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Spanning solar spectrum: A combined photochemical and thermochemical process for solar energy storage
Applied Energy 247 (2019) 116–126
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Spanning solar spectrum: A combined photochemical and thermochemical process for solar energy storage
T
⁎
Juan Fanga,b, Qibin Liua,b, , Shaopeng Guoa, Jing Leic, Hongguang Jina,b a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China b
H I GH L IG H T S
full spectrum of solar energy is utilised in the proposed system. • The discontinuous solar irradiation is stored as chemical energy. • The photochemical and thermochemical processes are jointly utilised. • Solar • The promising thermodynamic performance is numerically validated.
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar photochemical process Solar thermochemical process Solar energy storage Hybrid system
Storage in the form of chemical energy is crucial for efficient utilisation of solar energy. In recent years, solar photon-induced molecular isomerization energy storage, in which solar energy can be directly converted and stored as chemical energy through internal molecular isomerization reactions, has received increasing interest. It is a closed cycle without any CO2 emission. However, the absorption of reactants cannot cover the full spectrum of solar radiation, and only the ultraviolet and a portion of the visible spectrum of solar energy can be stored. Moreover, some absorbed photons are dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar photochemical efficiency. To address these problems, a new energy storage system which integrates the photochemical process with thermochemical process has been proposed to convert the full spectrum of solar energy into chemical energy. Concentrated sunlight enters the photochemical device. Norbornadiene derivatives present in the photochemical devices can be isomerized to the related quadricyclanes by absorbing the ultraviolet-visible light photons. Some absorbed photons are simultaneously stored in the chemical bonds of the quadricyclanes. The unabsorbed photons corresponding to the visible-infrared spectrum are transmitted to thermochemical reactors, providing heat for methanol decomposition. This portion of the solar energy is stored in the form of chemical energy of the products (H2 and CO). The heat dissipated in the photochemical process is transferred to the thermochemical reactors for methanol decomposition, resulting in a higher solar chemical efficiency. Results show that the average solar chemical efficiencies corresponding to the design condition and off-design condition are 75.38% and 49.78%, respectively. The results of comparison of the thermochemical performances verify that the proposed system is superior to both the single photochemical system and the single thermochemical system.
1. Introduction Renewable energy is crucial for developing low-carbon cities and a sustainable society. Solar energy is considered to be the most promising solution for achieving such a goal [1]. However, challenges associated with the intermittency of the solar flux have led to an increasing need for solar energy storage technologies [2]. In this context, solar ⁎
chemistry, which pertains to the conversion of solar energy and its storage as solar fuels, has received significant attention [3]. There are three main ways to convert solar energy into solar fuels: solar photochemical process [4], solar thermochemical process [5] and solar electrochemical process [6]. These approaches and their several variants, such as photon-induced CO2 reduction [7] or water splitting [8], thermo-induced decomposition of fossil fuels [9] or reforming with
Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail address: [email protected] (Q. Liu).
https://doi.org/10.1016/j.apenergy.2019.04.043 Received 1 December 2018; Received in revised form 30 March 2019; Accepted 10 April 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
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J. Fang, et al.
Nomenclature
Subscripts
E Q τ H n T DNI S h kT G R r P X k F A
iso relax
energy heat energy time enthalpy number of photons temperature solar direct nominal irradiation solar field area (m2) Plancks constant Bolzmanns constant Gibbs energy gas constant reaction rate (mol s−1 m−2) pressure conversion rate reaction rate constant mole flow rate (mol s−1) catalyst surface area (m2)
λ onset stored 1/2 D CH3O(2) S2, S2a STC MOST
energy loss related to the quantum yield energy loss related to the relaxation from parent* to the photoisomer the wavelength of the solar irradiation onset wavelength stored solar energy half life methanol decomposition intermediate in mechanisms reaction active site solar thermochemical molecular solar energy storage
Greek symbol η
efficiency
since the long-wavelength photons cannot be used [13]. In addition, some absorbed photons are dissipated as heat, resulting in an increase in the reaction temperature and a lower solar photochemical efficiency [24]. Several attempts have been made to circumvent these limits. Photon upconversion, which involves transforming two long-wavelength photons into one short-wavelength photon, seems to be a promising strategy, but efficient materials for upconversion need to be further investigated [25]. Kasper et al. combined a solar water heating system with the NBD-QC isomerization cycle to utilise the long-wavelength photons abandoned by the MOST system [26]. The solar water heating system can store a portion of the solar energy which cannot be stored in the MOST system in the form of sensible heat of water, but the storage time is short. To address these challenges, we proposed a solar energy storage system with photon-induced NBD-QC isomerization cycle and thermoinduced methanol decomposition to more effectively convert the full spectrum of solar radiation into chemical energy. As a kind of liquid fuel, methanol is easy to handle, compared with other hydrocarbons. Furthermore, methanol can be decomposed with middle- and lowtemperature heat (approximately 200–300 °C [27]), which can be provided by sunlight concentrated by using parabolic trough collectors. Through the solar thermochemical process, solar energy is converted and stored in the form of chemical energy of the products of methanol decomposition (CO and H2) [28]. The solar thermochemical efficiency is in the range 60–67% when the methanol conversion is 80–95% [29]. However, solar energy is first converted into heat which induces
water [10], have been intensively studied. Among these, the photoninduced molecular isomerization solar energy storage technology has attracted increasing interest in recent years [11]. The photon-induced molecular isomerization cycle involves two parts, namely, parents and photoisomers. Solar energy is stored through reversible molecular rearrangements between parents and photoisomers. When exposed to sunlight, the relaxed photoactive molecules, named parents, are photochemically converted into photoisomers and solar energy is stored in the energetic chemical bonds of the photoisomers. The photoisomers can convert back to the parents for the next cycle with the help of catalysts, thus recovering the stored solar energy in the form of heat in a controlled manner, which is the so-called molecular solar thermal energy storage (MOST) cycle (see Fig. 1) [12]. Börjesson et al. determined that these cycles are expected to achieve up to 10–12% solar energy storage efficiencies when relatively high photon energies (1.8–1.9 eV) are absorbed [13]. There are three main photoisomeric reactions: double-bond isomerization, organometallic ligand reorientation and electrocyclic reactions [14]. Double-bond isomerization, such as cis-trans photoisomerization in azobenzene derivatives, can absorb sunlight in the ultraviolet-visible region, but the low storage enthalpy discounts its value [15]. To improve the energy storage enthalpy, azobenzene/carbon nanotube material and functionalised graphene with azobenzene units were theoretically proposed by Kolpak and Grossman [16], but the challenging synthesis process hinders their application [17]. Organometallic compounds (fulvalene diruthenium) display high energy storage densities, but the high cost has led to a decline in research interest [18]. In contrast, electrocyclic isomerization reactions, such as norbornadiene-quadricyclane derivatives cycle (NBD-QC), show promise for solar energy storage owing to the high storage enthalpy, low molecular weight and commercially availability [19]. Moreover, there are several effective catalysts available for the backward reaction [20]. Therefore, NBD-QC is the focus of the present work. Recently, the MOST cycles have drawn much attention because they are closed systems where both the energy storage process and the release process occur within one entity, and other substances (such as O2) are not required. These closed cycles have many advantages compared with open systems (e.g. methane combustion [21]): easily obtainable recycling material and eco-friendly nature, without any CO2 emission [22]. However, a major problem with the MOST cycles is that the absorption by the parents does not cover the full spectrum of solar irradiation. Similar to the Shockley-Queisser limit for silicon solar cells [23], the MOST system also has a solar photochemical efficiency limit
Fig. 1. Photon-induced molecular isomerization cycle. 117
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is dissipated as heat, leading to an increase in temperature and a decrease in the solar chemical efficiency. To convert and store the full spectrum of solar energy as chemical energy and improve the solar chemical efficiency, a new system involving molecular isomerization process and methanol decomposition was proposed. The proposed system consists of three main parts: parabolic trough collectors, solar photochemical devices with heat exchangers and solar thermochemical reactors. A schematic of the system is shown in Fig. 2. Direct solar radiation is concentrated by using double-axis tracking parabolic trough collectors [30]. A concentrated solar beam first enters the molecular isomerization device, where norbornadiene derivatives (NBDs), driven by a pump, flow past. The chromophores of the NBDs can absorb the solar photons corresponding to the ultraviolet wavelength range and a portion of the visible spectrum, before the NBDs are converted into related quadricyclane derivatives (QCs). Some absorbed photons are simultaneously stored in the chemical bonds of the QCs, while the remaining absorbed photons are dissipated as heat, resulting in an increase in the temperature. Heat exchangers are employed to avoid the temperature rise caused by the focusing of light and utilise the solar energy more effectively. The heat dissipated in the molecular isomerization device is transferred to solar thermochemical reactors for methanol decomposition. The unabsorbed photons corresponding to the infrared range and a portion of the visible spectrum are transmitted to solar thermochemical reactors filled with a catalyst. Solar energy is converted into thermal energy in the reactors, which provides the heat for the decomposition of methanol. Then, methanol is decomposed into H2 and CO, and this portion of the solar energy is stored in the form of chemical energy of the products (H2 and CO), which can be further used in combustion to generate electricity or industrial raw materials. The main advantages of the proposed system are as follow.
methanol decomposition. The occurrence of two conversion processes increases the irreversible loss. Moreover, all the photons are converted into heat without any distinction. Therefore, we proposed a hybrid system, where photons with high energies can be converted and stored directly through a photochemical process, while the remainder of the solar energy is utilised by the thermo-induced methanol decomposition. The main contributions of this study are summarised as follow. (1) An energy storage system was proposed to realise full-spectrum solar energy conversion and storage. Based on the theory of cascade utilisation of energy, the photochemical process and thermochemical process were combined to achieve a higher solar chemical efficiency. (2) The discontinuous solar irradiation is stored as chemical energy. This system outputs multiple products which can be easily transported and stored. The high-energy photons are directly converted into chemical bonds of QCs via the photochemical process. The lowenergy photons are stored in the form of chemical energy of the methanol decomposition products during the solar thermochemical process. (3) In the photochemical process, the storage time and solar photochemical efficiency were studied. In the thermochemical process, the influences of methanol input temperature and the solar irradiation filtered by the photochemical devices on methanol decomposition were investigated. (4) The thermodynamic performances of the proposed system were compared with those of the single molecular isomerization system and the single methanol decomposition system, and the feasibility of the proposed system was validated. The rest of this paper is organized as follows. In Section 2, a new system is proposed to convert and store the full spectrum of solar energy as chemical energy. In Section 3, the thermodynamic performances of the proposed system are evaluated. Finally, the main conclusions of this study are summarised in Section 4.
(1) This system realises full-spectrum solar energy storage. The shortwavelength photons corresponding to the ultraviolet and visible range of the spectrum are converted and stored directly in photoninduced molecular isomerization devices. The long-wavelength photons abandoned by the solar photochemical process, corresponding to the infrared and the visible region of the spectrum, are utilised by the solar thermochemical process. (2) The low density and unstable solar energy is upgraded into the high density and stable chemical energy of multiple products. The shortwavelength photons are stored in the chemical bonds of QCs. The long-wavelength photons are stored in the products of methanol decomposition (H2 and CO), which can be easily transported and stored as gases.
2. Full-spectrum solar chemical energy storage systems 2.1. Proposed system Solar photon-induced molecular isomerization, in which solar energy can be stored in chemical bonds through reversible molecular rearrangements, has drawn much attention in recent years. However, only the ultraviolet and a portion of the visible light solar energy can be converted into chemical energy in this process. Some of the solar energy
Fig. 2. Mechanism of storage of the full spectrum of solar energy as chemical energy. 118
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days and for a full year are shown in Figs. 4 and 5.
(3) Heat exchangers are employed in the molecular isomerization devices which serve as channels for cooling the QCs in order to avoid back-conversion. The heat dissipated during the photochemical process is transferred to thermochemical reactors for methanol decomposition, which results in a higher solar chemical efficiency. (4) The system does not require additional light filters, resulting in a simpler structure. In the solar photochemical process, sunlight is divided into two parts by the chromophores present on the NBDs. (5) The system is eco-friendly. It does not produce CO2 during the solar photochemical process, and only generates a small amount of CO2 during the solar thermochemical process.
(1) Solar photochemical process A schematic diagram of the photon-induced molecular isomerization process is shown in Fig. 6. The parents (NBDs) absorb the solar photons to become activated parents (parents*), which may transform to their related photoisomers (QCs). By this process, ΔHstored is the amount of solar energy stored in the chemical bonds of QCs. The QCs can be converted back to the NBDs with the help of a catalyst, thereby releasing the stored solar energy. Some assumptions have been made for simplification, which are as follow.
2.2. Calculating model i All molecules act individually. ii The parents absorb all photons with energy higher than a threshold value, and the unabsorbed photons are transported to the thermochemical reactors. iii The stored energy in one activated parent is the energy difference between the parent molecule and the photoisomer (ΔHstored in Fig. 6), and the excess photon energy is dissipated as heat.
The energy and matter flows of the system are shown in Fig. 3. It can be concluded that the solar energy Qsolar and methanol are the only input energy and matter, respectively. The products are the solar energy stored in the chemical bonds of QCs and the chemical energy stored in CO and H2. It is worth noting that the NBDs and QCs can interconvert to each other in the intersystem. There are two main kinds of losses, namely, optical loss and heat loss. The optical losses of the parabolic trough collectors, solar photochemical device, jacketed glass tubes and metal reactor tubes of the solar thermochemical reactors are considered. The parabolic trough collectors reflect (reflectivity of 0.93 [31]) the incident solar beam to the molecular isomerization device. The solar concentration process involves the optical loss Qoptical-loss1. The remaining portion of solar energy is transmitted (transmittivity of 0.98) to the solar thermochemical process, with Qoptical-loss2 being the solar energy obstructed in the molecular isomerization devices. Because of the transmission of the glass envelops (transmittivity of 0.96 [31]) and the intercept of the metal tubes (intercept factor of 0.958 [31]), Qoptical-loss3 amount of solar energy is lost in the thermochemical process. Heat losses mainly occur in the solar thermochemical reactor through convection and irradiation; Qheat-loss2 and the heat exchange loss Qheat-loss1 are observed in the heat exchanger. The dissipated heat losses (Eiso + Erelax) associated with the photochemical section are transferred to the thermochemical reactors through the heat exchanger, and recycled for methanol decomposition. The solar direct nominal irradiation (DNI) and the ambient temperature were recorded by the BSRN300 meteorological station located in Langfang, Hebei (E116.65°, N39.58°). The hourly data for the typical
There are two main losses associated with the MOST process, Eiso and Erelax. Eiso is related to the quantum yield of the photoreaction, φiso, which is defined as φiso = niso/nabsorbed, where niso is the number of photons utilised in the isomerization events and nabsorbed represents the number of photons absorbed by the reactants; φiso ≠ 1. Erelax is attributed to the intrinsic property of the molecular isomerization process. When the parent is transformed to the photoisomer, energies Ea and E1 are lost (see Fig. 6). The two main losses can be computed as [26]
Eiso =
∫0
Erelax =
λ onset
∫0
nλ ·Eλ ·(1 − ϕiso) dλ
λ onset
nλ ·(Ea + E1)·ϕiso dλ
(1)
(2)
where nλ is the number of absorbed photons (s−1 m−1 nm−1) and Eλ is the energy of every photon at wavelength λ (J nm−1). Therefore, the theoretical stored solar energy and the photochemical efficiency are calculated as
Estored =
∫0
λ onset
nλ ·(Eλ − Ea − E1)·ϕiso dλ
Fig. 3. Energy and matter flows of the system. 119
(3)
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800 30
700
Ambient temperature /°C
DNI / Wm-2
600 500 400 300 Spring equinox Summer solstice Autumn equinox Winter solstice
200 100 0
6
8
10
12
14
16
18
20
25 20 Spring equinox Summer solstice Autumn equinox Winter solstice
15 10 5 0 -5
4
6
8
10
Time / h
12
14
16
18
20
22
Time / h
Without the barrier Ea, the photoisomers would convert back to the parents immediately, and the storage time would be very short. On the other hand, a large barrier, which is also the main contributor to the energy loss, will lead to a lower stored solar energy ΔHstored (see Fig. 6). Therefore, the storage time and photochemical efficiency ηMOST are mainly dependent on the energy barrier of the back-conversion process Ea. The relationship between ηMOST and Ea is shown in Eq. (4), and the storage time, quantified by the half-life, is computed by [13]
800 600 400 200
Ambient temperature /°C
DNI / Wm
-2
Fig. 4. Variations in the DNI (left) and ambient temperature (right) for typical days.
0 40 20
τ1/2 =
h × ln(2) kT × T × e−G / RT
(5)
0 0
2000
4000
6000
where h is Planck’s constant, kT is the Boltzmann constant, T is the storage temperature, G is the Gibbs energy of activation and R is the gas constant. Furthermore, the structures of the parents and photoisomers are similar because the NBDs are converted into the related QCs via internal rotations and rearrangements; no parameters which change the rigidity of the molecules are changed. Therefore, we assumed that the entropy of the isomerization reaction is negligible, which implies that the free energy equals the activation enthalpy based on the above reasoning.
8000
Time / h
Fig. 5. Variations in the DNI (upper) and ambient temperature (underside) for a full year.
(2) Solar thermochemical process The photons corresponding to the ultraviolet region and a portion of the visible region of the spectrum are absorbed during the photochemical process, while the infrared spectrum and the remaining portion of the visible spectrum are transmitted into the thermochemical reactor for methanol decomposition. The glass envelop is mounted to surround the metal reaction tube. The gap in the ring between the glass envelop and metal tube contains vacuum. Cu/Al2O3/ZnO catalyst is packed in the metal tube. The methanol fed from the heat exchanger flows though the porous catalyst bed, where it is decomposed into CO and H2 when the conditions are suitable. The heat transfers occurring between the inner and outer surfaces of the metal tube, the outer surface of the metal tube and the inner surface of the glass envelop, the inner and outer surfaces of the glass envelop, the outer surface of the glass envelop and the environment and in the catalyst bed are shown in Fig. 7. Moreover, the coupling model, involving solar-heat conversion, heat transfer, fluid flow, mass flow and the chemical reaction, has been studied by our group [32]. The theoretical details and experimental verifications have been intensively reported by us previously [33–35]. The parameters of the solar thermochemical reactors are shown in Table 1. The thermochemical reactor is divided into i infinitesimal units. For each unit, the kinematical model of methanol decomposition is given by [27,36]
Fig. 6. Schematic diagram of solar photon-induced molecular isomerization. t
ηMOST =
∫t12 Estored dt t
∫t12 (DNI ·S ) dt
(4)
where λonset is the onset wavelength (cut-off wavelength between the photochemical process and thermochemical process) (nm), DNI refers to the direct solar radiation and S is the area of the parabolic trough collectors. The loss Erelax includes the energy barrier Ea of the back reaction. 120
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thermochemical process:
XCH3 OH =
ηSTC =
FCH3 OH ,0 − FCH3 OH , i FCH3 OH ,0
t2
i ∑i= 1 t1
∫
(13)
[FCH3 OH ,0 Cp, i (Ti − Ti − 1) + rD, i ΔAcat , i ΔHr , i] dt t
∫t12 (DNI ·S ) dt
(14)
where FCH3 OH,0 is the molar flow rate of the input methanol; Cp,i represents the specific heat capacity of the ith unit; Ti and Ti-1 represent the temperatures of the species in the ith and i-1th units, respectively, and ΔHr,i is the reaction heat in the ith unit. (3) Performance of the proposed system The total solar chemical efficiency of the combined system is the ratio of the stored chemical energy to the input solar energy, and is equal to the sum of the solar photochemical efficiency and solar thermochemical efficiency i.e.
Fig. 7. Model of the solar thermochemical reactor.
ηcombine = ηMOST + ηSTC Table 1 Parameters of solar thermochemical reactors. Item
Unit
Value
Aperture width of the parabolic trough mirror Total length of the reactor Inner/outer diameter of the metal tube Inner/outer diameter of the glass envelope Catalyst bulk density Catalyst dimension Ambient temperature Pressure at the outlets of solar receivers/reactors
m m m m kg L−1 mm °C bar
4.2 12 0.046/0.050 0.075/0.080 1.15 Φ5 × 5 25 2.5
(15)
To evaluate the improvement in the thermodynamic performance of the proposed system compared with those of the single solar photochemical process and the single solar thermochemical process, the increase ratios ηincrease-MOST and ηSTC-MOST were calculated as
ηincrease − MOST =
ηincrease − STC =
Ecombined system − EMOST − sin gle EMOST − sin gle
(16)
Ecombined system − ESTC − sin gle ESTC − sin gle
(17)
(4) Validation of the calculation model
rD =
kD k ∗ (2) CH3 O
⎛ ⎝
PCH3 OH 0.5 PH 2
⎞ ⎛1 − ⎠⎝
2 P PH CO 2
kD PCH3 OH
⎞ CS 2 CS 2a ⎠
3 OH ⎞ ⎤ ⎡1 + k ∗ ⎛ PCH0.5 (1 + P 0.5(2a) PH0.52 ) H CH3 O(2) ⎢ ⎝ P H2 ⎠ ⎥ ⎣ ⎦
The numerical models of the photochemical and thermochemical processes have been validated by the respective experimental results. (6)
(1) Photon-induced molecular isomerization process
where CS2 and CS2a are the surface concentrations of active site 2 and active site 2a, respectively, mol m−2; kD is the reaction rate constant and K ∗ (2) KH(2a) are the absorption coefficients of the surface species,
For the photochemical process, the model has been validated [26]. In this case, λonset and φiso are 456 nm and 28%, respectively. We can obtain Eiso = 84 W m−2, Erelax = 21 W m−2 and Estored = 11 W m−2. Therefore, the photochemical efficiency is 1.1% for AM1.5 solar spectrum, which corresponds to the experimental value reported in Ref. [26]. Eiso ± Erelax ± Estored = 116 W m−2, which is in accordance with the solar energy below 456 nm. As a result, it is confirmed that Eqs. (1) and (2) describe the main losses occurring in the photon-induced molecular isomerization process.
CH3 O
bar−0.5, which can be calculated by
−E kD = kD∞ exp ⎛ D ⎞ ⎝ RTr ⎠ ⎜
K∗
CH3 O(2)
⎟
(7)
Δ ∗ ΔH ∗ (2) ⎛ SCH3 O(2) CH3 O ⎞ = exp ⎜ − R RTr ⎟ ⎝ ⎠
K H (2a) = exp ⎛ ⎝
ΔS H (2a)
⎜
R
−
(8)
ΔH H (2a) ⎞
(2) Validation of the solar thermochemical model
⎟
RTr
⎠
(9) The numerical model corresponding to the thermochemical process was validated by the experimental results reported by our research group [33]. We employed double-axis parabolic trough collectors in the proposed system, whereas the parabolic trough collectors used in the experiment were positioned in the East–West direction. Moreover, the parameters of the thermochemical reactors employed in the
where R represents the universal gas constant and Tr is the temperature of the solar thermochemical reactor. The other parameters are shown in Table 2. The molar flow rate of methanol in each infinitesimal unit FCH3 OH , i is computed as
FCH3 OH , i = FCH3 OH , i − 1 − rD, i ΔAcat , i
(10) Table 2 Parameters of the kinematical model for methanol decomposition.
where ΔAcat,i is the catalyst surface area of the infinitesimal reaction unit, which is calculated by
dAcat = Am dmcat
dmcat =
1 ρ πDin2 dL 4 cat
(11)
Rate constant or equilibrium constant
ΔSi /J mol−1 K−1 or ki∞/m2 mol−1 s−1
ΔHi or E /kJ mol−1
(12)
kD∞ (m2 mol−1 s−1) −0.5 k∗ ) (2) (bar
3.8 × 1020 30.0
170.0 −20.0
k H (2a) (bar−0.5)
−46.2
−50.0
CH3 O
The methanol conversion rate XCH3 OH and the solar thermochemical efficiency ηSTC were chosen to evaluate the performance of the 121
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utilise solar energy more efficiently and avoid the back-conversion caused by an increase in temperature due to the dissipating heat, this portion of solar energy is recycled to preheat the feed methanol as part of the thermochemical process by using a heat exchanger. Therefore, the input temperature of methanol is increased, and the effect of the input temperature on the performance of the thermochemical reaction was examined, as described in the following section.
experiments are listed in Table 3. Fig. 8 shows comparisons of the methanol conversion and solar thermochemical efficiency between the theoretical and experimental results. The results reveal good agreement, which indicate that the computing model used for the thermochemical process is valid. 3. Results and discussion In this section, the storage time and solar photochemical efficiency of the MOST system are presented. Based on the results, we further discuss the energy distribution in the MOST system. To recover the heat dissipated in the MOST process, heat exchangers were employed. The input temperature of the feed methanol in the thermochemical process was increased with the help of the heat exchangers. Therefore, the influence of the input temperature on the thermochemical process was investigated. Moreover, sunlight is filtered by the photochemical process. The influence of DNI on the thermochemical process was studied. In the proposed system, the thermodynamic performances corresponding to the design condition and off-design condition were investigated. Comparison of the performances of the proposed system and the single MOST system or single methanol decomposition system are presented.
3.2. Performance of the solar thermochemical process in the combined system The heat exchanger recycles the heat dissipated during the photochemical process and preheats the feed methanol as part of the thermochemical process, which increases the input temperature of methanol. Fig. 12 shows the effect of the input temperature on the methanol conversion rate and on the solar thermochemical efficiency. We can see that the methanol conversion rate is increased to 99% from 93% and that the solar thermochemical efficiency is increased to 69% from 63% when the input temperature is increased to 200 °C from 100 °C. The results show that the increase in the input temperature can enhance the thermodynamic performance of the thermochemical process. Therefore, heat exchangers not only serve as channels for the cooling species during the photochemical process but also improve the performances of the solar thermochemical process in the combined system. As an important parameter, the DNI affects the performance of the thermochemical process both in the individual thermochemical system and in the combined system. In the individual system, the full spectrum of solar energy is converted to heat, which is utilised for methanol decomposition. On the other hand, in the combined system, only the photons corresponding to the infrared region and a portion of the visible spectrum of the solar radiation, which cannot be absorbed during the photochemical process, are transported to the thermochemical reactors. Because sunlight is filtered by the photochemical process, the performance of the thermochemical process in the combined system is not as good as that in the single thermochemical system. To compare the difference in the performance of the thermochemical process between the individual system and the combined system, the methanol conversion rate and solar thermochemical efficiency were investigated as functions of the DNI. We can see from Fig. 13 that the methanol conversion rates of both the systems are directly proportional to the DNI. The conversion rate of the combined system is lower than that of the single thermochemical system, but the difference is small. The solar thermochemical efficiency also increases with the increase in DNI, but the increase ratio becomes smaller with the increase in DNI. The solar thermochemical efficiency of the combined system is also lower than that of the single thermochemical system, but the difference is smaller than 1%. It can be concluded that the variation in DNI significantly influences the thermochemical process in both the individual and combined systems. Although the performance of the thermochemical process in the combined system is not as good as that in the single thermochemical system, the difference between the values can be neglected. To further evaluate the performance of the combined system,
3.1. Performance of the photon-induced molecular isomerization process The storage time and solar energy conversion efficiency are the two most important parameters of the photochemical process. According to the analysis presented in Section 2.2, we can conclude that these depend mainly on the energy barrier of the back-conversion process, Ea. A small barrier leads to a short storage time. On the other hand, a large barrier will result in a high energy loss, which eventually reduces the solar energy conversion efficiency. If that is the case, what constitutes a suitable barrier? Here, the storage time is quantified by the half-life. First, we calculated the half-life as a function of Ea at different storage temperatures, which is shown in Fig. 9. It can be concluded that the half-life increases sharply with the decrease in the storage temperature and the increase in the energy barrier of the reverse reaction. At ambient temperature (25 °C), an energy barrier of 110 kJ mol−1 is sufficient for daily energy storage. The half-life of the photoisomer can reach up to 1400 days, with an energy barrier of 120 kJ mol−1, which is sufficient for seasonal energy storage. We also studied the solar photochemical efficiencies of different barriers as functions of the cut-off wavelength, and the results are shown in Fig. 10. The cut-off wavelength is the dividing line between the photochemical process and the thermochemical process. We assume that photons with energy higher than a threshold value (the photon energy at cut-off wavelength) can be absorbed for the photochemical process, and the quantum efficiency is 0.8. It can be concluded that the solar photochemical efficiency increases with the increase in the cut-off wavelength until a maximum point, before it decreases. The peak point increases with the decrease in the total energy loss (Ea + E1). The minimal amount of total energy loss for seasonal storage is 120 kJ mol−1 (Ea = 120 kJ mol−1 and E1 = 0 kJ mol−1) and for daily storage is 110 kJ/mol (Ea = 110 kJ mol−1 and E1 = 0 kJ mol−1), which yield maximum solar photochemical efficiencies of 10.41% and 8.93% at the cut-off wavelengths of 647 nm and 683 nm, respectively. According to the above analysis, the energy distribution of the molecular isomerization process corresponding to the design condition (DNI: 1000 Wm−2, ambient temperature: 25 °C) is shown in Fig. 11. In this case study, we assume that the total energy loss is 120 kJ mol−1, the quantum efficiency is 0.8 and the cut-off wavelength is 600 nm. In Fig. 11, we observe that 76.13% of the solar energy is transported to the solar thermochemical reactors for methanol decomposition. About 8.66% of the solar energy is converted and stored in the chemical bonds of the QCs, while 15.21% of the solar energy is absorbed but not stored, and is dissipated as heat during the molecular isomerization process. To
Table 3 Parameters of the solar thermochemical reactors used for validation.
122
Item
Unit
Value
Aperture width of the parabolic trough mirror Total length of the reactor Inner/outer diameter of the metal tube Inner/outer diameter of the glass envelope Catalyst bulk density Catalyst dimension Ambient temperature Mirror reflectivity
m m m m kg L−1 mm °C
2.5 4 0.031/0.035 0.051/0.054 1.7 Φ5 × 5 30 0.8
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Theoretical results
0.9 Methanol Conversion
Solar thermochemical efficiency / %
1.0 Experimental Results
0.8 0.7 0.6 0.5 0.4 0.3 0.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
70 65 60 55 50
Theoretical results
45
Experimental Results
40 35 30 25 20 1.0
1.5
Feeding rate of methanol / (l/h)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Feeding rate of methanol / (l/h)
Fig. 8. Variations of the methanol conversion (left) and solar thermochemical efficiency (right) with methanol feeding rate.
Estored Eiso Erelax Eunabsorbed
For solar thermochemical reaction 76.13%
8.66% 2.17%
13.04%
For preheating feed Fig. 9. Half-life of the photoisomer versus the energy barrier of the back-conversion reaction for different storage temperatures.
Fig. 11. Energy distribution of the photon-induced molecular isomerization process corresponding to the design condition (DNI: 1000 W m−2 and ambient temperature: 25 °C).
1.00
Conversion of methanol
0.98
0.68
0.96 0.94
0.66
0.92
0.64
0.90 Conversion of methanol Solar thermochemical efficiency
0.88 0.86 100
Fig. 10. Solar photochemical efficiency as a function of cut-off wavelength.
120
140
160
180
200
0.62
Solar thermochemical efficiency
0.70
0.60
Input temperature of methanol feed /°C the behaviour of the proposed system was studied as follows.
Fig. 12. Conversion of methanol and the solar thermochemical efficiency as functions of different input temperatures of the methanol feed (DNI: 1000 W m−2, ambient temperature: 25 °C and methanol feed rate: 0.33 mol s−1).
3.3. Behaviour of the proposed system In this section, we compared the thermodynamic performances of the proposed system with those of the single thermochemical system and single photochemical system for the design condition and off-design condition. The energy flow of the proposed system in the design condition (1000 W m−2) is shown in Fig. 14. Optical and heat losses are the two main losses observed in the system. The heat loss of the solar
thermochemical reactor accounts for 7.50% of the total input solar energy, and ranks the first among all the losses. The temperature difference between the reactors and the ambient leads to a large irradiation loss and a convection loss at the outer surface of the glass envelop. Then, the optical losses of the parabolic trough collectors and the
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Fig. 13. Conversion of methanol and the solar thermochemical efficiency for different DNIs. (Ambient temperature: 25 °C and methanol feed rate: 0.33 mol s−1).
thermochemical reactor rank second and third, respectively. The three main losses cannot be circumvented both in the single thermochemical system and in the combined system. When photochemical reactors are deployed in the combined system, the solar beam reflected from the parabolic trough collectors needs to go through two layers of glass, leading to increased optical losses in the combined system (about 3.25% of the input solar energy). The dissipated heat (Eiso + Erelax) of 13.85% in the photochemical section is transferred to the thermochemical reactors by the heat exchanger. It is demonstrated that 7.89% of the input solar energy can be stored by the photochemical process without significant reduction of the solar thermochemical efficiency (67.49%) corresponding to the methanol decomposition process. In the combined system, the total solar chemical efficiency is about 75.38% in the design condition. To establish the superiority of the proposed system in the off-design condition, we compared the amount of solar energy stored between 400 W m−2 and 800 W m−2 in the combined system with those stored in the single photochemical system and single thermochemical system. It can be concluded from Fig. 15 that the solar energy stored in the combined system is always higher than those stored in the single photochemical system and single thermochemical system. The increase ratio ηincreaseMOST increases with the increase in the DNI. However, the increase in the solar irradiance will lower the ratio ηincrease-STC. The reason is that the variation in DNI affects the thermochemical process more than the photochemical process.
Fig. 15. Solar energy stored in the proposed system in comparison with those stored in the single photochemical system and single thermochemical system under different DNIs (ambient temperature: 25 °C).
A comparison of the hourly stored solar energy for typical days between the proposed system and the single systems was performed to further evaluate the feasibility of the proposed system in the off-design condition, and the results are shown in Fig. 16 and Table 4. We assumed that methanol decomposes when the DNI is higher than 400 W m−2, because it is not possible to provide sufficient heat for methanol decomposition when the DNI is below 400 W m−2 [37]. Therefore, the combined system can significantly increase the running time, compared with that of the single thermochemical system across the four seasons. The running time of the proposed system even increases by 6 h in winter. The results also show that the proposed system can store much higher solar energy, compared with the other two systems. The average solar chemical efficiencies of the proposed system for the four typical days are also the highest. Such encouraging results indicate that the thermodynamic performance of the proposed system in the off-design condition is better than those of the two single systems. The annual behaviours of the three systems are also revealed in Fig. 17. The results show that the stored solar energy and the solar chemical efficiency of the combined system are higher than those of the
Fig. 14. Sankey diagram representing the energy flows in the proposed system for the design condition. (DNI: 1000 Wm−2 and ambient temperature: 25 °C). 124
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24 600
20 18
400
16 14 4
200
2 7
8
0
9 10 11 12 13 14 15 16 17 18 19 20
18
400
16 14 4
0
800
200
22
600
400
16 14 4
DNI / Wm-2
20 18
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
200
Winter solstice
24
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
500
20
400
18
300
16 4
0
0
700 600
22
200
2
2
0
800
26
Autumn equinox
24
Stored solar energy / kW
600
20
DNI / Wm-2
6
26
0
22
2
Stored solar energy / kW
0
Summer solstice
DNI / Wm-2
22
800
26
Spring equinox
Stored solar energy / kW
24
Stored solar energy / kW
800
EMOST-single ESTC-single ECombiend system
DNI / Wm-2
26
100
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
0
Time / h Fig. 16. Hourly stored solar energies corresponding to the typical days of the four seasons. Table 4 Comparison of the thermodynamic performances on the typical days of the four seasons. Typical days
Systems
Running hours/h
Incident solar energy/kW
Stored solar energy/kW
Average solar chemical efficiency
Spring
Single MOST system Single STC system Combined system
10 6 10
228.21 228.21 228.21
18.01 127.86 135.11
7.89% 56.03% 59.20%
Summer
Single MOST system Single STC system Combined system
14 10 14
330.78 330.78 330.78
26.10 203.44 212.37
7.89% 61.50% 64.20%
Autumn
Single MOST system Single STC system Combined system
12 7 12
278.06 278.06 278.06
21.94 147.31 156.87
7.89% 52.98% 56.42%
Winter
Single MOST system Single STC system Combined system
11 5 11
215.46 215.46 215.46
17.00 104.71 112.92
7.89% 48.60% 52.41%
remaining portion of the solar energy is dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar chemical efficiency. To address these issues, we proposed a hybrid system involving both the photochemical process and thermochemical process, in which the full spectrum of the solar irradiation is converted into chemical energy, resulting in a higher efficiency. The main results are as follow.
two single systems over a year. Compared with that of the single photochemical system, the increase rateηincrease-MOST is 4.75 over the whole year. Compared with that of the single thermochemical process, the increase rateηincrease-STC is 8.88% over a year. The average solar chemical efficiency of the single thermochemical system is 45.72%. In contrast, the efficiency of the proposed system is 49.78%, which suggests good performance of the proposed system in the off-design condition.
(1) A solar energy storage system integrating the molecular isomerization process with methanol decomposition is proposed to convert the full spectrum of solar irradiation into chemical energy. The system outputs multiple products which can be easily transported and stored. The short-wavelength photons are stored in the chemical bonds of quadricyclane. The long-wavelength photons are
4. Conclusions The ultraviolet region and a portion of the visible spectrum of the solar radiation can be stored by internal molecular rearrangements as part of photon-induced molecular isomerization cycles. However, the 125
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Solar-clean fuel distributed energy system with solar thermochemistry and chemical recuperation. Appl Energ 2018;225:380–91. [28] Bai Z, Liu Q, Gong L, Lei J. Investigation of a solar-biomass gasification system with the production of methanol and electricity: thermodynamic, economic and off-design operation. Appl Energ 2019. https://doi.org/10.1016/j.apenergy.2019.03.132. [29] Liu T, Liu Q, Xu D, Sui J. Performance investigation of a new distributed energy system integrated a solar thermochemical process with chemical recuperation. Appl Therm Eng 2017;119:387–95. [30] Manikandan GK, Iniyan S, Goic R. Enhancing the optical and thermal efficiency of a parabolic trough collector – a review. Appl Energ 2019;235:1524–40. [31] Valenzuela L, López-Martín R, Zarza E. Optical and thermal performance of largesize parabolic-trough solar collectors from outdoor experiments: a test method and a case study. Energy 2014;70:456–64. [32] Liu Q, Wang Y, Lei J, Jin H. 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Mid-temperature solar fuel process combining dual thermochemical reactions for effectively utilizing wider solar irradiance. Appl Energ 2017;185:1031–9.
Fig. 17. Stored solar energy and solar thermochemical efficiency of the proposed system, compared with those of the single photochemical system and single thermochemical system, for each month.
stored in the products of methanol decomposition (H2 and CO). (2) The half-life of the energy storage associated with the molecular isomerization process reaches up to 1400 days with 120 kJ mol−1 of barrier energy, which is sufficient for seasonal energy storage. In the single molecular isomerization process, about 8.66% of the solar energy is converted and stored in the chemical bonds of QCs, while 15.21% of the solar energy is dissipated as heat. (3) The influences of the input temperature and DNI on the performance of the thermochemical process were studied. Increasing the feed methanol temperature by using a heat exchanger can improve the methanol conversion and the solar thermochemical efficiency. Although sunlight is filtered by the photochemical process, the performance of the thermochemical process in the proposed system is not significantly weakened. (4) In the proposed system, 7.89% of the input solar energy can be stored in QCs during the photochemical process without significantly lowering the solar thermochemical efficiency (67.49%), and the total solar chemical efficiency is about 75.38% in the design condition. In the off-design condition, the average annual solar chemical efficiency is 49.78%. (5) The thermodynamic performances of the proposed system were compared with those of the single molecular isomerization system and single methanol decomposition system. The solar energy stored in the proposed system over a year and the annual solar chemical efficiency are higher than those of the two single systems, which confirm that the proposed system is superior to the other two systems. Acknowledgements The authors appreciate the financial support provided by the National Key Research and Development Program of China (2018YFB1502004)and the National Natural Science Foundation of China (No. 51722606). References [1] Li G. Design and development of a lens-walled compound parabolic concentrator – a review. J Therm Sci 2018;28:17–29. [2] Renno C. Thermal and electrical modelling of a CPV/T system varying its configuration. J Therm Sci 2018;28:123–32. [3] Meier A, Houaijia A, Monnerie N, Roeb M, Sattler C, Ravenswaay Jv, et al. Roadmap to solar fuels – strategy for industry involvement and market penetration. IEA/ SolarPACES; 2015. [4] Kwak BS, Chae J, Kang M. Design of a photochemical water electrolysis system based on a W-typed dye-sensitized serial solar module for high hydrogen production. Appl Energ 2014;125:189–96. [5] Sánchez Jiménez PE, Perejón A, Benítez Guerrero M, Valverde JM, Ortiz C, Pérez Maqueda LA. High-performance and low-cost macroporous calcium oxide based materials for thermochemical energy storage in concentrated solar power plants. Appl Energ 2019;235:543–52.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Spanning solar spectrum: A combined photochemical and thermochemical process for solar energy storage
T
⁎
Juan Fanga,b, Qibin Liua,b, , Shaopeng Guoa, Jing Leic, Hongguang Jina,b a
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China b
H I GH L IG H T S
full spectrum of solar energy is utilised in the proposed system. • The discontinuous solar irradiation is stored as chemical energy. • The photochemical and thermochemical processes are jointly utilised. • Solar • The promising thermodynamic performance is numerically validated.
A R T I C LE I N FO
A B S T R A C T
Keywords: Solar photochemical process Solar thermochemical process Solar energy storage Hybrid system
Storage in the form of chemical energy is crucial for efficient utilisation of solar energy. In recent years, solar photon-induced molecular isomerization energy storage, in which solar energy can be directly converted and stored as chemical energy through internal molecular isomerization reactions, has received increasing interest. It is a closed cycle without any CO2 emission. However, the absorption of reactants cannot cover the full spectrum of solar radiation, and only the ultraviolet and a portion of the visible spectrum of solar energy can be stored. Moreover, some absorbed photons are dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar photochemical efficiency. To address these problems, a new energy storage system which integrates the photochemical process with thermochemical process has been proposed to convert the full spectrum of solar energy into chemical energy. Concentrated sunlight enters the photochemical device. Norbornadiene derivatives present in the photochemical devices can be isomerized to the related quadricyclanes by absorbing the ultraviolet-visible light photons. Some absorbed photons are simultaneously stored in the chemical bonds of the quadricyclanes. The unabsorbed photons corresponding to the visible-infrared spectrum are transmitted to thermochemical reactors, providing heat for methanol decomposition. This portion of the solar energy is stored in the form of chemical energy of the products (H2 and CO). The heat dissipated in the photochemical process is transferred to the thermochemical reactors for methanol decomposition, resulting in a higher solar chemical efficiency. Results show that the average solar chemical efficiencies corresponding to the design condition and off-design condition are 75.38% and 49.78%, respectively. The results of comparison of the thermochemical performances verify that the proposed system is superior to both the single photochemical system and the single thermochemical system.
1. Introduction Renewable energy is crucial for developing low-carbon cities and a sustainable society. Solar energy is considered to be the most promising solution for achieving such a goal [1]. However, challenges associated with the intermittency of the solar flux have led to an increasing need for solar energy storage technologies [2]. In this context, solar ⁎
chemistry, which pertains to the conversion of solar energy and its storage as solar fuels, has received significant attention [3]. There are three main ways to convert solar energy into solar fuels: solar photochemical process [4], solar thermochemical process [5] and solar electrochemical process [6]. These approaches and their several variants, such as photon-induced CO2 reduction [7] or water splitting [8], thermo-induced decomposition of fossil fuels [9] or reforming with
Corresponding author at: Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail address: [email protected] (Q. Liu).
https://doi.org/10.1016/j.apenergy.2019.04.043 Received 1 December 2018; Received in revised form 30 March 2019; Accepted 10 April 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature
Subscripts
E Q τ H n T DNI S h kT G R r P X k F A
iso relax
energy heat energy time enthalpy number of photons temperature solar direct nominal irradiation solar field area (m2) Plancks constant Bolzmanns constant Gibbs energy gas constant reaction rate (mol s−1 m−2) pressure conversion rate reaction rate constant mole flow rate (mol s−1) catalyst surface area (m2)
λ onset stored 1/2 D CH3O(2) S2, S2a STC MOST
energy loss related to the quantum yield energy loss related to the relaxation from parent* to the photoisomer the wavelength of the solar irradiation onset wavelength stored solar energy half life methanol decomposition intermediate in mechanisms reaction active site solar thermochemical molecular solar energy storage
Greek symbol η
efficiency
since the long-wavelength photons cannot be used [13]. In addition, some absorbed photons are dissipated as heat, resulting in an increase in the reaction temperature and a lower solar photochemical efficiency [24]. Several attempts have been made to circumvent these limits. Photon upconversion, which involves transforming two long-wavelength photons into one short-wavelength photon, seems to be a promising strategy, but efficient materials for upconversion need to be further investigated [25]. Kasper et al. combined a solar water heating system with the NBD-QC isomerization cycle to utilise the long-wavelength photons abandoned by the MOST system [26]. The solar water heating system can store a portion of the solar energy which cannot be stored in the MOST system in the form of sensible heat of water, but the storage time is short. To address these challenges, we proposed a solar energy storage system with photon-induced NBD-QC isomerization cycle and thermoinduced methanol decomposition to more effectively convert the full spectrum of solar radiation into chemical energy. As a kind of liquid fuel, methanol is easy to handle, compared with other hydrocarbons. Furthermore, methanol can be decomposed with middle- and lowtemperature heat (approximately 200–300 °C [27]), which can be provided by sunlight concentrated by using parabolic trough collectors. Through the solar thermochemical process, solar energy is converted and stored in the form of chemical energy of the products of methanol decomposition (CO and H2) [28]. The solar thermochemical efficiency is in the range 60–67% when the methanol conversion is 80–95% [29]. However, solar energy is first converted into heat which induces
water [10], have been intensively studied. Among these, the photoninduced molecular isomerization solar energy storage technology has attracted increasing interest in recent years [11]. The photon-induced molecular isomerization cycle involves two parts, namely, parents and photoisomers. Solar energy is stored through reversible molecular rearrangements between parents and photoisomers. When exposed to sunlight, the relaxed photoactive molecules, named parents, are photochemically converted into photoisomers and solar energy is stored in the energetic chemical bonds of the photoisomers. The photoisomers can convert back to the parents for the next cycle with the help of catalysts, thus recovering the stored solar energy in the form of heat in a controlled manner, which is the so-called molecular solar thermal energy storage (MOST) cycle (see Fig. 1) [12]. Börjesson et al. determined that these cycles are expected to achieve up to 10–12% solar energy storage efficiencies when relatively high photon energies (1.8–1.9 eV) are absorbed [13]. There are three main photoisomeric reactions: double-bond isomerization, organometallic ligand reorientation and electrocyclic reactions [14]. Double-bond isomerization, such as cis-trans photoisomerization in azobenzene derivatives, can absorb sunlight in the ultraviolet-visible region, but the low storage enthalpy discounts its value [15]. To improve the energy storage enthalpy, azobenzene/carbon nanotube material and functionalised graphene with azobenzene units were theoretically proposed by Kolpak and Grossman [16], but the challenging synthesis process hinders their application [17]. Organometallic compounds (fulvalene diruthenium) display high energy storage densities, but the high cost has led to a decline in research interest [18]. In contrast, electrocyclic isomerization reactions, such as norbornadiene-quadricyclane derivatives cycle (NBD-QC), show promise for solar energy storage owing to the high storage enthalpy, low molecular weight and commercially availability [19]. Moreover, there are several effective catalysts available for the backward reaction [20]. Therefore, NBD-QC is the focus of the present work. Recently, the MOST cycles have drawn much attention because they are closed systems where both the energy storage process and the release process occur within one entity, and other substances (such as O2) are not required. These closed cycles have many advantages compared with open systems (e.g. methane combustion [21]): easily obtainable recycling material and eco-friendly nature, without any CO2 emission [22]. However, a major problem with the MOST cycles is that the absorption by the parents does not cover the full spectrum of solar irradiation. Similar to the Shockley-Queisser limit for silicon solar cells [23], the MOST system also has a solar photochemical efficiency limit
Fig. 1. Photon-induced molecular isomerization cycle. 117
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is dissipated as heat, leading to an increase in temperature and a decrease in the solar chemical efficiency. To convert and store the full spectrum of solar energy as chemical energy and improve the solar chemical efficiency, a new system involving molecular isomerization process and methanol decomposition was proposed. The proposed system consists of three main parts: parabolic trough collectors, solar photochemical devices with heat exchangers and solar thermochemical reactors. A schematic of the system is shown in Fig. 2. Direct solar radiation is concentrated by using double-axis tracking parabolic trough collectors [30]. A concentrated solar beam first enters the molecular isomerization device, where norbornadiene derivatives (NBDs), driven by a pump, flow past. The chromophores of the NBDs can absorb the solar photons corresponding to the ultraviolet wavelength range and a portion of the visible spectrum, before the NBDs are converted into related quadricyclane derivatives (QCs). Some absorbed photons are simultaneously stored in the chemical bonds of the QCs, while the remaining absorbed photons are dissipated as heat, resulting in an increase in the temperature. Heat exchangers are employed to avoid the temperature rise caused by the focusing of light and utilise the solar energy more effectively. The heat dissipated in the molecular isomerization device is transferred to solar thermochemical reactors for methanol decomposition. The unabsorbed photons corresponding to the infrared range and a portion of the visible spectrum are transmitted to solar thermochemical reactors filled with a catalyst. Solar energy is converted into thermal energy in the reactors, which provides the heat for the decomposition of methanol. Then, methanol is decomposed into H2 and CO, and this portion of the solar energy is stored in the form of chemical energy of the products (H2 and CO), which can be further used in combustion to generate electricity or industrial raw materials. The main advantages of the proposed system are as follow.
methanol decomposition. The occurrence of two conversion processes increases the irreversible loss. Moreover, all the photons are converted into heat without any distinction. Therefore, we proposed a hybrid system, where photons with high energies can be converted and stored directly through a photochemical process, while the remainder of the solar energy is utilised by the thermo-induced methanol decomposition. The main contributions of this study are summarised as follow. (1) An energy storage system was proposed to realise full-spectrum solar energy conversion and storage. Based on the theory of cascade utilisation of energy, the photochemical process and thermochemical process were combined to achieve a higher solar chemical efficiency. (2) The discontinuous solar irradiation is stored as chemical energy. This system outputs multiple products which can be easily transported and stored. The high-energy photons are directly converted into chemical bonds of QCs via the photochemical process. The lowenergy photons are stored in the form of chemical energy of the methanol decomposition products during the solar thermochemical process. (3) In the photochemical process, the storage time and solar photochemical efficiency were studied. In the thermochemical process, the influences of methanol input temperature and the solar irradiation filtered by the photochemical devices on methanol decomposition were investigated. (4) The thermodynamic performances of the proposed system were compared with those of the single molecular isomerization system and the single methanol decomposition system, and the feasibility of the proposed system was validated. The rest of this paper is organized as follows. In Section 2, a new system is proposed to convert and store the full spectrum of solar energy as chemical energy. In Section 3, the thermodynamic performances of the proposed system are evaluated. Finally, the main conclusions of this study are summarised in Section 4.
(1) This system realises full-spectrum solar energy storage. The shortwavelength photons corresponding to the ultraviolet and visible range of the spectrum are converted and stored directly in photoninduced molecular isomerization devices. The long-wavelength photons abandoned by the solar photochemical process, corresponding to the infrared and the visible region of the spectrum, are utilised by the solar thermochemical process. (2) The low density and unstable solar energy is upgraded into the high density and stable chemical energy of multiple products. The shortwavelength photons are stored in the chemical bonds of QCs. The long-wavelength photons are stored in the products of methanol decomposition (H2 and CO), which can be easily transported and stored as gases.
2. Full-spectrum solar chemical energy storage systems 2.1. Proposed system Solar photon-induced molecular isomerization, in which solar energy can be stored in chemical bonds through reversible molecular rearrangements, has drawn much attention in recent years. However, only the ultraviolet and a portion of the visible light solar energy can be converted into chemical energy in this process. Some of the solar energy
Fig. 2. Mechanism of storage of the full spectrum of solar energy as chemical energy. 118
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days and for a full year are shown in Figs. 4 and 5.
(3) Heat exchangers are employed in the molecular isomerization devices which serve as channels for cooling the QCs in order to avoid back-conversion. The heat dissipated during the photochemical process is transferred to thermochemical reactors for methanol decomposition, which results in a higher solar chemical efficiency. (4) The system does not require additional light filters, resulting in a simpler structure. In the solar photochemical process, sunlight is divided into two parts by the chromophores present on the NBDs. (5) The system is eco-friendly. It does not produce CO2 during the solar photochemical process, and only generates a small amount of CO2 during the solar thermochemical process.
(1) Solar photochemical process A schematic diagram of the photon-induced molecular isomerization process is shown in Fig. 6. The parents (NBDs) absorb the solar photons to become activated parents (parents*), which may transform to their related photoisomers (QCs). By this process, ΔHstored is the amount of solar energy stored in the chemical bonds of QCs. The QCs can be converted back to the NBDs with the help of a catalyst, thereby releasing the stored solar energy. Some assumptions have been made for simplification, which are as follow.
2.2. Calculating model i All molecules act individually. ii The parents absorb all photons with energy higher than a threshold value, and the unabsorbed photons are transported to the thermochemical reactors. iii The stored energy in one activated parent is the energy difference between the parent molecule and the photoisomer (ΔHstored in Fig. 6), and the excess photon energy is dissipated as heat.
The energy and matter flows of the system are shown in Fig. 3. It can be concluded that the solar energy Qsolar and methanol are the only input energy and matter, respectively. The products are the solar energy stored in the chemical bonds of QCs and the chemical energy stored in CO and H2. It is worth noting that the NBDs and QCs can interconvert to each other in the intersystem. There are two main kinds of losses, namely, optical loss and heat loss. The optical losses of the parabolic trough collectors, solar photochemical device, jacketed glass tubes and metal reactor tubes of the solar thermochemical reactors are considered. The parabolic trough collectors reflect (reflectivity of 0.93 [31]) the incident solar beam to the molecular isomerization device. The solar concentration process involves the optical loss Qoptical-loss1. The remaining portion of solar energy is transmitted (transmittivity of 0.98) to the solar thermochemical process, with Qoptical-loss2 being the solar energy obstructed in the molecular isomerization devices. Because of the transmission of the glass envelops (transmittivity of 0.96 [31]) and the intercept of the metal tubes (intercept factor of 0.958 [31]), Qoptical-loss3 amount of solar energy is lost in the thermochemical process. Heat losses mainly occur in the solar thermochemical reactor through convection and irradiation; Qheat-loss2 and the heat exchange loss Qheat-loss1 are observed in the heat exchanger. The dissipated heat losses (Eiso + Erelax) associated with the photochemical section are transferred to the thermochemical reactors through the heat exchanger, and recycled for methanol decomposition. The solar direct nominal irradiation (DNI) and the ambient temperature were recorded by the BSRN300 meteorological station located in Langfang, Hebei (E116.65°, N39.58°). The hourly data for the typical
There are two main losses associated with the MOST process, Eiso and Erelax. Eiso is related to the quantum yield of the photoreaction, φiso, which is defined as φiso = niso/nabsorbed, where niso is the number of photons utilised in the isomerization events and nabsorbed represents the number of photons absorbed by the reactants; φiso ≠ 1. Erelax is attributed to the intrinsic property of the molecular isomerization process. When the parent is transformed to the photoisomer, energies Ea and E1 are lost (see Fig. 6). The two main losses can be computed as [26]
Eiso =
∫0
Erelax =
λ onset
∫0
nλ ·Eλ ·(1 − ϕiso) dλ
λ onset
nλ ·(Ea + E1)·ϕiso dλ
(1)
(2)
where nλ is the number of absorbed photons (s−1 m−1 nm−1) and Eλ is the energy of every photon at wavelength λ (J nm−1). Therefore, the theoretical stored solar energy and the photochemical efficiency are calculated as
Estored =
∫0
λ onset
nλ ·(Eλ − Ea − E1)·ϕiso dλ
Fig. 3. Energy and matter flows of the system. 119
(3)
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800 30
700
Ambient temperature /°C
DNI / Wm-2
600 500 400 300 Spring equinox Summer solstice Autumn equinox Winter solstice
200 100 0
6
8
10
12
14
16
18
20
25 20 Spring equinox Summer solstice Autumn equinox Winter solstice
15 10 5 0 -5
4
6
8
10
Time / h
12
14
16
18
20
22
Time / h
Without the barrier Ea, the photoisomers would convert back to the parents immediately, and the storage time would be very short. On the other hand, a large barrier, which is also the main contributor to the energy loss, will lead to a lower stored solar energy ΔHstored (see Fig. 6). Therefore, the storage time and photochemical efficiency ηMOST are mainly dependent on the energy barrier of the back-conversion process Ea. The relationship between ηMOST and Ea is shown in Eq. (4), and the storage time, quantified by the half-life, is computed by [13]
800 600 400 200
Ambient temperature /°C
DNI / Wm
-2
Fig. 4. Variations in the DNI (left) and ambient temperature (right) for typical days.
0 40 20
τ1/2 =
h × ln(2) kT × T × e−G / RT
(5)
0 0
2000
4000
6000
where h is Planck’s constant, kT is the Boltzmann constant, T is the storage temperature, G is the Gibbs energy of activation and R is the gas constant. Furthermore, the structures of the parents and photoisomers are similar because the NBDs are converted into the related QCs via internal rotations and rearrangements; no parameters which change the rigidity of the molecules are changed. Therefore, we assumed that the entropy of the isomerization reaction is negligible, which implies that the free energy equals the activation enthalpy based on the above reasoning.
8000
Time / h
Fig. 5. Variations in the DNI (upper) and ambient temperature (underside) for a full year.
(2) Solar thermochemical process The photons corresponding to the ultraviolet region and a portion of the visible region of the spectrum are absorbed during the photochemical process, while the infrared spectrum and the remaining portion of the visible spectrum are transmitted into the thermochemical reactor for methanol decomposition. The glass envelop is mounted to surround the metal reaction tube. The gap in the ring between the glass envelop and metal tube contains vacuum. Cu/Al2O3/ZnO catalyst is packed in the metal tube. The methanol fed from the heat exchanger flows though the porous catalyst bed, where it is decomposed into CO and H2 when the conditions are suitable. The heat transfers occurring between the inner and outer surfaces of the metal tube, the outer surface of the metal tube and the inner surface of the glass envelop, the inner and outer surfaces of the glass envelop, the outer surface of the glass envelop and the environment and in the catalyst bed are shown in Fig. 7. Moreover, the coupling model, involving solar-heat conversion, heat transfer, fluid flow, mass flow and the chemical reaction, has been studied by our group [32]. The theoretical details and experimental verifications have been intensively reported by us previously [33–35]. The parameters of the solar thermochemical reactors are shown in Table 1. The thermochemical reactor is divided into i infinitesimal units. For each unit, the kinematical model of methanol decomposition is given by [27,36]
Fig. 6. Schematic diagram of solar photon-induced molecular isomerization. t
ηMOST =
∫t12 Estored dt t
∫t12 (DNI ·S ) dt
(4)
where λonset is the onset wavelength (cut-off wavelength between the photochemical process and thermochemical process) (nm), DNI refers to the direct solar radiation and S is the area of the parabolic trough collectors. The loss Erelax includes the energy barrier Ea of the back reaction. 120
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thermochemical process:
XCH3 OH =
ηSTC =
FCH3 OH ,0 − FCH3 OH , i FCH3 OH ,0
t2
i ∑i= 1 t1
∫
(13)
[FCH3 OH ,0 Cp, i (Ti − Ti − 1) + rD, i ΔAcat , i ΔHr , i] dt t
∫t12 (DNI ·S ) dt
(14)
where FCH3 OH,0 is the molar flow rate of the input methanol; Cp,i represents the specific heat capacity of the ith unit; Ti and Ti-1 represent the temperatures of the species in the ith and i-1th units, respectively, and ΔHr,i is the reaction heat in the ith unit. (3) Performance of the proposed system The total solar chemical efficiency of the combined system is the ratio of the stored chemical energy to the input solar energy, and is equal to the sum of the solar photochemical efficiency and solar thermochemical efficiency i.e.
Fig. 7. Model of the solar thermochemical reactor.
ηcombine = ηMOST + ηSTC Table 1 Parameters of solar thermochemical reactors. Item
Unit
Value
Aperture width of the parabolic trough mirror Total length of the reactor Inner/outer diameter of the metal tube Inner/outer diameter of the glass envelope Catalyst bulk density Catalyst dimension Ambient temperature Pressure at the outlets of solar receivers/reactors
m m m m kg L−1 mm °C bar
4.2 12 0.046/0.050 0.075/0.080 1.15 Φ5 × 5 25 2.5
(15)
To evaluate the improvement in the thermodynamic performance of the proposed system compared with those of the single solar photochemical process and the single solar thermochemical process, the increase ratios ηincrease-MOST and ηSTC-MOST were calculated as
ηincrease − MOST =
ηincrease − STC =
Ecombined system − EMOST − sin gle EMOST − sin gle
(16)
Ecombined system − ESTC − sin gle ESTC − sin gle
(17)
(4) Validation of the calculation model
rD =
kD k ∗ (2) CH3 O
⎛ ⎝
PCH3 OH 0.5 PH 2
⎞ ⎛1 − ⎠⎝
2 P PH CO 2
kD PCH3 OH
⎞ CS 2 CS 2a ⎠
3 OH ⎞ ⎤ ⎡1 + k ∗ ⎛ PCH0.5 (1 + P 0.5(2a) PH0.52 ) H CH3 O(2) ⎢ ⎝ P H2 ⎠ ⎥ ⎣ ⎦
The numerical models of the photochemical and thermochemical processes have been validated by the respective experimental results. (6)
(1) Photon-induced molecular isomerization process
where CS2 and CS2a are the surface concentrations of active site 2 and active site 2a, respectively, mol m−2; kD is the reaction rate constant and K ∗ (2) KH(2a) are the absorption coefficients of the surface species,
For the photochemical process, the model has been validated [26]. In this case, λonset and φiso are 456 nm and 28%, respectively. We can obtain Eiso = 84 W m−2, Erelax = 21 W m−2 and Estored = 11 W m−2. Therefore, the photochemical efficiency is 1.1% for AM1.5 solar spectrum, which corresponds to the experimental value reported in Ref. [26]. Eiso ± Erelax ± Estored = 116 W m−2, which is in accordance with the solar energy below 456 nm. As a result, it is confirmed that Eqs. (1) and (2) describe the main losses occurring in the photon-induced molecular isomerization process.
CH3 O
bar−0.5, which can be calculated by
−E kD = kD∞ exp ⎛ D ⎞ ⎝ RTr ⎠ ⎜
K∗
CH3 O(2)
⎟
(7)
Δ ∗ ΔH ∗ (2) ⎛ SCH3 O(2) CH3 O ⎞ = exp ⎜ − R RTr ⎟ ⎝ ⎠
K H (2a) = exp ⎛ ⎝
ΔS H (2a)
⎜
R
−
(8)
ΔH H (2a) ⎞
(2) Validation of the solar thermochemical model
⎟
RTr
⎠
(9) The numerical model corresponding to the thermochemical process was validated by the experimental results reported by our research group [33]. We employed double-axis parabolic trough collectors in the proposed system, whereas the parabolic trough collectors used in the experiment were positioned in the East–West direction. Moreover, the parameters of the thermochemical reactors employed in the
where R represents the universal gas constant and Tr is the temperature of the solar thermochemical reactor. The other parameters are shown in Table 2. The molar flow rate of methanol in each infinitesimal unit FCH3 OH , i is computed as
FCH3 OH , i = FCH3 OH , i − 1 − rD, i ΔAcat , i
(10) Table 2 Parameters of the kinematical model for methanol decomposition.
where ΔAcat,i is the catalyst surface area of the infinitesimal reaction unit, which is calculated by
dAcat = Am dmcat
dmcat =
1 ρ πDin2 dL 4 cat
(11)
Rate constant or equilibrium constant
ΔSi /J mol−1 K−1 or ki∞/m2 mol−1 s−1
ΔHi or E /kJ mol−1
(12)
kD∞ (m2 mol−1 s−1) −0.5 k∗ ) (2) (bar
3.8 × 1020 30.0
170.0 −20.0
k H (2a) (bar−0.5)
−46.2
−50.0
CH3 O
The methanol conversion rate XCH3 OH and the solar thermochemical efficiency ηSTC were chosen to evaluate the performance of the 121
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utilise solar energy more efficiently and avoid the back-conversion caused by an increase in temperature due to the dissipating heat, this portion of solar energy is recycled to preheat the feed methanol as part of the thermochemical process by using a heat exchanger. Therefore, the input temperature of methanol is increased, and the effect of the input temperature on the performance of the thermochemical reaction was examined, as described in the following section.
experiments are listed in Table 3. Fig. 8 shows comparisons of the methanol conversion and solar thermochemical efficiency between the theoretical and experimental results. The results reveal good agreement, which indicate that the computing model used for the thermochemical process is valid. 3. Results and discussion In this section, the storage time and solar photochemical efficiency of the MOST system are presented. Based on the results, we further discuss the energy distribution in the MOST system. To recover the heat dissipated in the MOST process, heat exchangers were employed. The input temperature of the feed methanol in the thermochemical process was increased with the help of the heat exchangers. Therefore, the influence of the input temperature on the thermochemical process was investigated. Moreover, sunlight is filtered by the photochemical process. The influence of DNI on the thermochemical process was studied. In the proposed system, the thermodynamic performances corresponding to the design condition and off-design condition were investigated. Comparison of the performances of the proposed system and the single MOST system or single methanol decomposition system are presented.
3.2. Performance of the solar thermochemical process in the combined system The heat exchanger recycles the heat dissipated during the photochemical process and preheats the feed methanol as part of the thermochemical process, which increases the input temperature of methanol. Fig. 12 shows the effect of the input temperature on the methanol conversion rate and on the solar thermochemical efficiency. We can see that the methanol conversion rate is increased to 99% from 93% and that the solar thermochemical efficiency is increased to 69% from 63% when the input temperature is increased to 200 °C from 100 °C. The results show that the increase in the input temperature can enhance the thermodynamic performance of the thermochemical process. Therefore, heat exchangers not only serve as channels for the cooling species during the photochemical process but also improve the performances of the solar thermochemical process in the combined system. As an important parameter, the DNI affects the performance of the thermochemical process both in the individual thermochemical system and in the combined system. In the individual system, the full spectrum of solar energy is converted to heat, which is utilised for methanol decomposition. On the other hand, in the combined system, only the photons corresponding to the infrared region and a portion of the visible spectrum of the solar radiation, which cannot be absorbed during the photochemical process, are transported to the thermochemical reactors. Because sunlight is filtered by the photochemical process, the performance of the thermochemical process in the combined system is not as good as that in the single thermochemical system. To compare the difference in the performance of the thermochemical process between the individual system and the combined system, the methanol conversion rate and solar thermochemical efficiency were investigated as functions of the DNI. We can see from Fig. 13 that the methanol conversion rates of both the systems are directly proportional to the DNI. The conversion rate of the combined system is lower than that of the single thermochemical system, but the difference is small. The solar thermochemical efficiency also increases with the increase in DNI, but the increase ratio becomes smaller with the increase in DNI. The solar thermochemical efficiency of the combined system is also lower than that of the single thermochemical system, but the difference is smaller than 1%. It can be concluded that the variation in DNI significantly influences the thermochemical process in both the individual and combined systems. Although the performance of the thermochemical process in the combined system is not as good as that in the single thermochemical system, the difference between the values can be neglected. To further evaluate the performance of the combined system,
3.1. Performance of the photon-induced molecular isomerization process The storage time and solar energy conversion efficiency are the two most important parameters of the photochemical process. According to the analysis presented in Section 2.2, we can conclude that these depend mainly on the energy barrier of the back-conversion process, Ea. A small barrier leads to a short storage time. On the other hand, a large barrier will result in a high energy loss, which eventually reduces the solar energy conversion efficiency. If that is the case, what constitutes a suitable barrier? Here, the storage time is quantified by the half-life. First, we calculated the half-life as a function of Ea at different storage temperatures, which is shown in Fig. 9. It can be concluded that the half-life increases sharply with the decrease in the storage temperature and the increase in the energy barrier of the reverse reaction. At ambient temperature (25 °C), an energy barrier of 110 kJ mol−1 is sufficient for daily energy storage. The half-life of the photoisomer can reach up to 1400 days, with an energy barrier of 120 kJ mol−1, which is sufficient for seasonal energy storage. We also studied the solar photochemical efficiencies of different barriers as functions of the cut-off wavelength, and the results are shown in Fig. 10. The cut-off wavelength is the dividing line between the photochemical process and the thermochemical process. We assume that photons with energy higher than a threshold value (the photon energy at cut-off wavelength) can be absorbed for the photochemical process, and the quantum efficiency is 0.8. It can be concluded that the solar photochemical efficiency increases with the increase in the cut-off wavelength until a maximum point, before it decreases. The peak point increases with the decrease in the total energy loss (Ea + E1). The minimal amount of total energy loss for seasonal storage is 120 kJ mol−1 (Ea = 120 kJ mol−1 and E1 = 0 kJ mol−1) and for daily storage is 110 kJ/mol (Ea = 110 kJ mol−1 and E1 = 0 kJ mol−1), which yield maximum solar photochemical efficiencies of 10.41% and 8.93% at the cut-off wavelengths of 647 nm and 683 nm, respectively. According to the above analysis, the energy distribution of the molecular isomerization process corresponding to the design condition (DNI: 1000 Wm−2, ambient temperature: 25 °C) is shown in Fig. 11. In this case study, we assume that the total energy loss is 120 kJ mol−1, the quantum efficiency is 0.8 and the cut-off wavelength is 600 nm. In Fig. 11, we observe that 76.13% of the solar energy is transported to the solar thermochemical reactors for methanol decomposition. About 8.66% of the solar energy is converted and stored in the chemical bonds of the QCs, while 15.21% of the solar energy is absorbed but not stored, and is dissipated as heat during the molecular isomerization process. To
Table 3 Parameters of the solar thermochemical reactors used for validation.
122
Item
Unit
Value
Aperture width of the parabolic trough mirror Total length of the reactor Inner/outer diameter of the metal tube Inner/outer diameter of the glass envelope Catalyst bulk density Catalyst dimension Ambient temperature Mirror reflectivity
m m m m kg L−1 mm °C
2.5 4 0.031/0.035 0.051/0.054 1.7 Φ5 × 5 30 0.8
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Theoretical results
0.9 Methanol Conversion
Solar thermochemical efficiency / %
1.0 Experimental Results
0.8 0.7 0.6 0.5 0.4 0.3 0.2
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
70 65 60 55 50
Theoretical results
45
Experimental Results
40 35 30 25 20 1.0
1.5
Feeding rate of methanol / (l/h)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Feeding rate of methanol / (l/h)
Fig. 8. Variations of the methanol conversion (left) and solar thermochemical efficiency (right) with methanol feeding rate.
Estored Eiso Erelax Eunabsorbed
For solar thermochemical reaction 76.13%
8.66% 2.17%
13.04%
For preheating feed Fig. 9. Half-life of the photoisomer versus the energy barrier of the back-conversion reaction for different storage temperatures.
Fig. 11. Energy distribution of the photon-induced molecular isomerization process corresponding to the design condition (DNI: 1000 W m−2 and ambient temperature: 25 °C).
1.00
Conversion of methanol
0.98
0.68
0.96 0.94
0.66
0.92
0.64
0.90 Conversion of methanol Solar thermochemical efficiency
0.88 0.86 100
Fig. 10. Solar photochemical efficiency as a function of cut-off wavelength.
120
140
160
180
200
0.62
Solar thermochemical efficiency
0.70
0.60
Input temperature of methanol feed /°C the behaviour of the proposed system was studied as follows.
Fig. 12. Conversion of methanol and the solar thermochemical efficiency as functions of different input temperatures of the methanol feed (DNI: 1000 W m−2, ambient temperature: 25 °C and methanol feed rate: 0.33 mol s−1).
3.3. Behaviour of the proposed system In this section, we compared the thermodynamic performances of the proposed system with those of the single thermochemical system and single photochemical system for the design condition and off-design condition. The energy flow of the proposed system in the design condition (1000 W m−2) is shown in Fig. 14. Optical and heat losses are the two main losses observed in the system. The heat loss of the solar
thermochemical reactor accounts for 7.50% of the total input solar energy, and ranks the first among all the losses. The temperature difference between the reactors and the ambient leads to a large irradiation loss and a convection loss at the outer surface of the glass envelop. Then, the optical losses of the parabolic trough collectors and the
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Fig. 13. Conversion of methanol and the solar thermochemical efficiency for different DNIs. (Ambient temperature: 25 °C and methanol feed rate: 0.33 mol s−1).
thermochemical reactor rank second and third, respectively. The three main losses cannot be circumvented both in the single thermochemical system and in the combined system. When photochemical reactors are deployed in the combined system, the solar beam reflected from the parabolic trough collectors needs to go through two layers of glass, leading to increased optical losses in the combined system (about 3.25% of the input solar energy). The dissipated heat (Eiso + Erelax) of 13.85% in the photochemical section is transferred to the thermochemical reactors by the heat exchanger. It is demonstrated that 7.89% of the input solar energy can be stored by the photochemical process without significant reduction of the solar thermochemical efficiency (67.49%) corresponding to the methanol decomposition process. In the combined system, the total solar chemical efficiency is about 75.38% in the design condition. To establish the superiority of the proposed system in the off-design condition, we compared the amount of solar energy stored between 400 W m−2 and 800 W m−2 in the combined system with those stored in the single photochemical system and single thermochemical system. It can be concluded from Fig. 15 that the solar energy stored in the combined system is always higher than those stored in the single photochemical system and single thermochemical system. The increase ratio ηincreaseMOST increases with the increase in the DNI. However, the increase in the solar irradiance will lower the ratio ηincrease-STC. The reason is that the variation in DNI affects the thermochemical process more than the photochemical process.
Fig. 15. Solar energy stored in the proposed system in comparison with those stored in the single photochemical system and single thermochemical system under different DNIs (ambient temperature: 25 °C).
A comparison of the hourly stored solar energy for typical days between the proposed system and the single systems was performed to further evaluate the feasibility of the proposed system in the off-design condition, and the results are shown in Fig. 16 and Table 4. We assumed that methanol decomposes when the DNI is higher than 400 W m−2, because it is not possible to provide sufficient heat for methanol decomposition when the DNI is below 400 W m−2 [37]. Therefore, the combined system can significantly increase the running time, compared with that of the single thermochemical system across the four seasons. The running time of the proposed system even increases by 6 h in winter. The results also show that the proposed system can store much higher solar energy, compared with the other two systems. The average solar chemical efficiencies of the proposed system for the four typical days are also the highest. Such encouraging results indicate that the thermodynamic performance of the proposed system in the off-design condition is better than those of the two single systems. The annual behaviours of the three systems are also revealed in Fig. 17. The results show that the stored solar energy and the solar chemical efficiency of the combined system are higher than those of the
Fig. 14. Sankey diagram representing the energy flows in the proposed system for the design condition. (DNI: 1000 Wm−2 and ambient temperature: 25 °C). 124
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24 600
20 18
400
16 14 4
200
2 7
8
0
9 10 11 12 13 14 15 16 17 18 19 20
18
400
16 14 4
0
800
200
22
600
400
16 14 4
DNI / Wm-2
20 18
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
200
Winter solstice
24
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
500
20
400
18
300
16 4
0
0
700 600
22
200
2
2
0
800
26
Autumn equinox
24
Stored solar energy / kW
600
20
DNI / Wm-2
6
26
0
22
2
Stored solar energy / kW
0
Summer solstice
DNI / Wm-2
22
800
26
Spring equinox
Stored solar energy / kW
24
Stored solar energy / kW
800
EMOST-single ESTC-single ECombiend system
DNI / Wm-2
26
100
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
0
Time / h Fig. 16. Hourly stored solar energies corresponding to the typical days of the four seasons. Table 4 Comparison of the thermodynamic performances on the typical days of the four seasons. Typical days
Systems
Running hours/h
Incident solar energy/kW
Stored solar energy/kW
Average solar chemical efficiency
Spring
Single MOST system Single STC system Combined system
10 6 10
228.21 228.21 228.21
18.01 127.86 135.11
7.89% 56.03% 59.20%
Summer
Single MOST system Single STC system Combined system
14 10 14
330.78 330.78 330.78
26.10 203.44 212.37
7.89% 61.50% 64.20%
Autumn
Single MOST system Single STC system Combined system
12 7 12
278.06 278.06 278.06
21.94 147.31 156.87
7.89% 52.98% 56.42%
Winter
Single MOST system Single STC system Combined system
11 5 11
215.46 215.46 215.46
17.00 104.71 112.92
7.89% 48.60% 52.41%
remaining portion of the solar energy is dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar chemical efficiency. To address these issues, we proposed a hybrid system involving both the photochemical process and thermochemical process, in which the full spectrum of the solar irradiation is converted into chemical energy, resulting in a higher efficiency. The main results are as follow.
two single systems over a year. Compared with that of the single photochemical system, the increase rateηincrease-MOST is 4.75 over the whole year. Compared with that of the single thermochemical process, the increase rateηincrease-STC is 8.88% over a year. The average solar chemical efficiency of the single thermochemical system is 45.72%. In contrast, the efficiency of the proposed system is 49.78%, which suggests good performance of the proposed system in the off-design condition.
(1) A solar energy storage system integrating the molecular isomerization process with methanol decomposition is proposed to convert the full spectrum of solar irradiation into chemical energy. The system outputs multiple products which can be easily transported and stored. The short-wavelength photons are stored in the chemical bonds of quadricyclane. The long-wavelength photons are
4. Conclusions The ultraviolet region and a portion of the visible spectrum of the solar radiation can be stored by internal molecular rearrangements as part of photon-induced molecular isomerization cycles. However, the 125
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Fig. 17. Stored solar energy and solar thermochemical efficiency of the proposed system, compared with those of the single photochemical system and single thermochemical system, for each month.
stored in the products of methanol decomposition (H2 and CO). (2) The half-life of the energy storage associated with the molecular isomerization process reaches up to 1400 days with 120 kJ mol−1 of barrier energy, which is sufficient for seasonal energy storage. In the single molecular isomerization process, about 8.66% of the solar energy is converted and stored in the chemical bonds of QCs, while 15.21% of the solar energy is dissipated as heat. (3) The influences of the input temperature and DNI on the performance of the thermochemical process were studied. Increasing the feed methanol temperature by using a heat exchanger can improve the methanol conversion and the solar thermochemical efficiency. Although sunlight is filtered by the photochemical process, the performance of the thermochemical process in the proposed system is not significantly weakened. (4) In the proposed system, 7.89% of the input solar energy can be stored in QCs during the photochemical process without significantly lowering the solar thermochemical efficiency (67.49%), and the total solar chemical efficiency is about 75.38% in the design condition. In the off-design condition, the average annual solar chemical efficiency is 49.78%. (5) The thermodynamic performances of the proposed system were compared with those of the single molecular isomerization system and single methanol decomposition system. The solar energy stored in the proposed system over a year and the annual solar chemical efficiency are higher than those of the two single systems, which confirm that the proposed system is superior to the other two systems. Acknowledgements The authors appreciate the financial support provided by the National Key Research and Development Program of China (2018YFB1502004)and the National Natural Science Foundation of China (No. 51722606). References [1] Li G. Design and development of a lens-walled compound parabolic concentrator – a review. J Therm Sci 2018;28:17–29. [2] Renno C. Thermal and electrical modelling of a CPV/T system varying its configuration. J Therm Sci 2018;28:123–32. [3] Meier A, Houaijia A, Monnerie N, Roeb M, Sattler C, Ravenswaay Jv, et al. Roadmap to solar fuels – strategy for industry involvement and market penetration. IEA/ SolarPACES; 2015. [4] Kwak BS, Chae J, Kang M. Design of a photochemical water electrolysis system based on a W-typed dye-sensitized serial solar module for high hydrogen production. Appl Energ 2014;125:189–96. [5] Sánchez Jiménez PE, Perejón A, Benítez Guerrero M, Valverde JM, Ortiz C, Pérez Maqueda LA. High-performance and low-cost macroporous calcium oxide based materials for thermochemical energy storage in concentrated solar power plants. Appl Energ 2019;235:543–52.
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