Smart Building Battery-Free Indoor Sensors Are Becoming Economically Interesting

Indoor multi-sensors for smart buildings quickly cost between 55 and 165 USD, which can quickly become expensive for large-scale applications. Ligna Energy from Sweden has developed QWEN, a battery-free sensor platform that costs around 10 USD for quantities of 10,000 to 50,000 units.

In times when energy guzzlers in particular are unnecessarily inefficient and cost-intensive, smart building applications are seen as one of the most realistic ways to achieve efficient and transparent building operation, for example in order to achieve sustainability goals. Reducing energy consumption through needs-based control using sensors and the associated reduction in costs can be a significant lever. Sensors for temperature, air quality, presence or light can be used to automatically activate heating, air conditioning and lighting only where they are actually needed. In office buildings with changing occupancy rates in particular, energy can be saved noticeably in this way.

A positive side effect is the condition monitoring of building technology, as networked sensors detect anomalies in ventilation, elevators or air conditioning systems at an early stage. Maintenance requirements can therefore be identified earlier. Failures can be announced and potentially prevented. Also on the plus side: smart buildings can increase comfort, for example through better air quality or dynamically adapted working environments. However, investing in large numbers of indoor multi-sensors can quickly run into money—for some companies, the smart building venture then becomes a showstopper.

When the Price is Hot

"End users told us that sensors are too expensive to use on a large scale," explains John Söderström, Marketing Director at Ligna Energy. "An indoor sensor can cost between 55 and 165 USD—just for the hardware. For a large-scale application, we need to come up with 10–15 USD per sensor." Ligna Energy, a Swedish provider of sustainable energy storage systems, has developed QWEN—a battery-free sensor platform for temperature and humidity.

Instead of a battery, QWEN uses a supercapacitor in combination with a low-power PV cell from Epishine, a manufacturer of printed solar cells. "We have tried to minimize the number of components," says Söderström in an interview at Embedded World 2026. The aim is to reduce both material costs (bill of materials) and housing costs by making the design thinner. A thin housing is cheaper to produce than a thick one.

Surprise in the Field Test

The first generation of QWEN was completed a year ago—shortly before Embedded World 2025—and has since been tested at various locations in Sweden. "None of them have had any downtime in a year," says Söderström. "And that surprised us ourselves." Sweden is relatively dark in winter and the offices are closed for several weeks in summer. "All these different phases of everyday office life—zero downtime."

Parallel to the internal test, a customer evaluated QWEN with a different system—different components, different protocol. "And now they have presented their data for a year, also with zero downtime." This customer is now planning to install between 10,000 and 50,000 sensors. "Their experience with indoor sensors of this size shows that the costs would be around 10 USD or less."

Energy Budget Calculation is Crucial

However, the battery-less approach requires a deeper understanding of the use case. "If you have a battery, as an engineer you add capacity to have a buffer—a safe space in case the environment is not as you thought it would be," explains Söderström. "If you don't have a battery, but a light cell and a supercapacitor, you have to understand how much light is available." How does the light vary over the course of a year—both daylight and artificial lighting? Is there any shading?

"You have to understand the environment you're going into," says Söderström. This also applies to batteries—manufacturers often state a battery life of five years, but in reality it is one to two years because the optimum environment is not given. With energy harvesting, however, you also have to use low-power components and minimize quiescent currents and leakage currents. "There are other things that draw energy from the architecture that you need to understand even more than if you have the safe capacity of a large battery."

QWEN is designed in such a way that it can bridge four to five days of darkness—such as a long weekend without lighting in the office. "But you have to understand the limitations of such a system," emphasizes Söderström. For safety reasons, some wireless protocols require a manual restart if a sensor is to go online again after a complete loss of power. "This is one of the challenges that engineers need to understand if they want to remove the battery completely."

Not Suitable for All Applications

Söderström emphasizes that battery-free sensors are not suitable for all applications. "It doesn't have many fancy features. You have to understand the disadvantages of not having a big battery." However, for simple data points such as temperature and humidity, which are widely used, there is a real benefit, he says. "In my estimation, we shouldn't have batteries for these types of sensors. There is no real need for continuous measurements every second. You can have potential downtime with these types of sensors."

However, batteries are necessary for other sensors. "I don't want to badmouth batteries. But for large-scale applications like this, you shouldn't have a lot of batteries that need to be replaced." The life cycle costs, lithium and other hazardous materials are problematic, he said. "If you just look at the IoT sensors already in use and how many batteries are in them—and then look five years ahead—it's a bit scary."

Ligna Energy sees the real market interest primarily in the area of transportation and asset tracking. "Moving objects or simply tracking something—logistics is an area where you track something." QWEN itself is stationary—for temperature and humidity at a fixed point. "But we have several projects in this application area. However, where we are furthest along in terms of actual signed contracts is more in the transportation sector."

QWEN is conceived as a reference design. "Of course, we are not the ones who are going to produce thousands of them. We are using it as a reference and something that someone else can adopt," says Söderström. (sb)

Link: More about QWEN

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