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From Scandinavia to St. Paul: the St. Paul RiverCentre Solar Thermal plant

Introduction: Warming a city

From the roof of the convention center in downtown St. Paul, Minnesota, one is surrounded by the irregular forms of the downtown buildings that comprise the city's center. On sunny days, long rows of solar thermal collectors on the St. Paul RiverCentre's roof supply water to those very buildings, in a system that encompasses the majority of the downtown area.

Source: District Energy St. Paul
Source: District Energy St. Paul

In March 2011, workers put the final touches on this 1,950 square meter solar thermal system. At 1 MW-thermal this comprises the largest solar thermal system in the Upper Midwest, and one of the largest in the United States. All in all, it is comprised of 100 metric tons of steel and 490 meters of pipe.

The system is the work of District Energy St. Paul (St. Paul, Minnesota, U.S.), the utility which operates the water and space heating system, the U.S. Department of Energy's Solar American Cities program and the St. Paul RiverCentre, and was meant from the beginning to be a showcase of what solar thermal can do on a large scale.

But for District Energy St. Paul, it is also a practical solution to assist with the heating needs of the city while offsetting fossil fuel and biomass generation.


The Solar America Cities opportunity

District Energy St. Paul Project Manager Nina Axelson states that the project was something that District Energy had been considering for some time.

Underground piping carrying heat from District Energy St. Paul's generation to the Minnesota State Capitol; Image source: District Energy St. Paul
Underground piping carrying heat from District Energy St. Paul's generation to the Minnesota State Capitol; Image source: District Energy St. Paul

"We knew that technically it was beyond feasible, it was a very viable option," states Axelson. "We had been hoping for that, for that sort of opportunity that would allow us to move a project of this scale forward here in the United States."

The opportunity came in the form of a USD 1 million grant from the U.S. Department of Energy through the Solar America Cities program. Just as District Energy St. Paul had been looking to implement a project like this, the Solar America Cities program was looking for a location where they could demonstrate the viability of the technology on a large scale.

District Energy St. Paul offered that scale. Their operations include a combined heat and power plant, powered primarily by biomass and fossil fuel generation, totaling 289 MW-thermal. The system, dating from the 1970's, supplies hot water to 80% of the city's downtown area.

Looking to Scandinavia

Minnesota is a state settled largely by Scandinavian immigrants, and retains cultural connections to Scandinavia - including District Energy St. Paul, which notes that its interest in solar thermal comes from looking at Scandinavian models.

"District Energy St. Paul is based on quite a few of the operating principles that we see in Denmark and Sweden as far as integrated energy models," explains Axelson. "A few of those components we already have in our system. Combined heat and power (CHP) we already have, biomass because of the CHP, we have district heat, which is a hot water loop and district cooling which is thermal storage."

Axelson also notes that a former District Energy St. Paul project manager was involved in a solar thermal project in Sweden 20 year prior, and that this is one of the factors that led the utility to pursue solar thermal.


Scandinavian products, American construction

In building the system, District Energy again looked to Scandinavia. The utility had difficulty finding Americans with the requisite experience to complete the project, and also with the necessary products.

Arcon Solar collector diagram; Image source: Arcon Solar A/S
Arcon Solar collector diagram; Image source: Arcon Solar A/S

While Axelson states that there are many high quality solar thermal panels manufactured in the U.S., in the end District Energy selected solar thermal panels manufactured by Arcon Solar A/S (Skørping, Denmark).

"We are doing a fairly high temperature system," states Axelson. "Most of what we found in the U.S. market after multiple RFPs and a bidding process, we couldn't find a U.S. product which matched our technical specs."

For construction, District Energy found a local engineering firm, CKDA (St. Paul, Minnesota, U.S.), which partnered with Ramboll Group A/S (Orestad, Denmark), a firm with experience in integrating solar with district energy systems.


Building the system: Snow

The construction process also presented challenges. Axelson states that after managing the "Buy American" provisions of the American Recovery and Reinvestment Act, and waiting for the RiverCentre to complete its new, white roof, construction began in the fall of 2010. This is not an opportune time to begin work on a construction project in Minnesota, with its cold, snowy winters.

Placement of solar thermal panels on the roof of the RiverCentre; Image source: District Energy St. Paul
Placement of solar thermal panels on the roof of the RiverCentre; Image source: District Energy St. Paul

"We started getting bad weather in Minnesota in October," recalls Axelson. "It got a little challenging for them to even complete the portion of the roof that they needed to."

And while she says that District Energy St. Paul was pleased with the speed of construction, she also notes that there was a 2-3 week delay around the holidays due to the severity of the snow.

"We had to figure out how to clear the snow and protect the safety of the construction workers up there," explains Axelson. "In the meantime they were able to work on the internal components of the system, so that we could get all the piping chases completed, get all the pumps and heat exchangers."

Construction resumed when the weather improved, and the system was operational by March 15th, 2011.


Inspiring others

Axelson says that she would "absolutely recommend" solar thermal to other companies. "As far as more businesses and other district heating companies, that's the core of why we did this. This is a market transformation project."

She notes that since District Energy completed the project, the utility has been invited to a number of conferences to relate its experiences, and have been asked to speak to facilities personnel to recommend the system.

"In my mind, District Energy and solar are like chocolate and peanut butter," notes Axelson. "I really think we should be putting them together in the U.S. years ago, but now people are starting to see these opportunities."


Real BTUs

However, the project is not just a showcase for Axelson and District Energy.

"Solar thermal we knew was a real practical technology that can produce real BTUs, it wouldn't be a shelf prize that would go up on our mantle," states Axelson. "We knew that it could be fully integrated and count on it to contribute to our system."

Axelson notes that a period of inclement weather in the spring dampened the short-term output of the system, but notes that such fluctuations are normal.

"We don't design these systems for monthly production, we design these systems for 20 years of production," she notes.

However, overall she states that the system has performed above expectations. The system was designed to be 1.07 MW-thermal, however Axelson states that it has produced in the range of 1.2 MW-thermal under peak conditions.

"I love this product. I love this technology that we have been able to showcase here, and I hope we see dozens more projects in the next years or so."

Image source: District Energy St. Paul
Image source: District Energy St. Paul