The present invention relates to nanofluids comprising nanoparticles from coffee, and processes to make such nanofluids. The invention also provides uses of such nanofluids for absorption of solar radiation in direct absorption solar collectors (DASCs) and as heat transfer fluids. Also provided is a DASC comprising a nanofluid of the invention.

Solar collectors are heat exchangers which are capable of transforming solar radiation to heat. Conventional solar collectors collect the sunlight by a black-body metal receiver. A reliable thermal contact is established between the receiver and a system of pipes mounted at the bottom of the receiver. Forced convection of a heat transfer fluid in the pipes transfers the heat further to a domestic hot water system. Depending on the temperature in the collector, the following heat transfer fluids have previously been used in solar collectors: water-glycol mixtures, mineral oils, molten salts or air in gas-based collectors.

The main disadvantage of conventional solar thermal technology is the thermal leaks from the receiver to the environment. They intensify with either an increase in receiver temperature or a reduction of the environmental temperature. Therefore, the technology has limited applicability in Northern climate conditions without proper thermal insulation. As the collector must be in direct optical contact with the sun, a transparent vacuum gap is the most efficient measure for the receiver’s thermal protection. However, vacuum protection is not entirely reliable and it significantly increases the price of solar collectors.

A direct absorption solar collector (DASC) is a new type of solar collector of a significantly different design. In DASCs (see Figure 1), the receiver is replaced by particles that are dispersed in a heat transfer fluid. The particle-fluid mixture flows in a channel with a transparent top surface. In this case, the volumetric absorption of solar radiation reduces the temperature at the transparent top and limits any thermal losses to the environment.

The concentration of the particles in a DASC has to make the optical depth of the system corresponding to the thickness of the DASC. For practical purposes, the thickness is often within an interval from 1 to 2 cm [3], This gives the total extinction coefficient in the interval from 100 to 400 1/m. The corresponding mass concentration of particles is generally from 0.01 % to 0.5%.

The prototype DASCs demonstrated superior thermal performance relative to collectors with surface-based receivers. For example, the average efficiency of a surface-based collector was 15% lower than the efficiency of an equivalent DASC with an iron oxide particle-fluid [2], The efficiency of a tubular DASC with a fluid with dispersed carbon particles was 6-38% better than the efficiency of a surface-based collector [4],

Different types of particle-fluids have previously been considered in prototype DASCs. Ham et al. [2] recorded the maximum thermal efficiency of 70% for a DASC with iron oxide particles. Kulkarni et al. [5] used a mixture of aluminum oxide and cobalt oxide particles in a spiral DASC and got the maximum thermal efficiency of 50%. An extensive computational study by Gorji and Ranjbar [6] demonstrated that the efficiency of DASCs becomes 60%, 50%, and 30% when fluids based on iron oxide, graphite, and silver particles were used. Karami et al. [7] demonstrated experimentally that a rectangular DASC could gain the maximum thermal efficiency of 70% when particles of copper oxide were used.

It follows from the above comments that particle-fluids are very effective in the direct absorption of solar collectors. The thermal efficiency is often in the interval of 50-70%. These high values are comparable to the efficiency of conventional solar collectors with vacuum-gap thermal insulation, but the design of a DASC is much simpler.

However, currently-used particle-fluids are highly toxic [8], These particle-fluids can contaminate water supplies or pollute the environment in the case of a leak. In addition, fluids made with chemically-produced particles of metals, their oxides, and semiconductors are expensive. The average price of a commercial particle-fluid is about 2500 EUR/I [9] which is about 100 times more costly than conventional water-glycol mixtures.

The problem of particle-fluid toxicity has previously been addressed [12], They proposed compositions of an “eco-friendly” particle-fluid. However, the definition of environmental safety is not entirely correct in these works as they focused on using organic fluids to disperse the particles. Therefore, the problem of particle-fluid toxicity has not been solved for these particlefluids, and standard biological tests do not prove their environmental safety. Moreover, these particle-fluids were developed for applications other than direct absorption solar collectors. The Applicant has now developed a particle-fluid comprising particles from coffee suitable for use in DASCs.

Instant coffees comprise microparticles which are derived from ground roasted coffee beans. The Applicant has tested instant coffees from a wide range of geographical sources, and all of these instant coffees have been found to comprise insoluble microparticles which will have been derived from coffee beans. The microparticles were found to have size distributions which are primarily in the range of 1-10 pm. The Applicant tested these microparticles for use as solar radiation absorbers, but they were found not to be effective in solar collectors.

However, following sonication of the microparticles from solutions of instant coffee, nanoparticles in the size range of 30-300 nm were produced with enhanced solar absorption properties. Such nanoparticles are non-toxic and are biodegradable. Additionally, nanofluids comprising such nanoparticles were found to be stable for several months. In particular, a direct absorption solar collector (DASC) comprising nanoparticles of the invention was found to be 1 .3 times more efficient than a flat-plate solar collector.

Different methods to produce submicron particles of coffee have been suggested [16-17], Ball milling or high-pressure homogenization has been proposed to make dry particles of coffee with sizes 1 to 1000 nm. However, the nanoparticles produced in these methods were prepared for dispersion in a water-milk mixture to make a beverage.

Coffee-based colloids have previously been used in DASCs [15], In these latter studies, the light-absorbing heat transfer fluid was based on brewed coffee. These fluids are not suitable for use in DASCs because the colloids are laden with micro-sized particles that will sediment in a static condition and settle after thermal cycles. Moreover, no precise procedure has been developed for the accurate control of particle concentration in brewed coffee, nor have biotoxicity tests been conducted for the colloids.

It is an object of the invention therefore to provide novel environmentally-friendly, biodegradable and stable nanofluids and processes to make such nanofluids. It is another object of the invention to provide the use of such nanofluids for absorption of solar radiation in direct absorption solar collectors (DASCs) and as heat transfer fluids. In one embodiment, the invention provides a nanofluid comprising nanoparticles in a solution, wherein:

(a) at least 70% of the nanoparticles in the nanofluid have a size below 300 nm; and

(b) the nanoparticles are derived from or obtained from coffee.

Preferably, the coffee is instant coffee, ground roasted coffee beans or an aqueous solution thereof. Most preferably, the coffee is instant coffee, e.g. a sonicated aqueous solution of instant coffee.

The nanofluid is suitable for use as a solar radiation absorber.

In another embodiment, the invention provides a process for producing a nanofluid, the process comprising the steps:

(a) obtaining an aqueous solution comprising instant coffee;

(b) applying a fragmenting force to the aqueous solution; thereby producing a nanofluid comprising nanoparticles of the invention.

The invention also provides the use of a nanofluid of the invention as a solar absorber or as a heat-transfer liquid. In another embodiment, the invention provides a direct absorption solar collector (DASC) comprising a nanofluid of the invention.

In one embodiment, the invention provides a nanofluid comprising nanoparticles. The particles in the nanofluid preferably have a mean particle size in the range 30-300 nm. Preferably, the mean particle size is in the range 70-200 nm, and more preferably, the mean particle size is below 100 nm. Preferably, in a size distribution curve, the major peak of the particles in the nanofluid is between 70-90 nm, more preferably at about 80 nm.

In some embodiments, the nanofluid is substantially free from particles having a particle size of greater than 1000 nm, more preferably greater than 500 nm, and more preferably greater than 300 nm. Preferably, at least 70% of the particles in the nanofluid have a size of less than 300 nm. Preferably, at least 70% (more preferably at least 80%, 90%, or 95%) of the nanoparticles in the nanofluid have a size in the range of 30-300 nm.

The size of the nanoparticles may be determined by any suitable technique, e.g. laser diffraction or scanning/transmission electron microscopy of dried samples. Preferably, the size of the nanoparticles is measured by laser diffraction. The particle size is preferably determined on the basis of the longest dimension of the nanoparticle.

The concentration of the nanoparticles (e.g. in a DASC) is selected so as to make the optical depth of the system corresponding to the thickness of DASC. For practical purposes, the thickness is often within an interval from 1 to 2 cm.

The mass concentration is a ratio of the mass of the particles to the mass of the entire nanofluid. Preferably, the mass concentration of nanoparticles is from 0.01 % to 0.5% wt, e.g. from 0.01 % to 0.1 % wt, or from 0.1 % to 0.5% wt.

Light absorption by the nanofluids of the invention may be measured by the UV-VIS technique.

Another important property of the nanofluid of the invention is its extinction coefficient. This parameter demonstrates the degree to which the nanofluid absorbs thermal radiation. In some embodiments, the extinction coefficient is in the range 100-400 1/m. In other embodiments, the extinction coefficient is 400-700 1/m.

With regard to the stability of the nanofluid of the invention, preferably at least 70% of the nanoparticles in the nanofluid have a size of less than 300 nm after the nanofluid has been kept static at 21 °C for 2 weeks.

Preferably, the nanofluid of the invention has an EC50 of 5-7 %, based on a 24 hour exposure of Daphnia magna.

The nanoparticles are insoluble in aqueous solutions.

The nanoparticles are derived from or are obtained from coffee. As used herein, the term “derived from or are obtained from coffee” means that the nanoparticles are obtained or are obtainable from ground roasted coffee beans, (solid) instant coffee, or an aqueous solution of ground roasted coffee beans or an aqueous solution of instant coffee.

In particular, the nanoparticles are fragments of roasted coffee beans which are present in instant coffee or in aqueous solutions thereof, as an inherent result of the process for the production of all instant coffees from ground roasted coffee beans. Preferably, the nanoparticles are sonicated fragments of ground roasted coffee beans or sonicated instant coffee.

The nanofluid is a suspension of nanoparticles in a solution. The nanofluid comprises an aqueous solution. The aqueous solution is preferably water, more preferably distilled water.

The nanofluid may additionally comprise one or more of the following: (i) an agglomeration inhibitor; (ii) a defoaming agent; (iii) an antifreeze; and (iv) a fungicide.

One or more agglomeration inhibitors may be used to prevent or inhibit agglomeration of the nanoparticles, thus increasing the stability of the nanofluid and enhancing its usable life. Examples of suitable agglomeration inhibitors include sodium dodecyl sulphate (SDS), a surfactant (e.g. Triton™ X-100) and polyoxyethylenated sorbitan monooleate (e.g. Tween® 80). Preferably, the agglomeration inhibitor is SDS, e.g. 0.05-0.2% wt, preferably about 0.1 % wt SDS.

One or more defoaming agents may be used to prevent or eliminate the production of foam (e.g. from the use of SDS). Examples of suitable defoaming agents include polydimethylsiloxane (e.g. silicone oil) and commercial defoamers from Karcher®. Preferably, the defoaming agent is Karcher® Foam Stop (e.g. 0.3-0.5% wt, preferably about 0.4%).

One or more antifreezes may be used to prevent or inhibit the freezing of the nanofluid. Examples of suitable antifreezes include methanol, ethanol, ethylene glycol, and propylene glycol. Preferably, the antifreeze is ethylene glycol, e.g. 8-12% wt, preferably about 10% wt.

One or more fungicides may be used to prevent or inhibit the growth of fungi in the nanofluid. Examples of suitable fungicides include citronella oil and copper sulphate. Preferably, the fungicide is copper sulphate, e.g. 1.5-2.5 ppm, preferably about 2.0 ppm.

In yet other embodiments, the invention provides a process to produce a nanofluid of the invention. In particular, the invention provides a process for producing a nanofluid, the process comprising the steps:

(a) obtaining an aqueous solution comprising (dissolved) instant coffee; and

(b) applying a fragmenting force to the aqueous solution; thereby producing a nanofluid comprising nanoparticles of the invention. Step (a) comprises obtaining an aqueous solution of instant coffee, e.g. dissolving solid instant coffee in an aqueous solution.

Instant coffee comprises microparticles which are derived from ground roasted coffee beans. The Applicant has tested instant coffees from a wide range of geographical sources (see the Examples herein), and all of these instant coffees have been found to comprise microparticles which will have been derived from roasted coffee beans. Most of the microparticles were found to have size distributions which are primarily in the range of 1-10 pm.

Instant coffee is made by applying super-heated water to ground roasted coffee beans and then filtering out the coffee beans to produce a concentrated aqueous coffee solution. Instant coffee is produced by removing water from this concentrated aqueous coffee solution (e.g. by evaporation or freeze-drying). Microparticles of roasted coffee beans are, however, not removed by this process and hence they remain in the instant coffee.

In some embodiments, therefore, the instant coffee has been obtained by a process comprising the steps:

(i) grinding roasted coffee beans;

(ii) applying super-heated water to the ground roasted coffee beans;

(iii) filtering out the ground roasted coffee beans to produce a concentrated aqueous coffee solution; and

(iv) removing water from the concentrated aqueous coffee solution to produce instant coffee; wherein microparticles of roasted coffee beans are retained in the instant coffee.

The filtering step is one which does not remove microparticles (e.g. particles in the range of 1-10 pm) of ground roasted coffee beans from the aqueous coffee solution. Thus the microparticles of roasted coffee beans are retained in the solid instant coffee.

Preferably, the concentration of instant coffee in the aqueous solution (wt/vol) is 0.1 -5.0 %, more preferably less than 0.1 -2.0 %, and most preferably 0.1 -1.0 %.

The aqueous solution is preferably water, more preferably distilled water. Preferably, Step (a) is performed at a temperature which aids the dissolution of the instant coffee in the aqueous solution, e.g. 85-90°C. After Step (a), the aqueous solution is preferably allowed to cool, e.g. to 55-65°C.

Preferably, Step (a) includes the step of adding an agglomeration inhibitor (e.g. sodium dodecyl sulphate (SDS)) to the aqueous solution of instant coffee.

Step (b) comprises applying a fragmenting force to the aqueous solution. The aim of this step is to reduce the mean particle size of the microparticles in the aqueous solution, preferably to a range of 30-300 nm, and more preferably to the range 70-200 nm. The force is preferably a force which is sufficient to achieve this.

The fragmenting force may be any force which is suitable to achieve the fragmentation of the coffee microparticles into nanoparticles of the above-mentioned size ranges. Examples of such forces include sonication, high-pressure homogenization and laser ablation.

Preferably, the fragmenting force is sonication. Sonication is the process of disrupting or homogenizing something, usually a chemical solution or biological medium, with sound waves. Preferably, the sonication frequency is an ultrasound frequency, e.g. 20-25 kHz, more preferably about 22 kHz. Preferably, the sonication power is, for example 400-800 W, more preferably about 600W. Preferably, the intensity of the sonication is 200-300 W/cm ² , more preferably about 250 W/cm ² . Preferably, the sonication time is, for example, 10-30 minutes, more preferably about 20 minutes. In a particularly preferred embodiment, the sonication is at 600 W, 22 kHz; the intensity is 250 W/cm ² ; and the sonication procedure is 20 minutes long.

In some embodiments, the process comprises Step (c), which comprises:

(c) isolating particles from the aqueous solution which have a size distribution in the range 30-300 nm.

In some embodiments, the process comprises Step (d), which comprises:

(d) combining a nanofluid of the invention with one or more (preferably all) of the following:

(i) an agglomeration inhibitor;

(ii) a defoaming agent;

(iii) an antifreeze; and

(iv) a fungicide. The invention also provides a nanofluid obtained or obtainable by a process of the invention.

Preferably, the process steps are carried out in the order specified.

In yet a further embodiment, the invention provides the use of a nanofluid of the invention as a solar absorber or as a heat-transfer liquid.

In another embodiment, the invention provides a direct absorption solar collector (DASC) comprising a nanofluid of the invention. Preferably, the direct absorption solar collector comprises a fluid-filled chamber, wherein one side of the chamber comprises a transparent surface, and wherein the fluid is a nanofluid of the invention. The chamber may also comprise an inlet and an outlet, thus enabling the passage of the nanofluid into and out of the chamber.

In another embodiment, the invention provides a direct absorption solar collector (DASC).

The fundamental concept of a direct absorption solar collector (DASC) [4, 23-25] is straightforward: a dark fluid absorbs solar radiation. The fluid captures solar heat volumetrically, restricting the temperature of the receiving surface of the solar collector and reducing thermal losses to the surroundings. Moreover, promising advancements can be applied to DASCs by tailoring the lower surface of the fluid reservoir (e.g., implementing mirrors [24], selective absorption, and wettability modifications [25]). Direct absorption solar collectors are competitors to conventional solar collectors based on surface absorption.

The essential part of the DASC is the fluid responsible for capturing solar radiation. Initially, Indian ink was utilized in pioneering research on DASCs [23], However, this (and similar) fluid contains microscopic particles. During the operation of a solar collector, the working fluid undergoes multiple thermal cycles and static flow regimes. The microparticles can therefore settle, reducing the absorption of the DASC, and in some instances, the settled particles may obstruct the flow channels. More stable nanofluids [26] are presently used in DASCs.

Numerous laboratory studies [5, 24, 27] and small-scale field tests [4, 28-32] have been documented. These investigations involved nanofluids composed of metal oxide nanoparticles (e.g., iron, aluminium), gold, and carbon. Two configurations of transparent top surfaces were examined: tubular [4-5, 23, 27-28] and massive flat surfaces [24, 29-32], The maximum thermal efficiency of these collectors ranged from 46% to 87%. DASCs surpassed commercial flat-plate solar collectors, particularly those operating in Northern climates with limited insolation and high heat loss [33],

However, full-scale prototypes or commercial DASCs are not presently on the market. A significant obstacle to further technological advancements is the toxicity associated with nanoparticles. The acute toxic concentration (EC ₅ o Daphnia Magna) for nanofluids falls below 10-4% wt., significantly lower than that of conventional heat transfer fluids (e.g., approximately 6% wt.) [34], While some studies have employed organic particles [15] in DASCs, they were likely micron-sized, and their toxicity was not assessed.

Another challenge concerning DASCs is the need for a design methodology for thermal systems incorporating volumetric absorption. Material compatibility, thermal insulation, pumping costs, and seals for transparent systems have not been systematically explored and optimized within the existing body of research literature.

It is yet another object of the invention, therefore, to provide a DASC which overcomes one or more of the above-mentioned deficiencies.

In another embodiment, therefore, the invention provides a direct absorption solar collector (DASC) comprising a base plate, an assembly frame and a transparent plate, wherein the base plate, the assembly frame and the transparent plate form a collector cavity, wherein the DASC comprises one or more or all of the following features:

(a) the collector cavity is divided into a plurality of parallel channels;

(b) the DASC comprises inlet and outlet ports which are provided on opposite sides of the DASC;

(c) the transparent plate is plastic (preferably polycarbonate);

(d) the transparent plate and/or base plate are sealed to the assembly frame using stud bolts;

(e) the area of the transparent plate is 1.0 - 2.0 m ² .

As used herein, the term “base plate” refers to the plate which forms one boundary of the collector cavity and that is arranged opposite to the transparent plate. In some embodiments, the base plate may be reflective or non-reflective. In some embodiments, the base plate is preferably made from plastic (e.g. polyethylene or polycarbonate), aluminium or stainless steel. As used herein, the term “assembly frame” refers to the casing of the DASC. The assembly frame may be distinct from the base plate or formed as an integral part of it. The assembly frame is preferably made of polyethylene (PE) or other materials which are used for the base plate. In some embodiments, the assembly frame is at least 2 cm thick, preferably at least 2.5 cm, 3.0 cm, 3.5 cm, 4.0 cm, 4.5 cm or at least 5.0 cm thick. In some embodiments, the assembly frame is insulated, for example, to prevent thermal loss to the surroundings. The insulation may take the form of a foam, preferably a mineral foam (e.g. Jackofoam XPS®).

In some preferred embodiments, the thermal insulation of the assembly frame is achieved using about 3 cm of mineral foam (Jackofoam XPS®).

As used herein, the term “transparent plate” refers to the plate that is arranged to first contact and allow passage of the incident solar radiation. The term “top plate” may be used interchangeably herein with the term “transparent plate”. The transparent plate is preferably made of polycarbonate (PC or hardened glass). In some embodiments, the transparent plate has a thickness of at least 0.5cm, preferably at least 1 .0 cm, 1 .5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm or 4.0 cm. Most preferably, the transparent plate has a thickness of about 1 .0 cm.

In some embodiments, the DASC may further comprise a second transparent plate which is arranged parallel with the first transparent plate, wherein the two plates are non-coplanar. The two transparent plates are juxtaposed, with parallel planes. The second transparent plate may be made of the same material or a different material from the first transparent plate. Preferably, the second transparent plate is made from PC. In some embodiments, the second transparent plate is removable.

In some embodiments, the first and second transparent plates may be arranged such that an air gap is formed between said first and second transparent plates. Such an air gap may provide insulation to the plates. In some embodiments, the air gap is at least 0.25 cm, preferably at least 0.5 cm, 1.0 cm, 1 .5 cm or 2.0 cm. Most preferably, the air gap is about 1 .0 cm. In some embodiments, the air gap region between the first and second transparent plates may further comprise one or more ventilation channels to avoid condensation forming one or both of the transparent plates. Preferably, the surface area (i.e. solar radiation collection area) of the transparent plate is 1.0 - 2.0 m ² , more preferably about 1.6 m ² .

The base plate, the assembly frame and the transparent plate form a collector cavity which is sealed (apart from the inlet and outlet ports) in order to contain a collector fluid. As used herein, the term “collector cavity” refers to the region of the DASC that contains, retains and facilitates the translation of a collector fluid that stores solar radiation. In some preferred embodiments, the collector fluid is a nanofluid, preferably a nanofluid comprising nanoparticles from coffee, as described herein. In some embodiments, the flow rate of the collector fluid within the collector cavity is 1-10 l/min, preferably 5-10 l/min, most preferably about 5 l/min. In some embodiments, the flow rate is above 7.5l/min, e.g. 7.5-10 l/min.

Advantageously, the division of the collector cavity into a number of channels reduces the pressure drop compared to configurations without channels and prevents vortices forming at the inlet port of the collector. In particular, it has been found that dividing the collector cavity into three channels provides a particularly good thermal efficiency. In some embodiments, therefore, the collector cavity comprises a plurality of channels, preferably a plurality of parallel channels. Preferably, the channels are not connected in series. In some embodiments, the collector cavity comprises 2, 3, 4, 5, 6, or more channels. Most preferably, the collector cavity comprises 2-4, more preferably 3 channels. In some embodiments, the channels are parallel channels, thus facilitating parallel fluid flow. The channels are in liquid communication with each other. In some preferred embodiments, the collector cavity is divided into a plurality (e.g. 2, 3, 4, 5 or more) of parallel channels that run parallel to the length of the longest side of the DASC.

In some embodiments, the plurality of channels run parallel to each other and are equal in width and depth. Preferably, the channels each have a width of 10 cm - 50 cm, more preferably 20 cm - 40 cm and most preferably about 30 cm. Preferably, the channels have a depth of 0.5 cm - 5.0 cm, more preferably about 1 .0 cm.

In some embodiments, the channels may be separated from one another by one or more baffles, preferably rectangular baffles. Preferably, one or more of the rectangular baffles are about 0.5 cm, 1.0 cm, 1 .5 cm, 2.0 cm, 2.5 cm or 3.0 cm in width. Most preferably, one or more of the rectangular baffles are about 2.0 cm in width.

The DASC may further comprise one or more inlet and outlet ports. As used herein, the term “ports” refers to openings in the DASC that permit the entry of the collector fluid (i.e. via the inlet port) and the exit of collector fluid (i.e. via the outlet port). In some embodiments, the inlet and outlet ports are arranged on the same end or on the same side of the DASC. More preferably, the inlet and outlet ports are arranged on opposites ends or opposite sides of the DASC. The arrangement of the inlet and outlet ports on opposite ends or sides of the DASC advantageously serves to provide a natural convection that facilitates pumping of the collector fluid.

In use, the DASC of the present invention may be set up in any suitable arrangement known to the skilled person in order to collect solar radiation. In preferred embodiments, the DASC of the invention is mounted with a tilt angle of 40-50°, more preferably about 45°, to the sun.

In some embodiments, the transparent plate and/or base plate are sealed to the assembly frame using stud bolts. Preferably, each of the channels are individually sealed using stud bolts.

The disclosure of each reference set forth herein is specifically incorporated herein by reference in its entirety.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 . Schematic diagram of a direct absorption solar collector (DASC).

Figure 2. Particle size distributions at different stages of production in a 0.1 % wt. sample. (I) dissolution in hot water, (II) mixing with SDS, (III) ultrasonic treatment.

Figure 3. Particle size distribution for nanofluids produced of different sorts of instant coffee.

Figure 4. Extinction coefficient as a function of particle concentration.

Figure 5. Particle size distribution of the nanoparticles in a long-term stability assay of a nanofluid of the invention.

Figure 6. Relative extinction coefficient as a function of time (hours) for nanofluid versus coffee colloid.

Figure 7. Relative extinction coefficient for brewed coffee colloid and nanofluid.

Figure 8. Efficiency of nanofluid versus conventional nanofluid based on inorganic nanoparticles.

Figure 9. Streamlines of flow velocity in the midline cross-section of the DASC with a single flow channel (Figure 9A) and three flow channels (Figure 9B).

Figure 10. A DASC prototype of the invention (left) and connection scheme (right).

EXAMPLES

The present invention is further illustrated by the following Examples, in which parts and percentages are by weight and degrees are Celsius, unless otherwise stated. It should be understood that these Examples while indicating preferred embodiments of the invention, are given by way of illustration only. From the above discussion and these Examples, one skilled in the art can ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, various modifications of the invention in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims.

Example 1 : Production of nanofluid

A nanofluid was produced from instant coffee by cracking the particles of coffee with ultrasound. The best results were obtained using Nescafe Gull (available in Norway). It is estimated that the current (2022) price of a coffee-based nanofluid is in the range EUR 5-15 per litre; this is comparable to the price of a water-glycol heat transfer fluid.

To synthesize the nanofluid, we first dissolved a given mass of instant coffee in hot tap water at 85-90°C. The concentration of coffee powder was between 0.1% wt. and 2.0 % wt. Once the sample cooled down to 60°C, we added 0.1 % wt. of sodium dodecyl sulphate (SDS) and stirred the sample until the SDS dissolved.

The next step of production was the fragmentation of particles by ultrasound (US). For this purpose, we used an immersed US probe operating at 600 W, 22 kHz. The intensity of US- treatment was 250 W/cm ² . The sonication procedure was 20 minutes long.

Figure 2 illustrates the evolution of particle size in the sample at different production stages. The results were obtained using the laser diffraction method (Malvern Mastersizer 2000™).

At stage (I) of dissolution, there are two maxima of the particle size distribution function: a major peak at 2 pm and a secondary maximum at about 200 nm. The produced suspension is not stable as the average particle size is well above 500 nm. The particle size distribution does not become finer when the SDS is added at stage (II). In this case, we also observed a third peak at 80 pm. The last maximum is probably caused by a partial agglomeration of 2 pm particles due to a sudden change in the surface tension. The ultrasonic fragmentation of the particles at stage (III) shifts the particle size distribution to values below 100 nm. The major maximum becomes at approximately 80 nm. Example 2: Characterization of particle sizes from different coffees

The obtained particle sizes were similar for different coffee and regions where the powder was produced. Figure 3 shows particle size distribution for nanofluids produced using our method from different types of instant coffee purchased in Norway (N), the US (U), France (F), and Italy (I). It follows from the figure that most of the sonicated instant coffees produced a mean particle size in the interval 70-200 nm. This indicates that our method is applicable to multiple commercial types of instant coffee. In addition to the particle sizes presented in Figure 3, we did a trial test of a decaffeinated coffee produced in Norway. The average particle size was 90 nm for this measurement.

Example 3: Extinction coefficients of the nanofluids

Another important property of the nanofluid is the extinction coefficient. This parameter demonstrates the degree to which the fluid absorbs thermal radiation.

Figure 4 demonstrates how the extinction coefficient depends on the concentration of instant coffee. The dependence is non-linear for the concentration above 0.5% wt. The extinction coefficient increases from about 50 1/m for water up to 400-500 1/m for 2.0% nanofluid. The ultrasonic treatment deteriorates the optical absorption as it produces more nano-sized particles which absorb less in the infrared part of the spectrum. However, the stability of the nanofluid, i.e. the size of the particles, is the most important factor affecting the performance of DASC. The desired extinction coefficient may be set by using a US-treated nanofluid produced with a higher concentration of coffee. In Table 1 below, we demonstrate how the extinction coefficient depends on the sort of coffee at the concentration of 0.6% wt. As follows from Table 1 , the extinction coefficient is of the same order of magnitude for all the samples. The values are in the interval 145-280 1/m.

Table 1. Extinction coefficients in different coffee samples

Example 4: Long-term stability

The produced nanofluid was tested for its long-term stability. A freshly-characterized nanofluid sample was kept in a static state and normal conditions for 2 weeks. The particle size distribution was measured thereafter.

As shown in Figure 5, the minor extremum of the distribution function at about 800 nm was not present in the matured sample. This happened due to the gravitational settling of the non- Brownian particles. The rest of the sample remained stable with the sizes well under 100 nm.

Example 5: Toxicity of the nanofluids

Toxicity tests were carried out using a standard procedure (OECD 202 (2004), SOP K9 ID 17723) of the Norwegian Institute for Water Research (NIVA) [18], The first test aimed to determine the effective concentration (EC50) for 24-hour exposure of Daphnia magna. According to the test, EC50=6.0% for our nanofluid. This is comparable to the toxicity of other heat transfer fluids often used in conventional solar collectors. For example, EC50=5.6% for ethylene glycol [19],

The metal-based nanofluids that demonstrated excellent thermal performance in DASCs have far greater toxicity. EC50=1.4-10’ ⁴ % for the nanofluid with 148 nm silver nanoparticles, EC50=5.5' 10' ⁶ % 200 nm CuO nanoparticles [8],

In addition, NIVA conducted a toxicity test using Raphidocelis subcapitata [20], The 24-hour effective concentration was 4.8%.

Example 6: Stability of the nanofluids

A detailed study of the static stability of the nanofluid was conducted, considering the temperature history of optical absorption. The study was conducted using the UV-VIS technique. In this experiment, heavy agglomerated clusters of nanoparticles settled to the bottom of the nanofluid column, and the optical absorption was reduced. For the analysis, we produced a coffee-based nanofluid with an extinction coefficient of ~540 nr ¹ . This value was chosen to match the extinction coefficient with a referent coffee colloid we produced following methodology from Alberghini et al. [15], A standard optical cell was filled with freshly produced nanofluid and kept in a static condition inside the UV-VIS instrument for 48 hours. The extinction coefficient was measured episodically. The measurement results are presented in the Figure 6 in terms of a relative extinction coefficient given by the ratio of an instantaneous value of the coefficient to the initial value. The data in Figure 6 shows that the extinction coefficient does not significantly alter with time. The coffee-based nanofluid was stable and, therefore, capable of sustaining an uninterrupted operation of a direct absorption solar collector after shut-down periods.

The results for the referent coffee colloid are also given. As shown in Figure 6, the extinction coefficient for the reference colloid was reduced by about 30% within two days.

Example 7: Thermal cycles

We conducted a series of cycling experiments to clarify whether the coffee-based nanofluid is tolerant to dynamic thermal loads with peak temperatures close to the boiling point. For this purpose, we boiled 1 I of deionized water at standard conditions using a hot plate. A tall steel cuvette with a freshly produced nanofluid (extinction ~540 nr ¹ ) was placed in the volume of boiling water. The bottom of the cuvette was far from the wall contacting with the hot plate. The neck of the cuvette was 5 cm above the water level. An infrared thermometer controlled the temperature of the nanofluid. Once the temperature of the sample increased to 100 °C, the sample was kept in boiling water for 5 minutes. It was then removed and placed into the 1-1 bath with cold water at 12 °C until the sample and cold water were of equal temperature. Then the heating and cooling were repeated ten times. Using this procedure, we also studied a referent sample of a brewed coffee colloid made following Alberghini et al. [15], Finally, we conducted a UV-VIS analysis of the samples. The experimental results regarding the relative extinction coefficient are presented in Figure 7. The extinction coefficient was reduced by 2% for the coffee nanofluid, while for the reference coffee colloid, by 20%. This demonstrated that the thermal cycles destabilized the reference colloid but not the coffee-based nanofluid.

Example 8: Comparison of nanofluid with a conventional nanofluid based on inorganic nanoparticles

Additional experiments were carried out in a direct absorption solar collector (DASC), comparing the nanofluid with a conventional nanofluid based on inorganic nanoparticles. This experiment tested the DASC in field conditions measuring its efficiency according to [21], The conventional nanofluid was produced using carbon black particles (CB) from Timcal™. The size of the nanoparticles in a dry powder was 51 nm [22], The CB nanofluid was produced using a conventional two-step method when the carbon nanoparticles were dispersed in a watersurfactant mixture using ultrasound (600 W). To match the extinction coefficient of the CB nanofluid with the coffee-based nanofluid, the maximum concentration of the CB particles was 0.006 %wt. We used 0.1 %wt. polyvinylpyrrolidone (PVP) as a surfactant. SLS determined the resulting maximum size of agglomerated nanoparticles as 263 nm. The thermal efficiency of the DASC is presented in Figure 8. In this Figure, we compare the efficiency of the coffee-based experiments with the CB nanofluid. The coffee-based nanofluid demonstrated a performance equivalent to the cases with the CB nanofluid. However, we note the environmental compatibility and the reduced cost of the coffee-based nanofluid.

Example 9: DASC design

The collector’s top surface area (A) was set as 1 .6 x 1 .0 m (HxW). The volume flow of nanofluid in the collector was altered in the interval of 5 to 10 l/min. The thickness of the nanofluid layer within the DASC was determined by balancing the maximum collector mass and the pressure drop. The current regulations applicable in the construction sector (referenced as [35]) can be utilized to limit the collector's mass. The optimization of the pressure drop followed the methodology proposed by e.g. [36], These requirements were satisfied when the thickness of the layer reached approximately 1 cm. The cold nanofluid entered the DASC through a horizontal port at the bottom of the solar collector, while a corresponding port at the top of the collector facilitated the exit of the nanofluid.

The internal flow patterns in the DASC were determined using a computational fluid dynamics (CFD) model. The simulation approach was simplified, assuming a homogeneous nanofluid in the collector. Therefore, steady, laminar, and incompressible Navier-Stokes equations were formulated for the model:

According to Beer-Lambert's law, the last term in Equation 2 presents the volumetric absorption of solar heat [4],

The model was discretized by a trimmed mesher using the STAR-CCM+; the average size of the computational cells was 10 mm. The mesh was refined along the light-path setting the cell thickness to 0.5 mm. The equations were solved using the SIMPLE technique. Field tests

To compare the performance of the DASC to a referent commercial solar collector, we used the flat-plate solar collector KPG1 from Regulus. The surface area of the collectors was made equal. For this, 0.3 m ² of the reference collector area was blocked by a reflecting panel. The average ambient temperature in the tests was 15°C and an average insolation q _o =3OO W/m ² . These values were set as boundaries in the computational fluid dynamics (CFD) model.

CFD Optimization

We optimized the internal flow of the DASC using the CFD-model. Parallel channels along the height of the collector distributed the flow. 2-cm rectangular baffles separated the channels. Different geometries with one to five channels were evaluated in the model. The heat transfer coefficient to the environment was estimated as ~ 6 W/m ² K. The objective of the simulations was to minimize the pressure drop and to maximize the thermal efficiency of the collector [4],

T] = pQc _p &T/q ₀ A, (3) where AT is the temperature difference between the outlet and the inlet, and A is the top area of the DASC. The simulation results are presented in Figures 9A and 9B.

A large vortex was observed at the entrance and the bottom of the single-channel geometry. The vortex increased the residence time in the collector and then the temperature of the top surface. The respective enhancement of the thermal loss was detected. In Table 2, we present how the flow geometry influenced the thermal efficiency of the collectors. The table shows that the best efficiency was predicted for the three-channel geometry.

Table 2 Thermal efficiency for the different number of channels. K=800 nr ¹ , Q=2.4 l/min

Example 10: Production of prototype DASC

Prototype

A full-scale prototype of a direct absorption solar collector (DASC) was designed and manufactured in cooperation with Odda Plast AS (Norway). The collector was a polyethylene- made (PE) tray (940x1940 mm) with three rectangular channels (266x10 mm) separated by walls. The thickness of the collector’s bottom was 3 cm, and that of the side walls was 4 cm. The top wall of the channels, facing the sun, was made of a 1-cm thick transparent polycarbonate (PC), which covered the entire top surface of the collector. PC was sealed with rubber gaskets and high-temperature silicone around the perimeter and was pressed against the collector body by a PE frame held by bolts. The collector was thermally insulated on all sides with a 3-cm mineral foam (Jackofoam XPS®), and a second PC sheet was located on its front surface. The air gap between the first and second sheets was 1 cm thick and connected with the atmosphere for ventilation. The total mass of the collector was 80 kg, and the maximum operating overpressures and temperatures of nanofluid were 0.2 bar and 80 °C, respectively. Two similar DASCs were manufactured in total.

For experiments, the direct absorption solar collector was installed outdoors on a metal frame at an angle of 45 degrees to the horizon. A reference flat-plate solar collector (FPC) was installed next to it. Both collectors had independent circulation loops, including pumps, expansion tanks, and sensor systems. The FPC used tap water as a coolant. Both collectors were connected via heat exchangers to the hot water tank. This tank formed a secondary loop. The heat from this loop was discharged into the air in a separate standing greenhouse through convectors. More details about the hydraulic system of the solar field are presented in [33], The DASC prototype and connection scheme for the collectors are shown in Figure 10.

Performance

The work aimed to evaluate the thermal efficiency of the direct absorption collector in variable climate conditions. We also tested how the extinction of the nanofluid, its flow rate, and the thermal insulation of the front surface affect the collector efficiency. In the beginning, moderate foaming of the nanofluid due to using SDS was observed. However, it did not complicate the operation of the DASC prototype.

The results of the tests are presented in Figure 8. Here, we demonstrate how the collector efficiency changed with an increase in the temperature difference between the coolant and the environment. In this figure, the dotted line indicates the efficiency of the reference flat-plate solar collector, calculated following the manual. Our tests showed that the actual FPC efficiency was 5-7% lower depending on weather conditions. On average, its efficiency was about 65%.

The direct absorption solar collector showed an efficiency comparable to a flat-plate solar collector if the extinction of the nanofluid did not exceed 400 nr ¹ . However, an increase in extinction up to 700 nr ¹ led to an increase in collector efficiency by up to 87%. At the same time, the DASC remained, on average, 10-15% more efficient than the FPC. Also, we note that the nanofluid flow rate significantly affects the DASC efficiency. Setting the flow rate above 7.5 L/min boosted the DASC efficiency additionally by up to 10%. The thermal insulation of the front surface is also important. Despite the worse light transmission when using a second PC glass, its absence significantly increases heat losses. As a result, it led to a deterioration in DASC efficiency by 5-10%.

Conclusion

We developed a low-cost and environmentally-friendly nanofluid for the direct absorption solar collectors, optimized the design of the collector using the numerical model, and tested the prototype of the collector in the field. The field tests confirmed that the DASC is a viable technology for solar thermal applications. Surprisingly, we found, that the best efficiency of our collector was above 80%, the efficiency of the DASC was 15% higher efficient than the efficiency of a commercial flat-plate collector in equivalent climate conditions.

Nomenclature c _p specific heat [J/kg ■ K] q ₀ incident heat [ W/m ² ] g acceleration due to gravity [ m/s ² ] T temperature [K] K extinction coefficient [ nr ¹ ] u flow velocity [ m/s ] k thermal conductivity [ W/m K ] q thermal efficiency [ ]

/ lightpath [ m ] p dynamic viscosity [ Pa s ] p pressure [ Pa ] p density [ kg/m ³ ]

Q volume flow rate [ m ³ /s ] (p particle concentration [ ]

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