flash vessel tends to be lower because those devices are more compact than kettle reboiler because of the smaller residence time of the first device compared to the second.Everything said also explains why the plant configuration with kettle reboiler and  equal to 20°C (KR & ΔT=20 °C) is the worst in terms of productivity.
productive operating hours reach values of around 1000 h/year and 1400 h/year, respectively for Avellino and Qurayyah, plants, but results do not show objectively justifiable differences between the various simulated configurations.Total electrical energy consumed by pumps is a parameter of little interest during plant design steps, but in this paper, it is fundamental since it is the only factor that can potentially generate  2 emissions in plant operational hours.As can be seen in Figure 3 and Figure 4, there is no net difference in electrical consumption between configurations with different steam production devices.On the other hand, evident differences are found in configurations with different .In particular, the configurations with lower  are the most energy intensive.This result is related to the amount of circulating flowrate, which, considering same thermal energy collected in the solar field, in the configuration with  = 10 ° must necessarily be double that the one circulating in configurations with  = 20 °.Therefore, "ΔT=10 °C" configurations have double energy consumption compared to the "ΔT=20 °C" ones (around 2400 kWh/year vs 1200 kWh/year for Avellino plant, and around 45000 kWh/year vs 22000 kWh/year for Qurayyah plant).Anyway, taking the configuration "FV & ΔT=10 °C" as a reference for optimal production, all other configurations need to carry out thermal integrations to achieve the same steam productivity.Thermal integrations are generally made through a traditional medium-low pressure steam generator, i.e., a firetube boiler, for which a primary energy consumption of 650 kWh for each ton of steam produced can be assumed.Considering a firetube boiler having an overall conversion efficiency of 90% and burning natural gas, with an emission factor of 0.24 kgCO2/kWh, Tables 3 and 4 show the  2 emission savings for the reference configuration, comparing it to the other analysed ones.On the other hand, the reference configuration has a higher electricity consumption, so extra emissions due to electricity purchased from the grid are also calculated.Electrical grid emission factors equal to 0.424 kgCO2/kWh for Italy and 0.568 kgCO2/kWh for Saudi Arabia are considered (Tables 5 and 6).As shown  7 and Table 8, for both the Avellino and Qurayyah plants, the reference configuration always allows overall emission savings compared to all the other configurations analysed.Therefore, this configuration represents the best choice also from an environmental point of view.In the Tables 7 and  8 a balance between 0 2 emission savings due to the less thermal integration and extra 0 2 emissions due to the extra electricity-purchasing in the reference configuration, for both Avellino and Qurayyah plants, is shown.6. Conclusions This paper deals with the technical problem related to the production of dry saturated steam for industrial use by exploiting thermal energy produced by MT-Power technology high vacuum solar collectors.In detail, two possible dry saturated steam production technologies were investigated: direct production through a flash vessel, and indirect production through kettle reboiler.This study was carried out with dynamic analyses conducted using the TRNSYS® simulation software.Results showed that the optimal accessory device, to combining with an ultra-high vacuum collectors' solar field, to carry out the sensible-to-latent transformation is the flash vessel, as it allows the solar field to work at relatively low average temperatures and involves higher collectors' efficiency and less heat losses.Moreover, for the same reason, it results that is more convenient to keep the plant  as low as possible.With same solar field , plant configuration with flash vessel has a productivity of +18%.While, with the same steam-device, configurations with a solar field  equal to 10 °C have a productivity of +10%, but also a higher electricity consumption of about +100%.Comparing the two TVP Solar plants, the one placed in Avellino has a unitary steam productivity of about 0.7 / 2 /, while the other in Qurayyah has unitary productivity of 1 / 2 /.
Future studies will analyse the increase in plant productivity due to installation of a pressurized thermal storage tank, which accumulates the excess thermal energy produced during the day and then return it in the hours with lack of insolation.Moreover, the results obtained in this study will lead to the implementation in the Avellino facility of a flash vessel, which will be used as an experimental setup for the upcoming investigations.

Figure 3 .Figure 4 .
Figure 3. Avellino plant: yearly steam production, total annual working hours, and yearly electricity consumption by pumps

Table 1 .
Main TRNSYS® "Types" describing solar field, used in logic plant schemes implemented.

Table 2 .
Main TRNSYS® "Types" describing steam devices, used in logic plant schemes implemented.

Table 3 .
Comparison between  2 emissions due to thermal integration -Avellino plant

Table 4 .
Comparison between  2 emissions due to thermal integration -Qurayyah plant

Table 5 .
Comparison between  2 emissions due to grid electricity usage -Avellino plant

Table 6 .
Comparison between  2 emissions due to grid electricity usage -Qurayyah plant

Table 7 .
2 emission balance for Avellino plant configurations

Table 8 .
2 emission balance for Qurayyah plant configurations