Technical Insight
Simplifying air quality monitoring
Conventional sensors for assessing the air quality in buildings are wired into the mains, making them expensive to install and reposition. But portability and battery powering is possible with Gas Sensing Solutions� modules that feature mid-infrared LEDs and photodetectors. Richard Stevenson reports.
We're not always highly productive at work. Instead, despite our best efforts, we fall short. We might blame a poor night's sleep, a lunch that was a little heavier than it should have been, or maybe, just maybe, a stuffy office that causes us to yawn and fail to maintain our concentration.
If the latter is to blame for sub-optimal productivity, help is on its way. In several countries, legislation is being introduced to set minimum standards for air quality at work, while in the UK, similar requirements will be set for schools.
In many workplaces, it is the operation of the heating, air-conditioning and ventilation system that determines air quality. To prevent fatigue-inducing carbon dioxide levels from getting too high, air is pulled in from outside to increase the proportion of oxygen within the building.
It is possible to always draw in enough air from outside to deliver sufficient air quality. But that's expensive: Air taken from outside often has to be heated, so it is better for the amount of fresh air that is introduced to track the level of CO2 in the building, which tallies with its occupancy. Do that and it is possible to trim the cost of air conditioning by up to 25 percent.
To realise these significant savings, a network of sensors have to be deployed to monitor carbon dioxide levels throughout the building. The expense associated with this is not only governed by the price of the sensors, but also their cost of installation. Traditionally, sensors have to be hard-wired to mains electricity, and if the layout of the building is altered "“ not an uncommon occurrence in offices "“ wiring must be stripped out before the sensors are repositioned and re-wired.
Superior sensing
Fortunately, there is more sensible way for monitoring the air quality. Recently, Scottish start-up Gas Sensing Solutions (GSS) launched the world's first CO2 sensor that does not have to be hooked up to the mains, but can run off AA batteries. Running off just one of them, this type of sensor, which incorporates an antimonide-based light emitter and photodetector, can take a reading every 2 minutes for 10 years thanks to its vastly superior efficiency over the incumbent technology.
The conventional sensors, which are under threat from those produced by GSS, typically combine a tungsten filament lamp for a light source with a pyroelectric or thermopile detector. The bulb, which consumes 100 mW or more, provides a broadband source that excites a CO2 asymmetric stretching vibrational mode. By monitoring the amount of light absorbed by this mode that occurs at 4.26 �m, it is possible to determine the CO2 level in the air.
GSS' sensor works on the same operating principal. However, the LED is far more frugal than the bulb, drawing just 3.3 mW, and this solid-state source takes far less time to deliver a stable output.
"With a light bulb, you might have to wait a minute for it to stabilise, but with an LED it stabilises almost instantaneously," explains the CEO of GSS, Des Gibson. Slashing the stabilisation time holds the key to the vast improvement in the key figure of merit, the energy per measurement. "In our case, it's millijoules, whereas in a standard thermal source, it's hundreds-to-thousands of millijoules. So it's a radical improvement."
The tremendous energy saving is not the only area where the antimonide-based device outscores the incumbent. The established CO2 sensor requires optical filtering, but thanks to the relatively narrow emission and detection profiles of the emitter and detector, the GSS design is filter-free. "This reduces cost, because these filters can be expensive," explains Gibson, who adds that they are also restrictive, requiring parallel light to work effectively. "We can use a neat, folded design that wouldn't necessarily be amenable to a filter-based approach."
Multiple markets
In addition to the sensor market for monitoring air quality in buildings, which is valued at �100 million, GSS is targeting other markets. They include the automotive sector that could be worth even more. "Within cars, CO2 monitoring is coming in as an anti-drowsiness system," says Gibson. "That's linked to the air-conditioning system: It will automatically control the in-cabin CO2 level to a point where the driver isn't going to fall asleep."
Opportunities also exist for CO2 sensors in horticulture. It is possible to accelerate plant growth by up to 40 percent by optimising CO2 concentration, humidity, temperature and light level. What's more, CO2 sensors can help divers stay underwater for longer by switching to re-breathing systems. They were first used in the military by Special Forces, but are now being used for recreation. "You need to be able to monitor CO2 very accurately, because it's a chemical scrubber system, so you need to make sure that you are removing the CO2 as you recycle," explains Gibson, who reveals that GSS sells quite extensively into that market.
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The core technology of GSS is the combination of mid-infrared LED and photodiode
Sensors for all these applications are assembled at the company's facilities in Cumbernauld, a few miles northeast of Glasgow. But production begins at GSS' facility at the West of Scotland Science Park, Glasgow, where III-V epiwafers are grown in a Veeco GEN 3 MBE reactor on 4-inch GaAs substrates, before they are dispatched to a local foundry, Compound Semiconductor Global, where they are processed to GSS specifications. Another subcontractor then dices these wafers, mounts LED and photodiode chips on bridgeboards and wire bonds them, before shipping these assemblies back to GSS. Here, all the components are brought together "“ including injection-molded plastic optics from China "“ and a portfolio of sensors are put together (see box, "The GSS range of sensors" for details of various products).
"Another piece of intellectual property is the calibration process," explains Gibson. "We calibrate the assembly for temperature and CO2. We 100 percent test and we download the calibration data into the firmware, so each sensor has its own unique calibration."
Getting going
Following its founding in 2006 with funding from by�Tweed Renaissance Investment Capital�and�The Scottish Co-Investment Fund, GSS worked with various partners to develop its product. The UK has a very strong track record in III-V mid-infrared material research and development, and GSS tapped into this, quickly establishing ties with several universities. This includes a strong link with Glasgow University, which has supported efforts in device design and fabrication.
Initially, GSS outsourced the epitaxial process for making mid-infrared LEDs and photodiodes. However, the partner could not ramp up the supply of these wafers to a high enough level to cope with high-volume product manufacture, so the Scottish start-up went in search of an alternative supplier.
This failed. "If someone else had been producing mid-infrared LED-photodiodes, we would have happily brought them from them, but they weren't. So we were left in a position where we had to establish our own development and production capability," explains Gibson.
As well as having to find the funds to finance the introduction of high-volume production implementation, GSS had to bring in the epitaxial expertise to grow the epiwafers. They did this via a knowledge transfer partnership with Glasgow University, who provided an MBE grower. This know-how has now been transferred to GSS.
Moving to in-house epiwafer production appears to have been a good move for the company, because it can continue to develop its devices with fewer restrictions. Although there are no plans to replace the LED with a laser "“ it costs more to make and emits a far narrower bandwidth, so expensive hardware is needed to stabilise the cavities "“ the Scottish start-up is involved in a project funded by the UK's Technology Strategy Board (TSB) to investigate the potential of other routes to increased device efficiencies.
� The COZIR sensors produced by GSS are a low-power product. The 20 mm path length version can detect CO2 levels of 0.2 percent to 100 percent, while the 70 mm version monitors CO2 levels up to 1 percent
Second string
The LEDs and photodiodes made at GSS have dimensions of 1.7 mm by 1.5 mm, and sensor shipments currently are at about 50,000 per year. This figure is well short of the capacity of the MBE tool, which is capable of producing 1.1 million LEDs and photodiodes per annum. So, to address this reactor redundancy, last November Gibson and his co-workers set-up Quantum Device Solutions, which is a trading name within GSS. �
"We have a unique capability in terms of mid-infrared III-V material combinations, and we are finding that there are lots of opportunities to sell on the worldwide market. It's almost now at a level where it pays for the running of the machine," explains Gibson.
The time that is available for fulfilling these external orders will steadily reduce as production of the portfolio of GSS sensors increases over the next few years. "We're now ramping into the hundreds of thousands per year, and ultimately this market will go into the low millions," explains Gibson.
One of the companies that GSS is shipping to is Schneider Electric Industries, the biggest operator in building control systems. The two firms have worked together to develop a specific sensor, which Schneider uses with its own electronics.
Second string
The LEDs and photodiodes made at GSS have dimensions of 1.7 mm by 1.5 mm, and sensor shipments currently are at about 50,000 per year. This figure is well short of the capacity of the MBE tool, which is capable of producing 1.1 million LEDs and photodiodes per annum. So, to address this reactor redundancy, last November Gibson and his co-workers set-up Quantum Device Solutions, which is a trading name within GSS. �
"We have a unique capability in terms of mid-infrared III-V material combinations, and we are finding that there are lots of opportunities to sell on the worldwide market. It's almost now at a level where it pays for the running of the machine," explains Gibson.
The time that is available for fulfilling these external orders will steadily reduce as production of the portfolio of GSS sensors increases over the next few years. "We're now ramping into the hundreds of thousands per year, and ultimately this market will go into the low millions," explains Gibson.
One of the companies that GSS is shipping to is Schneider Electric Industries, the biggest operator in building control systems. The two firms have worked together to develop a specific sensor, which Schneider uses with its own electronics.
� A Veeco MBE reactor is used to produce photodiodes and sensors at GSS, and provide worldwide epiwafer services
This additional revenue stream that results from shipping III-V devices, rather than sensing modules, looks set to grow. "We are seeing a lot of interest in people purely buying LED-photodiodes from us in extremely high volumes," says Gibson, who reveals that there are firms that are keen to use GSS chips in their own automotive air-conditioning systems. This approach could benefit both parties: The air-conditioning unit maker does not want to have to acquire the expertise in growing mid-infrared III-Vs, while GCS does not wish to take on the liability associated with a safety-critical sensor for anti-drowsiness. "Second-tier suppliers are well set-up to handle those liabilities," claims Gibson.
GSS is also working to make the sensor technology less dependent on batteries, which can degrade with time. Alternative approaches include using a photovoltaic cell to charge a capacitor or a battery. En Ocean Alliance, a Siemens spinout, is already marketing a GSS sensor powered with a silicon solar cell, and efforts are underway to provide power from other types of photovoltaic device.
"Silicon doesn't work very well in diffuse lighting," says Gibson. "Once you drop below 100 lux, it just stops working." Dye-sensitized solar cells and organic photovoltaics don't suffer from the same fate,and latter class of device is being developed for a GSS sensor in a TSB programme involving the London start-up Solar Press.
Yet another area of development being pursued by GSS is the development of sensors for other gases. The company's mid-infrared LED and photodiode technology can make devices operating from 2.5 �m to 6.0 �m, a spectral range where many gases have absorption features. They include hydrocarbons, such as methane, an explosive gas that GSS is making a sensor for. Such a sensor can improve safety in the oil and gas industries and landfill sites, thanks to its very low operating voltage that prevents the generation of explosion-triggering sparks.
There are clearly many lucrative opportunities for GSS. So, if it can get the balance right between making sensors, supplying its chips to other sensor makers, and maintaining its epiwafer services business, it promises to be a big success story for Scotland.
This additional revenue stream that results from shipping III-V devices, rather than sensing modules, looks set to grow. "We are seeing a lot of interest in people purely buying LED-photodiodes from us in extremely high volumes," says Gibson, who reveals that there are firms that are keen to use GSS chips in their own automotive air-conditioning systems. This approach could benefit both parties: The air-conditioning unit maker does not want to have to acquire the expertise in growing mid-infrared III-Vs, while GCS does not wish to take on the liability associated with a safety-critical sensor for anti-drowsiness. "Second-tier suppliers are well set-up to handle those liabilities," claims Gibson.
GSS is also working to make the sensor technology less dependent on batteries, which can degrade with time. Alternative approaches include using a photovoltaic cell to charge a capacitor or a battery. En Ocean Alliance, a Siemens spinout, is already marketing a GSS sensor powered with a silicon solar cell, and efforts are underway to provide power from other types of photovoltaic device.
"Silicon doesn't work very well in diffuse lighting," says Gibson. "Once you drop below 100 lux, it just stops working." Dye-sensitized solar cells and organic photovoltaics don't suffer from the same fate,and latter class of device is being developed for a GSS sensor in a TSB programme involving the London start-up Solar Press.
Yet another area of development being pursued by GSS is the development of sensors for other gases. The company's mid-infrared LED and photodiode technology can make devices operating from 2.5 �m to 6.0 �m, a spectral range where many gases have absorption features. They include hydrocarbons, such as methane, an explosive gas that GSS is making a sensor for. Such a sensor can improve safety in the oil and gas industries and landfill sites, thanks to its very low operating voltage that prevents the generation of explosion-triggering sparks.
There are clearly many lucrative opportunities for GSS. So, if it can get the balance right between making sensors, supplying its chips to other sensor makers, and maintaining its epiwafer services business, it promises to be a big success story for Scotland.
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The Sprint IR is capable of 20 readings per second
The GSS range of sensors
The portfolio of GSS sensors can be divided into�three classes:
COZIR,the low-power product. This is available with a 20 mm and 70 mm path length. The former targets industrial safety and medical markets and measures CO2 levels between 0.2 percent and 100 percent, while the version with the 70 mm path length is suitable for building control and horticultural markets, and monitors CO2 concentrations up to�1 percent.
Sprint IR,which is capable of 20 readings per second and can measure CO2 levels ranging from either: 0-5 percent, 0-20 percent, 0-60 percent or 0-100 percent.
MISIR,a lower-cost product with alternative optics. This sensor requires more power, and can be wired into the mains. Its benefit over the incumbent sensor technology is greater longevity, which stems from the use of III-V emitters and detectors.
The GSS range of sensors
The portfolio of GSS sensors can be divided into�three classes:
COZIR,the low-power product. This is available with a 20 mm and 70 mm path length. The former targets industrial safety and medical markets and measures CO2 levels between 0.2 percent and 100 percent, while the version with the 70 mm path length is suitable for building control and horticultural markets, and monitors CO2 concentrations up to�1 percent.
Sprint IR,which is capable of 20 readings per second and can measure CO2 levels ranging from either: 0-5 percent, 0-20 percent, 0-60 percent or 0-100 percent.
MISIR,a lower-cost product with alternative optics. This sensor requires more power, and can be wired into the mains. Its benefit over the incumbent sensor technology is greater longevity, which stems from the use of III-V emitters and detectors.
