Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
PYRHELIOMETER
Document Type and Number:
WIPO Patent Application WO/2016/140567
Kind Code:
A1
Abstract:
The invention relates to a pyrheliometer (1) comprising a tubular housing (2) having at one end an entrance opening (6) covered by a front window (5), a heating member (7) situated near and thermally coupled to the front window and at a second end (8) of the housing a sensor (9). The sensor is thermally insulated from the front window in such a manner that a temperature difference between the sensor and the front window is at least 1°C for each W of heating power applied to the heating member (7) at ambient conditions in the absence of convective air flow. A sensor window (10) is situated in proximity to the sensor (9), and is thermally coupled to the sensor. The sensor window has a transmission of radiation below a wavelength of 4000 nm of at least 0.8, preferably between a wavelength of 200 and 4000 nm, while substantially blocking radiation above 4000 nm. The sensor is sensitive to radiation above as well as below 4000 nm.

Inventors:
VAN DEN BOS CORNELIS JAN (NL)
HOEKSEMA ERIC RICHARD (NL)
Application Number:
PCT/NL2016/050130
Publication Date:
September 09, 2016
Filing Date:
February 23, 2016
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
HUKSEFLUX HOLDING B V (NL)
International Classes:
G01J1/42; G01J5/04; G01J5/06; G01J5/08
Foreign References:
US3489008A1970-01-13
DE102011013975A12012-09-20
US20130135465A12013-05-30
Other References:
ANONYMOUS: "DR02 pyrheliometer | Hukseflux", 2 December 2014 (2014-12-02), XP055231420, Retrieved from the Internet [retrieved on 20151125]
ANONYMOUS: "DR01 pyrheliometer | Hukseflux", 2 December 2014 (2014-12-02), XP055231422, Retrieved from the Internet [retrieved on 20151125]
ROGER A. PAQUIN: "Materials for optical systems", OPTICAL ENGINEERING HANDBOOK, 1 January 1999 (1999-01-01), XP055231434, Retrieved from the Internet [retrieved on 20151126]
Attorney, Agent or Firm:
NEDERLANDSCH OCTROOIBUREAU (2502 LS The Hague, NL)
Download PDF:
Claims:
Claims

1. Pyrheliometer (1) comprising a tubular housing (2) having at one end an entrance opening (6) covered by a front window (5), a heating member (7) situated near and thermally coupled to the front window and at a second end (8) of the housing a thermo-electric sensor (9), wherein the sensor (9) is thermally insulated from the front window in such a manner that a temperature difference between the sensor and the front window is at least 1°C for each W of heating power applied to the heating member (7) at ambient conditions in the absence of convective air flow, a sensor window (10) being situated in proximity to the sensor (9), the sensor window (10) being thermally coupled to the sensor and having a transmission of radiation below a wavelength of 4000 nm of at least 0.8, preferably between a wavelength of 200 and 4000 nm, while substantially blocking radiation above 4000 nm, the sensor being sensitive to radiation above as well as below 4000 nm. 2. Pyrheliometer (1) according to claim 1, wherein the front window (5) has a thermal conductivity of at least 20 W/(m.K), preferably at least 30 W/(m.K).

3 Pyrheliometer (1) according to claim 2, wherein the front window (5) has a spectral transmission between 100 nm and 7000 nm, preferably between 200 nm and 6000 nm.

4. Pyrheliometer (1) according to any of the preceding claims, one of the windows (5, 10) comprising sapphire, the other window (10,5) comprising glass or quartz.

5. Pyrheliometer (1) according to any of the preceding claims, the heater (7) having a power of between 0.2 W and 15 W, preferably between 0.5 W and 4 W.

6. Pyrheliometer (1) according to any of the preceding claims, the housing (2) being made of metal, having a length of between 10 cm and 50 cm, and a diameter between 2 cm and 20 cm. 7. Pyrheliometer (1) according to any of the preceding claims, the sensor window (10) being with its edges (11) connected to an internal perimeter (12) of the housing (2).

8. Pyrheliometer (1) according to any of the preceding claims, wherein the window (10) comprises a diffusor with a transmission of at least 0.2..

Description:
Pyrheliometer

Field of the invention The invention relates to a pyrheliometer comprising a tubular housing having at one end an entrance opening covered by a front window.

Background of the invention Zero offsets (i.e. signals not related to the quantity that is to be measured) and the deposition of water on the front instrument window are important factors determining the reliability of measurements with pyrheliometers, which are radiation detectors for measuring direct beam solar irradiance. In traditional broad-band pyrheliometers, zero offsets are a source of measurement uncertainty. These offsets are linked to the use of "broad band" sensors, that are sensitive over a large wavelength range from the UV to the far-infra-red Pyrheliometers that comply with the ISO 9060 standard must have a flat spectral response in the 0.3 to 3 micron spectral range. They employ thermo-electric sensors. Using thermal-electric sensors (as opposed to photo- electric sensors), zero offsets, i.e. signals not related to the quantity to be measured, are a significant source of measurement uncertainty.

Reduction of zero offsets is useful because it improves measurement accuracy. The best known definition for zero offset, but not the only one, is the "zero offset" specification as defined by the ISO 9060 standard which classifies pyrheliometers. It is defined as the offset when heating or cooling the instrument with a fixed temperature rate of change of 5 K/hr. This temperature change produces internal temperature differences in the instrument. These differences not only cause far-infra-red radiation exchange but also generate energy flows to or from the sensor. Both mechanisms cause zero offsets, adding up to the total "zero offset".

Heating a pyrheliometer, for example using an electrical resistor close to the front window or heated ventilation are known to independently produce zero offsets by the same mechanisms. Offsets caused by heating are not specifically mentioned or defined in the ISO 9060 standard. In practice they are an integral part of the measurement, and therefore part of the measured "zero offset". For one instrument model there may be offset specifications with heating and without heating.

Deposited water on pyrheliometer windows leads to unpredictable and potentially very large but non-quantifiable errors; it reduces the "data availability". Deposition of rain and snow are quite common, but this usually goes together with cloudy conditions under which the measurement errors are small. Most pyrheliometers are located in moderate climate zones. Deposition of dew or frost on the window in the early morning regularly causes large errors. Water condenses on the window because at night the entire instrument cools down to a temperature below dew point by far-infra-red radiation exchange with the sky.

A pyrheliometer with water deposited on its window operates beyond its rated conditions. Prevention of deposition of water or fast removal of deposited water is useful because a dry window is the rated condition for a reliable measurement. A dry window also is unattractive for dust to stick to.

Lower zero offsets may be attained by improving thermal coupling between the thermal sensor, the instrument metal body and all elements in the field of view of the sensor such as apertures. Better thermal coupling results in smaller temperature differences between these parts and thereby reduces far-infra-red radiation exchange.

The part of the zero offset caused by energy flows to or from the sensor may be reduced by symmetrically coupling the sensor to the instrument body, or by using a sensor with a low heat capacity. Some sensors employ a so-called compensation element.

Another way to reduce zero offsets is by the use of "narrow band" pyrheliometers. These may employ detectors that are neither sensitive to far-infra-red radiation exchange (typically above 3000 nm), nor to thermal gradient offsets. US2013/0135465 gives an example. The detector recommended in this configuration is a based on silicon photodiodes, and is not sensitive above 1200 nm. Zero offsets and thermal coupling are no issue in such instruments. Although it claims to measure the same quantity, i.e. direct solar radiation, in fact this instrument type is no pyrheliometer according to the ISO standards, because that requires sensitivity from 300 nm up to higher wavelengths, typically up to 4000 nm. Zero offsets are caused by far infra red radiation exchange between the sensor and its environment, typically in the range above 4000 nm. Sensors of pyrheliometers that comply with the standard, sensitive between 1200 and 4000 nm, are usually also sensitive above 4000 nm, and thus sensitive to offsets. Heating a pyrheliometer window might help in preventing deposition of dew and frost. A heated window should have a temperature above dew point, so that moisture in the ambient air does not condense on it. In case water is deposited, heating accelerates evaporation of dew and rain, and promotes the process of sublimating or melting of snow and frost. To promote sublimation and melting, higher temperatures are beneficial. Melting requires a window temperature above 0 °C. The simplest option would be to directly heat a pyrheliometer, i.e. internally or with a heater connected to the instrument body, as opposed to externally via ventilation air. Using traditional pyrheliometers, already at low power levels, where heating is not yet effective to prevent humidity from condensing on the instrument window, the added zero offsets caused by direct heating are beyond the specification limits of the ISO 9060 standard.

In some cases direct heating is nevertheless used, for instance in pyrheliometer model DR01 as marketed by Hukseflux Thermal Sensors B.V. Delft, Netherlands, where it is typically switched on at night only when offsets do not matter because there is no sun. In the known pyrheliometer, a heater is connected to the instrument front aperture and window The zero offset caused by 0.5 W direct heating is 1 W/m 2 which is within the specification limits of ISO 9060 for the accuracy class. However, using traditional pyrheliometer designs, low power direct heating alone is not very effective against dew or frost deposition. Higher heating at 2 W is more effective, but leads to zero offsets of 4 W/m 2 , beyond the specification limits of the class. Applying direct heating at higher power, for example to promote evaporation and sublimation or to melt snow or ice is possible, but creates still larger errors and therefore is not mentioned in any standard. Following common practice with pyranometers, prevention of deposition of water and accelerated evaporation of precipitated water may be achieved by external ventilation of the pyrheliometer optionally in combination with indirect heating. The same external ventilation may have the added benefit that water droplets or snow are blown away. External ventilation has several disadvantages. Firstly, ventilation requires maintenance, for instance cleaning and replacing air filters. In many environments air filters need weekly maintenance for the system to remain effective. Secondly, the heating of ventilation air is relatively ineffective, requiring for example 13 W heating plus ventilation power for a total air temperature rise of the order of 1 °C. The high power consumption is a problem because pyrheliometers are often used at remote sites where power is not available. Also many outdoor applications only allow 12 V power supply for safety, so that power loss and voltage drop along the power supply cable becomes a problem.

Low power consumption is essential for use of pyranometers as these instruments are often applied in remote locations where mains power is not available.

It is an object of the present invention to provide a radiation detector, in particular a broadband pyrheliometer, in which zero offsets are reduced. It is a further object of the present invention to provide a radiation detector in which deposition of dew, ice or snow are reduced at a low power consumption.

Summary of the invention

Hereto a radiation detector according to the invention comprises a heating member situated near and thermally coupled to the front window and at a second end of the housing a thermoelectric sensor, wherein the sensor is thermally insulated from the front window in such a manner that a temperature difference between the sensor and the front window is at least 1°C for each W of heating power applied to the heating member at ambient conditions in the absence of convective air flow, a sensor window being situated in proximity to the sensor, the sensor window being thermally coupled to the sensor and having a transmission of radiation below a wavelength of 4000 nm of at least 0.8, preferably between a wavelength of 200 and 4000 nm, while substantially blocking radiation above 4000 nm, the sensor being sensitive to radiation above as well as below 4000 nm.

With the term 'thermo-electric sensor" as used herein, a sensor is intended having an absorbing sensor surface (which may be black). Radiation falling on the absorbing sensor surface causes heat flux though the sensor, which heat flux causes a temperature difference, which is converted into an electrical signal, for instance using a thermopile that is thermally coupled with the sensor surface.

The inventive aspect of the present invention involves predominantly heating the front window while thermally insulating it from the sensor area. This saves power, which is relevant for low power remote stations in which these instruments are often used. The zero offsets that would be caused by the temperature difference between front window and the sensor are compensated according to the invention by using an additional window near the sensor, allowing zero offsets to be reduced from 2 W/m 2 or higher to below 0.25W/m 2 or lower at 2 W heating power. he use of one or more sensor windows in addition to a known front window, located relatively close to the detector, acting as a far-infra-red radiation screen between sensor and the other parts of the instrument that would otherwise be in the field of view of the sensor, surprisingly allows direct heating the of the pyrheliometer front window while maintaining a low zero- offset error.

The use of the added sensor window suppresses the zero offset by creating a better thermal connection between the pyrheliometer sensor and all parts within its field of view. The inventors have realized that this improved performance allows direct heating of the pyrheliometer front window i.e. internally or with a heater connected to the instrument body, as opposed to externally via ventilation air while the keeping zero offset within the specification limits of the accuracy class or the user requirement.

Direct heating of a front window that is thermally insulated requires relatively little power compared to heating the entire instrument, and no significant maintenance.

In one embodiment of a pyrheliometeraccording to the invention, the front window has a thermal conductivity of at least 20 W/(m.K), preferably at least 30 W/(m.K). Use of a window having a high conductivity allows effective removal of moisture or water/ice from the front window at low power. As the detector window shields the detector from far- infrared radiation, the front window can be selected to have a relatively high conductivity. In a preferred embodiment of a pyrheliometer according to the invention, the front window has a spectral transmission between 100 nm and 7000 nm, preferably between 200 nm and 6000 nm.

For the front window as well as the added sensor window, materials can be used with a spectral range beyond that of a pyrheliometer, extending beyond 4 x 10 "6 m, but with a higher thermal conductivity provided that this is in combination with another window made of window material which limits the spectral range to the required range for the pyrheliometer. This enables use of still higher heating levels, while the zero offset remains within the specification limits of the accuracy class or the user requirement.

In a pyrheliometer, typical choices for sensor window materials are glass (transmitting from 0.3 to 3 x 10 "6 m approximately) and quartz (transmitting from 0.2 to 4 x 10 "6 m

approximately). A good optional front window material with an extended spectral range and higher thermal conductivity is sapphire (transmitting from 0.2 to 6 x 10 "6 m approximately). The sapphire has thermal conductivity of the order of 30 W/(m K), glass of 1.1 W/(m K), quartz of 1.4 W/(m K).

As an example: a typical pyrheliometer such as Hukseflux' model DR01 will at 0.5 W direct heating at the front window produce an offset of around 1 W/m 2 . This is still within the specification limits of ISO 9060 of 3 W/m 2 , but the heating is not effective against frost. Heating at 2 W, which is reasonably effective against frost deposition, produces an offset of 4 W/m 2 , which is considered too high not only when compared to the specification limits of the standard, but also by the user. According to the invention, a 2-window construction reduces any offsets by at least a factor 1.5 while the resulting offset is well within the specification limits of the standard even at higher heating power levels. Use of higher thermal conductivity materials such as sapphire if used for the sensor window again reduces the zero offset induced by heating by at approximately a factor 2, based on its thermal conductivity. If used for the front window, the use of sapphire potentially leads to lower power heating having the same effectivity.

The invention may be combined with direct or indirect heating of the instrument. Typically the heating will be limited to a specified maximum level of permissible heating, in W, at which it still performs within certain target zero offset limits, in W/m 2 , for example the maximum limits as specified by the user or in the classification system.

The invention may be combined with traditional features of pyrheliometers such as indirect heating and external ventilation. More windows may be added. Internal ventilation of the instrument may be applied.

The sensor window and front window may also perform a filtering function.

The sensor window may also be a reflector in which the radiation passes through a cover material which then acts as a window and radiation screen.

The invention may be combined with model-based zero offset corrections for example from analysis of temperature measurements in the instrument.

It may be combined with other measures to prevent zero offsets that are not included in the classification system, for example measures to reduce offsets induced by thermal shocks, such as increasing body weight or insulating the instrument body from contact with the ambient air or insulating the heated front window from the rest of the instrument. An embodiment of a radiation detector in accordance with the invention will by way of non- limiting example be explained in detail with reference to the accompanying sole drawing.

Brief description of the drawing

The drawing shows a perspective view of a directly heated pyrheliometer comprising a sensor window and a front window.

Detailed description of the invention

The figure shows a radiation detector in the form of a pyrheliometer 1 having a tubular metal housing 2. At an entrance side 3 of the housing 2, a front window 5 and aperture 4 cover the entrance opening 6. A direct heater 7, such as a strip-shaped electrical resistor, is in conductive contact with the front window 5 for heating the front window and prevention and/or removal of moisture or ice from the front window.

At a second end 8, a radiation sensor 9 is situated, shielded by sensor window 10. The sensor window 10 is situated within aperture 11 that is in contact with the internal perimeter 12 of the tubular housing 2. The window 10 has a relatively high transmission of radiation, preferably 0.8 or higher. The window 10 may comprise a diffusor, in which case the transmission is 0.2 or higher.

At a typical heating power of 2 W, which generally is still acceptable for low power measurement stations, the temperature rise of the second end 8 of the tubular housing 2 that includes the sensor 9 of the instrument is 3 °C relative to ambient temperature, while the front window 5, insulated from the main body 8 , rises by 6 °C. These figures apply to ambient conditions without convective air (zero m/s wind speed). The insulation between the instrument body 8 and heated front window hence amounts to 3 °C temperature difference per W heating power in stationary air. In general the thermal insulation between front window 5 and sensor 9 is greater than 1 °C / W.