Thermal storage integrated with biomass water heating systems is a “hot topic,” that has been debated “over and over,” according to Scott Nichols, owner and president of New Hampshire-based Tarm Biomass. But members of the biomass heating industry, he says, are reaching a point where they are finding some common understanding on the technology and its benefits to the market.
“We use thermal storage primarily to reduce emissions and increase [biomass boiler] efficiency [as well as] increase service life and reduce downtime,” Nichols said on March 31 during a panel discussion on the pros and cons of thermal storage at the Northeast Biomass Heating Expo (NEBHx) in Burlington Vt., sponsored by the Biomass Thermal Energy Council.
The focus topic for the panel session was boiler systems that use cord wood, pellets or wood chips to heat water for space heating and domestic water use, and the ongoing debate over whether thermal storage via an independent hot water tank is appropriate for use in all biomass heating systems. The panel session explored how thermal storage can deliver benefits, and some of the places where it would not be beneficial to pair thermal storage with a biomass water heating system.
Biomass boilers operate differently than, for example, natural gas-fueled boilers, session moderator Adam Sherman told Renewable Energy World. Sherman is manager of the Biomass Energy Resource Center, a part of the Vermont Energy Investment Corporation.
“Wood boilers, whether they are wood chip, cord wood or pellets, don’t tend to turn on and off, and turn down to a very low output as efficiently as a natural gas system,” Sherman said. “Gas-fueled systems can turn on and off really fast, and they can immediately respond to load in a building for a call for hot water, and they can be turned down to very low outputs without dramatically impacting efficiency.”
According to Sherman, thermal storage has distinct advantages that can help integrate biomass boiler equipment efficiently into a building’s systems.
The storage functionality is incorporated into the boiler system in the form of a buffer tank — an independent tank that is highly insulated and stores large quantities of water at a constant temperature. That stored hot water then stands ready for use by a facility’s system for space heating or domestic water use.
According to water storage tank provider HydroFlex Systems of Pennsylvania, standard water storage tanks are constructed with two layers of one-inch polyisocyanurate foil faced insulation and embossed aluminum, and they can range from 100 gallon to 5,000 gallon capacity.
Dan Wilson, vice president of Wilson Engineering and panelist for the conference session, said that the thermal storage provides a buffer between the boiler and the heat demand in biomass boiler systems.
“The purpose of that is, we want the system to be as responsive as possible,” he said. “We also want to reduce wear on our boilers from cycling.”
He added that including the buffer tank with biomass heating systems expands the amount of fossil fuel that is being offset by the system and improves the overall efficiency of the boiler.
According to Wilson, the specifics of where and how thermal storage works in the different types of biomass heating systems are system-dependent.
“A large wood chip system providing district heating is different than a pellet boiler that has the ability to start and stop pretty easily, or even a cord-wood boiler that has a fixed load of charge of energy that you want to be able to use or burn in a highly efficient manner,” he said.
Thermal storage “is not one-size fits all,” Sherman said, adding that there have been a number of government agencies that are interested in promoting wood heating, but they want to enforce safeguards to make sure these systems are highly efficient and have low emissions.
“They are leaning toward mandating that thermal storage be used in these systems,” he said. “There are times where, for all the benefits of thermal storage, you might be actually getting more drawbacks.”
Biomass heating is a growing market that is helping some states meet their emissions reductions programs.
Speaking at NEBHx on March 31, Val Stori, a research analyst with the Clean Energy States Alliance (CESA), said that there are 12 states with renewable thermal provisions in their renewable portfolio standards, and five of those states include biomass specifically — Vermont, Massachusetts, New Hampshire, Wisconsin and Indiana.
According to Stori, states have very different positions on whether thermal storage should or should not be part of a biomass thermal system and part of state program requirements.
“I haven’t seen a trend in thermal storage other than it is an ongoing debate,” she said.
Stori added that CESA is working on a report to present the facts of thermal storage in the context of the current biomass heating market.
“What remains unclear, and what I think policy makers want more information on is how do the benefits from thermal storage compare to benefits from high modulation rates or proper system sizing or other factors that we don’t have data on,” she said.
Lead image: Hot water tap. Credit: Shuttestock.