Currently in Sweden, municipal wastewater sludge is spread on land as fertilizer. But, a two-thirds reduction in this current method of dealing with sludge will be required if a newly proposed law is passed.
In the future, prompted by concerns about contamination from heavy metals and other pollutants, Swedish wastewater sludge will need to be dried, which requires heat.
A new solar heat application
This potential new industry use for solar heat sparked the interest of Isak Svensson, a new civil engineering graduate hired at the Swedish solar thermal firm Absolicon to work on water-related industrial solar heat applications, following a master’s in environmental engineering.
Svensson performed an initial cost-benefit analysis of drying sludge using concentrated solar to supply the hot air at a temperature of 140 C to dry it.
“This is kind of a new application for us,” he said. “We use water in the collectors and can produce steam, but you can also use heat exchangers to get hot air if you want that. It depends on the application, but in the sludge drying that we’re looking at now, we’ll use hot air. In my thesis project for Absolicon, that I presented at the SolarPACES conference, I looked at this application for a specific treatment plant and an economic analysis based on multiple factors, like putting a price on carbon dioxide greenhouse gas emissions, etc.”
The project consortium, that includes Huber Waste Water Solutions, a manufacturer of different wastewater treatment applications, is the recipient of $45,000 USD funding from the Swedish Innovation agency to assess renewable options for sludge drying – and to increase Swedish participation in Horizon Europe by making an application for an EU grant to build a real scale demonstrator to prove the concept.
Demo stage next for solar wastewater treatment
“Currently this project is early stage – our next step here is to conduct a more detailed feasibility study,” he said. “We’re doing this with Huber Waste Water Solutions. They have this belt dryer that we think has promising technical specs for integration with the solar collectors, so we’re going to conduct a feasibility study with them and then apply for another grant at Horizon Europe to build a real scale demonstrator, once we have the results of that feasibility study.“
In his master thesis Svensson compared three options to ensure year-round operation:100% biogas, 50/50 biogas and solar, and 100% solar – with an intermediate sludge storage. After doing the math for three alternatives, he determined that a solar-biogas hybrid would enable the wastewater drying to operate most economically around the clock.
“In Denmark, there’s been lots of development in heat storages where they actually build a really large pit in the ground and store hot water in it,” Svensson explained. “They can make this thermal storage so large that you can have seasonal thermal storage; actually utilize it during winter as well. You can store the heat for nighttime, but also the different seasons.”
The next step is to conduct a more detailed feasibility study to evaluate technical integration and optimize the economics and then develop a solar sludge drying demonstrator to show proof of concept in Sweden and then develop this turn key setup that production line partners can market in different parts of the world.
Absolicon operates the largest solar thermal installation using concentrating solar in Sweden to supply heat to the local district heating network.
They also provide solar installations for industry customers throughout Europe like breweries and chemical producers. Depending on the facilities and available installation area the solar fields can be installed on ground or in an elevated or rooftop installation, like the installation for Colgate Palmolive in Greece. Their full turn key systems for an onsite installation include collectors, pumps and control systems.
Learning from PV
One reason that solar PV grew rapidly is the technology lends itself to mass-production and diverse markets. Individual solar panels are small and scalable. Homeowners can install just a few panels for a roof, and developers can install multiple shipments of panels to build utility-scale solar farms. Online estimation tools are commercially available for end users.
Similarly, the Absolicon website includes an online estimating tool so individual industrial heat customers can get personalized savings estimates for ditching current heat sources and switching to solar heat.
Absolicon’s Field Simulator allows potential customers to input the position and size of their potential installation, and accounts for local DNI to produce an estimated cost and output: “The results are displayed in real-time and a summary is sent to your email so that you can study the calculations in peace and quiet.”
“In our example, we know that drying wastewater sludge requires about .8 kilowatt-hours of thermal energy per kilogram of water evaporated,”
“So if you know how much sludge you have, it’s quite easy to calculate how much energy you would need to evaporate it. Then depending on how dry you want it, it is quite easy to see how large a solar field you would require.”
A solar collector every six minutes
Absolicon appears to be the first solar thermal collector manufacturer to try to make the heat-generating form of solar as mass-produceable and as easy to order as PV.
The firm recently developed an automated assembly line for its parabolic trough solar collectors, working with robotics firm ABB, that has accelerated manufacturing time from three units a day to one every six minutes. The goal is to have production line partners in every country that can produce the collectors for these types of applications.
Absolicon estimates that by producing one solar collector every six minutes with ABB’s robotics, they can make their solar trough collectors cost-competitive with fossil energy for industrial heat. So Absolicon would be well-placed to be a global supplier of mass-produced solar collectors for this and many other industrial heat applications.