There is great opportunity to develop power supplies for autonomous application on the small scale. For example, remote environmental sensors may be powered through the harvesting of ambient thermal energy and heating of a thermoelectric generator. This work investigates a small-scale (centimeters) solar thermal collector designed for this application. The absorber is coated with a unique selective coating and then studied in a low pressure environment to increase performance. A numerical model that is used to predict the performance of the collector plate is developed. This is validated based on benchtop testing of a fabricated collector plate in a low-pressure enclosure. Model results indicate that simulated solar input of about 800 W/m2 results in a collector plate temperature of 298 K in ambient conditions and up to 388 K in vacuum. The model also predicts the various losses in W/m2 K from the plate to the surroundings. Plate temperature is validated through the experimental work showing that the model is useful to the future design of these small-scale solar thermal energy collectors.
There is both need and opportunity to develop small-scale autonomous power supplies that can operate sensors or other devices that require power input for continuous operation. Examples include remote environmental sensors that can monitor water, air, or other critical conditions. Solar thermal power generation for small-scale devices is one means to achieve a low-cost solution that may be deployed to these remote locations.
In these solar thermal systems, maximizing absorption while limiting losses due to convection and radiation is critical in achieving high temperature values. Typically, these systems use a formal collector plate or absorber that is heated by the incoming solar energy. A selective absorber coating is used to enhance absorption and limit reradiation losses. In addition to this plate coating, additional steps that improve collector operation can include transparent thermal insulation. This reduces convection and conduction losses from the heated collector plate, while still allowing incoming solar energy to heat the absorber plate surface. This work examines thermal insulation in solar thermal applications.
In general, there are many types of insulation available on the market. The basic consideration in selecting any particular insulation material is to reduce the flow of heat from one point to the other. Specific parameters such as thermal conductivity, operating temperature, combustibility, chemical stability, mechanical strength and durability, and cost are also considered in the material selection process.
The thermal conductivity of traditional thermal insulation materials like mineral wool, expanded polystyrene (EPS), and extruded polystyrene (XPS) is in the range of 0.033 W/mK to 0.040 W/mK, while polyurethane (PUR) has thermal conductivity ranging from 0.020 W/mK to 0.030 W/mK [
Another growing insulation material is aerogel. One advantage of this material is its light weight (about 90% porosity). Further, silica aerogel granulate nanostructured material has been reported to have high solar transmittance and low thermal conductivity and is commercially available with thermal conductivity as low as 0.012 W/mK and thickness in the mm range [
Other techniques being actively explored for flat plate solar thermal applications include vacuum insulation panels [
In the absence of sufficient vacuum, there is the possibility to fill the volume above a solar collector plate with a low thermal conductivity gas such as argon (Ar), krypton (Kr), and xenon (Xe). This way the gas-filled panel enhances insulation qualities as gas conductivity is lowered. In both vacuum insulation and gas-filled panel, hermetic sealing is critical. For micro scale applications, there are many established vacuum packaging/encapsulation techniques with airtight seals [
This work investigates vacuum insulation technology with a small-scale flat plate solar thermal collector to reduce energy losses from the plate to a minimum. Specifically the effect of low pressure on the thermal performance of the collector plate is modeled and then validated through experimental effort. In final form, the assembled solar collector will capture incoming solar energy and provide power for autonomous sensors or other systems that may be sustainably operated. Figure
Solar collector system for environmental energy scavenging.
This section describes numerical and experimental investigations conducted to thermally characterize the performance of solar collector plates in low thermal conductive environments. First, the numerical methods that provide insight and expectations for the operation of these plates in a low pressure environment are presented. Second, the fabrication of the plates is reviewed.
This section reviews the approach and methods employed to produce a numerical model of the small-scale STC (solar thermal collector) within a low conductivity space. This begins with an overview of temperature in evacuated spaces and proceeds to describe the equations required to model an overall heat transfer coefficient associated with these environments. These equations formed the foundation for numerical results in this present work.
In an enclosed volume, such as flat plate collectors, the pressure (density) of the gas between the plate and cover glass (top transparent cover, Figure
The average diameter,
With an increase in
Heat transfer by molecular flow is different from boundary layer (continuum flow) regime where the temperature of the hot plate and the gas in contact with it is assumed to have same temperature values. In the continuum regime, the pressure is near atmospheric and so the mean free path is very small. Hence, heat energy is more easily transferred by collision of molecules. The Nusselt number is usually used in correlating heat transfer in the boundary layer regime [
A transition regime exits in which heat transfer is not exactly governed by molecular flow nor by continuum flow. This intermediate regime is further classified into slip and transition regimes [
Flow regimes versus Knudsen number.
Knudsen number | Pressure (Torr) | Flow regime |
---|---|---|
Kn < 0 . 001 |
|
Continuum flow |
0.001 < Kn < 0.1 | 0.09 < |
Slip flow |
0.1 < Kn < 10 | 0.0009 < |
Transitional flow |
10 < Kn |
|
Molecular flow |
As the pressure is lowered, natural convection within the enclosure is also lowered. The main source of heat transfer therefore becomes conduction (by gas molecules) and radiation. At a sufficiently low pressure (the molecular flow regime), natural convection is completely eliminated. The thermal conductivity of gas varies with temperature and pressure. Kaminski [
Overall,
Pressure versus thermal conductivity of air at different temperatures.
The useful energy harvested by a solar collector is determined by the ability of the surface to absorb incident radiation as well as the capacity of the body to limit long wavelength radiation from the surface. Further, the convection losses from the collector plate to the ambient air limit the overall useful energy gain. Equation (
Equation (
The temperature of the absorber element rises as radiation is absorbed. Hence, the temperature varies with time. However, to simplify this analysis, a steady-state (thermal equilibrium) condition is assumed. Hence, this section presents the steady-state temperature conditions for a solar collector element.
At thermal equilibrium, the energy absorbed by the collector is equal to that lost from the surface such that there is no net energy gain as shown in (
The ability to achieve high temperature values is critical for a system that relies on thermal energy as input. The goal is, then, to design a system that maximizes temperature gain from the sun’s heat energy. Typically, in a flat plate solar collector, there is no optical concentrating device. Hence, the area of the collector
If the collector’s overall heat loss coefficient is negligible (i.e.,
Figure
Steady-state temperature contour plot of a surface exposed to 750 W/m2 and ambient temperature
In this work, nickel-tin (Ni-Sn) coating has been selected as the collector absorber material. We have previously demonstrated this collector in small-scale application in ambient conditions [
Despite the negligible thermal losses predicted at low pressures, in real world applications, the amount of heat absorbed by the collector is reduced by those losses from the collector. The effect of losses on the collector plate can be specifically studied through the use of established collector plate models for overall heat loss effects. Results of this study are shown in Figure
Effect of overall heat transfer coefficient
If steady-state condition is assumed, the net useful heat gain absorbed by the collector is zero. Equation (
The overall heat loss coefficient,
An equation for
Further, the back-loss and edge-loss coefficients can be solved using (
The fabrication and testing of a solar selective absorber coating in atmospheric pressure conditions have been reported in prior published work. Fabrication of the collector plates that were tested as part of this effort is also reviewed and has been published in prior work [
Fabrication of the solar thermal collector plates began with selection of a copper plate to serve as the substrate. 200
Thickness of the black selective absorber coating was applied based on literature indicating effective thicknesses in the range of 100 to 200 nm [
Copper solar collector plate with intermediate 10 pm Ni layer (left) and final nickel-tin selective coating (right).
This section describes tests conducted to thermally characterize the performance of collector plates in low thermally conductive environments. Two tests were conducted. The first was conducted under atmospheric conditions similar to previously reported tests [
A halogen lamp was used to simulate solar radiation in this experiment. The simulator lamp used was a Sun System R SS-2 MH 400 W lamp. All tests were conducted by exposing the collector plate to incident radiation from the lamp. Like the sun, the intensity of the radiation from the lamp varied with distance. As the distance from the lamp increased, the intensity of output decreased. A Hukseflux SR11 pyranometer was used to validate the intensity of the radiation at various distances from the simulator lamp. This information was used to select an appropriate distance from the lamp which simulated flux density closely approximating real world availability. A
An intensity of approximately 796 W/m2 was selected. The pyranometer was used to determine the required distance from the simulator lamp. The experiment was placed in a vacuum chamber to minimize heat losses during experimentation. The vacuum chamber was cast and designed specifically for these tests of small-scale collectors. A highly transmissive glass window provided top cover to the chamber. The transmissivity of the chamber glass cover was verified by passing the simulated solar radiation through the lid and measuring its flux density via Pyranometer. The result showed that the glass had a transmissivity of 0.98. This amounted to a radiation intensity of approximately 780 W/m2 reaching sample surfaces inside the chamber. Other features of the chamber included different CF style bulkhead fittings that allowed pass-through of wires as needed to fully operate and characterize the collector plates in the altered environment.
A fiberglass material (Garolite G-7 from McMaster-Carr) was used as a frame to support the collector plate at the center of the chamber. The frame was suspended as shown in Figure
Vacuum chamber with suspended fiberglass frame for collector plate mounting.
The temperature of the collector plate and chamber environment when exposed to incident radiation were monitored using thermocouples (TCs) (SA1-k-120 from Omega Engineering). The first TC was placed on the back side of the collector plate. This monitored the plate temperature during operation. The second TC was suspended within the chamber to directly monitor the temperature within the chamber. A thermocouple feedthrough (TFT3KY00008B from Lesker, USA) was used to fit the TCs through the vacuum chamber wall. This was useful in maintaining the isolated environment. National Instruments LabVIEW (using a cDAQ-9174 data logger) was used to record all TC data to a computer for analysis. Figure
Top-down view of collector plate (STC) mounted in a vacuum chamber ready for testing.
With collector plate mounted within the vacuum chamber and TCs connected, a vacuum pump was connected with pressure sensor to vary the working condition of the collector environment. This readied the plates for testing.
First, results from the numerical analysis are presented. Figure
It can be seen from Figure
Small-scale solar collector specification variables.
Variable | Range |
---|---|
Ambient temperature, |
298 K |
Absorber plate temperature, |
363–388 K |
Absorber plate emittance, |
0.1 |
Glass cover emittance, |
0.90 |
Collector tilt angle, |
0° |
Collector length, |
0.04 m |
Collector width, |
0.04 m |
Number of cover, |
1 |
Insulation material | Vacuum |
Wind heat transfer coefficient, |
1–10 W/m2 K |
Top loss coefficient, |
1.68–4.40 W/m2 K |
Total loss coefficient, |
3.10–7.86 W/m2 K |
This section presents results of experimental tests conducted to validate the numerical analysis. Temperature of the collector plates was monitored when exposed to radiation flux as described in Section
Figure
Collector plate operating temperature at different atmospheric pressures.
To further confirm that temperature gain recorded under partial vacuum conditions was primarily due to space evacuation, air molecules were reintroduced into the chamber after steady-state conditions were reached. The vacuum release points are as shown in Figure
Small-scale energy scavenging through solar thermal application has great potential to provide power to a variety of sensors or other remote devices that may be fully autonomous in their operation. In this effort, a small-scale solar thermal collector (STC) is fabricated and tested in a simulated low vacuum environment. In parallel, a numerical model is developed that can be utilized to design and predict operation of STCs of varying sizes and operating environments.
The STC itself was fabricated using a copper substrate and tin-nickel coating to form a selective surface. Tests were conducted to verify the operation temperature of collector plates when exposed to simulated solar radiation. Results showed an improvement in the stagnation temperature of the collector plate when operated in a partial vacuum environment (715 mTorr, 95.3 Pa) compared to results obtained under atmospheric pressure. Total increase was about 16% for a collector plate with surface area of 40 by 40 mm. The model proved a useful tool for future development efforts based on comparison to the experimental results.
Continuing and future work will examine the temperature of the plate under higher vacuum conditions. Effort is also underway to examine the effect of other operation environments with low thermal conductivity gases like argon, xenon and krypton. This way, rather than maintaining a vacuum environment, other gases (with thermal conductivity lower than air) may be utilized to replace air molecules within the operation environment.
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the support of this work by the NSF via Grant no. ECCS-1053729.