Have you ever wondered what kind of mechanical system you would build if the entire focus of the project was to maximize energy efficiency and minimize operating expenses and carbon footprint?
In the commercial realm, that daydream might include what’s called “combined heat and power,” (CHP), also known as “Co-generation” (Co-gen). These systems continuously supply electricity to a facility while simultaneously using the waste heat from the power generation process for space heating, domestic hot water, and sometimes air conditioning.
The heart of a CHP unit is a natural gas or LP-powered reciprocating (piston) engine connected to a generator. It may look like a back-up generator, however, the biggest difference is that the CHP unit operates 24/7, and instead of being cooled by a conventional radiator and coolant circuit, the CHP unit expels heat into the building’s hydronic heating systems.
Above: An indoor 150kW CHP unit at a prison in MA.
Heat from the engine and exhaust gas is rejected into a large heat exchanger, which interfaces with the facility’s various heat needs. As an example, a 35kW CHP system will produce 200,000 BTUs per hour and a 250kW CHP system will produce 1,300,000 BTUs per hour.
“The smallest system we’ve installed is a 35 kW unit, and the largest was a 1 MW system,” said Rob McMenimon, owner of Co-Energy America, a Boston-based designer and builder of commercial co-gen systems across the Northeast. “The small system serves a hotel, and the large system is installed at a 1 Million square-foot school and community center.”
The 25-person company was founded in the 1990s, and has more than 90 systems in the field supplying over 12 MW to a wide variety of commercial and industrial facilities. Co-Energy America provides turnkey systems, from feasibility study through engineering and installation, and works with the customer to secure utility rebates after completion.
But power and heat are only two parts of the equation. When a full-scale co-gen system is used in a temperate climate, space cooling can also be provided through the use of one or more absorption chillers.
While there are different types of absorption chillers that utilize various chemical systems, they all work on a similar principle. Inside the chiller, within a low-pressure system, an absorption fluid is evaporated, removing heat from the chilled water. A heat source, in this case hot water, is used to regenerate, or liquefy the absorption solution. The entire process is comprised of four or more steps, but what it boils down to (pun intended), is that hot water enters the chiller, and cooled water leaves the chiller.
“This is called a “tri-gen” system,” explained McMenimon. “It’s a way of using the heat in the summer months, and sometimes year-round if the facility needs constant cooling like a data center. More often than not, co-gen systems are sized for the total heat load of a structure, and depending on the facility, there are more uses than you’d expect.”
“Typically, heating the building through a hydronic system accounts for the bulk of the hot water load,” he continued. “But DHW and commercial laundries add to the thermal usage. We’ve done plenty of pool heating applications, and dehumidification can be accomplished through the use of a heated desiccant wheel. Snowmelt is easily integrated as well.”
Each co-gen application and sizing is different based on the facility’s energy use and what the building owner is looking to accomplish. The load that the co-gen system is designed to handle can vary greatly, too. It can be designed to supply part of the heating or cooling load, or the entire load. The same applies for the electrical need.
Obviously, co-gen systems aren’t common because a number of factors must be present to justify a co-gen application, and this is what Co-Energy America determines during a feasibility study. The upfront capital expense for a system can be large, but typically payback within 2-4 years.
Co-gen systems provide the biggest return on investment where gas prices are low and electric prices are high. Large metro centers across the North East are a good example, and this is where most of Co-Energy’s applications are located. Further, where there is a strain on the existing power grid, utilities may offer co-gen incentive programs.
Above: A look inside a 250kW CHP outdoor enclosure showing the control panel and pump module.
If ample onsite power generation is critically needed (for example, a hospital or data center), CHP can quickly become a consideration. Office building, educational facilities, nursing homes, hotels and public buildings are all prime candidates.
Some, if not all states offer a variety of incentive programs to help offset the cost of co-gen installations. These can cover between 25 and 50 percent of the installed cost. To find out what’s available in your state, search www.EPA.gov for “The CHP Partnership.”
No boiler, no bueno
There’s one extremely important component of a co-gen system that we haven’t covered yet. Boilers are every bit as important in a co-gen application as in a typical heating system.
Should the CHP unit go offline for any reason, hot water needs to be readily available. In addition to that, some co-gen systems are designed to only supply a portion of the heating load. In that situation, a boiler, or multiple boilers, pick up where the CHP unit leaves off.
Again, because CHP systems often appear in mission-critical or medical facilities, boiler redundancy can be extremely important. It’s not uncommon for one boiler, or set of boilers, to be sized for the entire heating and cooling load of the facility, with an additional boiler as a failsafe. And with numerous condensing boilers serving a single system, modulation can be optimized at part-load conditions.
But that’s not the only reason that high-efficiency boilers are the winning ticket in a co-gen system. Boilers in a co-gen system are very likely to sit unused for extended periods of time. A high-mass boiler would take much longer to reach setpoint, but if it were kept at or near setpoint while not in use, the standby heat loss would be substantial.
In order to be cost-effective, CHP units need to run long hours, regardless of whether or not heating or cooling capacity is needed. Spring and fall could potentially create low-load situations where the generator may create more heat than the HVAC system can use. In such an instance, the co-gen system needs a heat sink, or somewhere else to dump BTUs. Some sort of heat rejection system needs to be installed, or the CHP unit is forced to shut down. Typically, a small cooling tower or closed-loop fluid cooler is used.
Once the system is purchased and installed, CHP provides the closest thing to “free heat.” The heat is a byproduct of energy production, so facility owners with co-gen systems ought to get creative with how they want to spend their spare BTUs.
“Because we’re involved from the inception of the project, we help the owner realize the full potential of a co-gen system,” said McMenimon. “Not every building in the U.S. is a good candidate, but when co-gen makes sense, it usually makes a lot of sense.”