Energy harvesting with solar cells for wireless alarm nodes

Energiförbrukningen i elektronikprodukter minskar samtidigt som prestandan för solceller hela tiden ökar. Nya solcellstyper med lägre priser skapar nya möjligheter för använding även i små elektronikprodukter. Att samla energi från omgivningen, t.ex. från ljuset i ett rum, och använda det för att driva små elektronikprodukter kallas i ett vidare begrepp för ”energy harvesting” (SV: energiskördning). Moderna larmsystem består av trådlösa larmnoder, med olika energiförbrukning, som installeras på olika platser med olika ljusförhållanden. En vanligt förekommande trådlös larmnod är magnetkontakten, vilken ofta installeras i fönster och på dörrar. Fönsterläget är särskillt intressant eftersom där finns goda ljusförhållanden, vilket ökar potentialen i användning av solceller. Denna avhandling behandlar de tre delsystemen som typiskt ingår i ett ”energy harvesting”-system; solceller, batterier och kraftelektronik. Olika alternativ för de tre delsystemen övervägs och jämförs, med applikation för trådlösa larmnoder i åtanke. Litteraturstudier kompletteras med undersökningar av marknaden för att estimera vad de olika systemen kan kosta. Mätningar på tre typer av solceller görs både i hemmiljö och i labbmiljö för att estimera hur mycket energi som kan skördas i naturligt och artificiellt ljus. Slutligen demonstreras konceptet av att skörda energi till larmnoder genom att bygga en prototyp för magnetkontakten. Resultatet från undersökningen visar att det går att bygga ett ”energy-harvesting”-system på många sätt och att valet av lösning starkt beror på hur mycket lösningen får kosta. Funktionaliteten av prototypen demonstreras, men fortsatta tester krävs för att avgöra hur den beter sig året runt.The power consumption of electronic devices is decreasing at the same time as the performance of solar cells is increasing. Solar cells based on new technologies with lower costs are becoming commercially available, creating new opportunities for use in small electronic products. The concept of capturing ambient energy, such as light, and using it to power small electronic devices is known as energy harvesting. Modern home alarm systems consist of wireless nodes with different power requirements and which are placed in different locations with different light conditions. One common wireless alarm node is the magnet contact, which is often placed on windows or on doors. The window location is very interesting as it provides good conditions for harvesting energy with solar cells. This thesis investigates the three subsystems which are typically part of an energy harvesting system; solar cells, energy storage and power electronics. Different options for the three subsystems are considered and compared keeping the application in mind, harvesting energy for wireless alarm nodes. The literature studies are complemented by investigations of the market to get a rough estimate of the cost of each system. Measurements on three types of solar cells are made both in a home environment and in a lab environment to estimate the amount of energy that can be harvested in natural and artificial light. Finally, the concept of harvesting energy with solar cells for wireless alarm nodes is demonstrated by building a prototype for the magnet contact alarm node. The result from the investigation shows that there are many ways to design an energy harvesting system, and that the solution very much depends on the maximum acceptable cost. The functionality of the prototype is demonstrated, although further tests are required to assess the year-round behavior.Solar cells are becoming better and cheaper while the power consumption of small electronic products is decreasing. These advancements can be utilized for future home alarm systems to reduce or potentially eliminate the need for batteries. You may have noticed that pocket calculators sometimes have a dark rectangular window with a few vertical lines in it. These are solar cells which are used to harvest energy from the ambient light to reduce the need for batteries. Solar powered calculators have existed since the 1970s, and now wireless alarm systems powered by solar cells may be next in line. The combination of low power consumption and well-lit placement of window sensors makes it interesting to investigate if they can be powered by solar cells. When designing a system for harvesting energy using solar cells there are a number of decisions which need to be made. First of all you need to decide which type of solar cells to use. Silicon is the traditional material of which the majority of solar cells are based on. It can be treated in different ways to create either crystalline silicon or amorphous silicon. The latter is used to make cheaper solar cells at the cost of lower performance, the type that is traditionally found in pocket calculators. New types of solar cells, referred to as third generation solar cells, have recently started to appear commercially. Organic- and dyesensitized solar cells are two third generation solar cells which challenge amorphous silicon solar cells by also promising low costs. The next decision that needs to be made is which type of energy storage to use. For the alarm sensors to keep functioning during the night when there is no natural light, they need to store energy during the day or rely on a backup battery during the night. Two types of devices suitable for storing energy include supercapacitors and rechargeable batteries. Each type has its pros and cons that need to be considered with the specific alarm sensor in mind. The third decision which needs to be made is regarding the electrical solution. To extract maximum energy from the solar cells it is required that they work at the optimal voltage level at every instant. This can be achieved with a power converter controlled according to a MPPT scheme (maximum power point tracking). The choice is between using a MPPT chip to achieve a high efficiency but at an increased cost, or accepting that the solar cells will work at a lower efficiency maintaining a low cost. In my master thesis I investigate these subjects and apply them to wireless alarm sensors. My work also includes measurements made in both a home environment and in a lab environment to estimate the amount of energy that can be harvested in natural and artificial light. Finally I demonstrate the concept of a solar cell driven window alarm sensor by building a prototype