A perimeter walkway allows observation of equipment. Photo © Luis Asín

Sited on industrially zoned land chosen for its visibility and proximity to customers, the heating plant powers an 8-mile network of insulated and pressurized hot-water piping—both supply and return—that the company has laid under city streets. The network can provide central heating and sanitary hot water to 6,400 apartment units (about 20 percent of the city’s housing stock) and 40 additional public and private buildings. The system benefits from the compact urban fabric of Spanish cities, where large apartment buildings are the norm.

The plant’s three biomass boilers have a current total capacity of 12 thermal megawatts (MWt), but there is space for an additional unit (demountable sections of the roof permit such upgrades). A backup gas boiler producing 6 MWt provides supplemental power during maintenance, emergencies, and peaks in demand. Other equipment includes feeders that transport the biomass fuel of wood chips from underground silos to boilers, pumps and controls for maintaining network water pressure, and automated compressed-air systems for cleaning boilers and removing the particle dust gathered by exhaust filters. A translucent chimney at one end of the building emits exhaust comprising chiefly water vapor, with carbon dioxide and other gases (though nitrogen oxide emissions are trapped by the boilers, according to DH EcoEnergías).

In buildings receiving service, substations transmit heat by thermal interchange to existing hot-water-circulation systems. Users are billed only for the energy consumed, like other utilities. Savings on heating bills can be as much as 25 percent, the company says, due to the relatively low and stable prices of locally produced wood chips compared to the volatility of the fossil-fuel market, as well as to efficiencies of scale and in the use of advanced technologies.

The key figures for the district heating plant, and the reason it has received grants from various government agencies, are its savings in fossil fuel consumption and CO₂ emissions. DH EcoEnergías calculates that the plant, when running at full capacity, cuts emissions by 95.5 percent, with a reduction of 25,468 tons of non-neutral carbon dioxide equivalent (CO₂) per 115.9 gigawatt/hours (GW/h) a year.

But these statistics require a closer look. The European Union’s Green Deal assumes that biomass obtained from forests is carbon neutral, since the CO₂ it releases into the atmosphere is recaptured by new forest growth. These assumptions have been challenged by Greenpeace and other environmental groups, who warn of the difference in time cycles of rapid fuel consumption and slow forest growth, the potential damage to wooded habitats, and a new-growth forest’s reduced capacity to absorb carbon. In the United States, the Partnership for Policy Integrity has cited evidence that biomass fuel emits far more actual CO₂ than gas or coal per megawatt of energy produced.

These caveats are mitigated in the case of Spain, where the production of biomass stems from new programs that incentivize clearing the country’s extensive forests of dead wood and excess underbrush, which increase the virulence of forest fires. Decades ago, before rural depopulation, this task fell to local farmers gathering firewood. At the same time, in other projects with the architects, DH EcoEnergías proposes to supplement biomass with geothermal and solar energy sources.

Rodriguez gives credit to López for seeing the value of architectural design in his energy plants. Trained as an engineer, he first worked with the architects as a sustainability consultant. Rodriguez sees their designs for DH EcoEnergías as an opportunity to reclaim “the architectonic nobility that industrial buildings had in the time of Peter Behrens.” Spain has stood out in the recent past for its remarkable public architecture, from cultural centers and transportation hubs to public housing and urban design. As environmental concerns have become a pressing public priority, it is gratifying that this spirit continues to thrive in the design of something as down and dirty as a power plant.

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