The scale of district energy systems provides
opportunities to generate and deliver energy to school buildings in a more sustainable manner.
Although we may tell the story to our children, not many of us really remember going to a school with a wood-fired stove in the corner of the classroom. Once the standard method for school heat, they long ago were replaced with what we may remember: steam radiators that crackled and popped on a cold day. These radiators represented the first application of district energy, systems involving energy plants that are designed to deliver thermal or electric utilities to multiple buildings, in our schools. Today, these central systems can provide heat, cooling and even power to everything from simple elementary school classrooms to complex university research facilities in a quiet, reliable and energy-efficient manner.
Because of many advantages associated with central utility systems, school campuses, from large universities to elementary schools, have used district energy for decades. District energy facilities enable thermal and electric utilities to be generated with greater efficiency and higher system reliability, while requiring fewer maintenance and operations personnel. However, in order to be affordable and effective, they require multiple buildings to be closely situated. School campuses are the perfect fit. Today, however, the sustainable building movement is changing how these systems are viewed.
In 1993, the U.S. Green Building Council (USGBC) met for the first time. Five years later, it introduced the LEED program, which enabled buildings to receive the "brand" of LEED and make a statement to its stakeholders about green values. In a short period of time, the LEED program has taken off in popularity.
Today, more than 1.4 billion square feet of building space has been certified through the LEED program; another 6 billion square feet is seeking LEED certification.
In addition, countries around the world have adopted variations of the LEED program for local use. Many institutions have mandated that all new construction on their campuses be LEED-certified, and some potential students say they take into account a university’s green rating when deciding where to attend. In addition, many K-12 systems have sought LEED certification for their projects to demonstrate a commitment to sustainable design.
When the USGBC developed the first rating system, however, the impact of district energy was not addressed. For the most part, the rating system concerned itself only with what was occurring on a proposed building’s construction site. The assumption was building owners had little ability to affect how utilities were generated off-site. District energy was treated the same as traditional utilities such as power and gas.
However, the USGBC found that not addressing district energy within its LEED guidelines created many questions for project designers. In 2008, the USGBC released "Required Treatment of District Thermal Energy in LEED-NC Version 1," its first guideline to address applying the LEED program to proposed buildings that would receive district energy.
In 2010, it released an update to this guide, "Treatment of District or Campus Thermal Energy in LEED V2 and LEED 2009—Design & Construction Version 2.
It also released the first version of a separate guide, "Treatment of District or Campus Thermal Energy in LEED for Existing Buildings: Operations and Maintenance Version 1.0," to address district energy under the rating system for existing buildings. All of these guides are available free at USGBC.org.
The International District Energy Association (IDEA) (www.districtenergy.org) has been working with the USGBC’s guideline writing committee from the early days of the first guide’s development as the representative expert on the district energy industry. As a result, aspects of district energy systems align well with core goals of the USGBC.
Understanding these aspects and incorporating them in district energy systems on campus will create scenarios in which connecting a proposed building to district energy increases the level of LEED certification that can be achieved.
These three opportunities—renewable energy, combined heat and power (CHP), and thermal energy storage—often are expensive to install and impractical to maintain within a proposed building’s site.
Because of the scale of district energy, these challenges can be overcome when installing them within the district energy system itself. As such, district energy often is the ideal choice for buildings pursuing LEED certification.
Renewable energy has been around for centuries. Power from wind and water movement and solar thermal were used by ancient civilizations. Today, solar energy, wind and biomass all offer promises of reducing dependency on traditional nonrenewable energy sources such as coal, oil and natural gas.
The problem with traditional renewable-energy sources is that they often are cost-prohibitive and difficult to use and maintain. Wind and solar are not in constant supply, and biomass feedstocks often come from immature markets with unreliable supply streams and large price swings—neither of which is conducive to attracting normal sources of private capital investment.
In concept at least, renewable energy makes a lot of sense. If schools are able to effectively harness the energy of the sun, wind or tidal wave movement, or harvest energy from waste streams, the problems with energy security, supply, pollution and global warming could in theory all go away.
As a result, the U.S. government and many state governments encourage the development of renewable-energy markets in the form of tax incentives, grants and regulation. However, governments are not the only entities trying to encourage the development of renewable energy markets.
The USGBC hopes to accelerate the maturity of renewable energy markets and technologies through its LEED program. The group will do this by rewarding to building projects that use renewable energy a significant number of points toward their LEED rating goals.
LEED and Renewables
The USGBC recognizes that encouraging the use of renewables helps achieve its goals. It also recognizes that in order to sufficiently encourage the use of renewables, significant incentives are required within the LEED rating system to overcome first cost, operating cost, and long payback periods.
As a result, the USGBC gives points for renewable energy in two separate credit categories within the LEED rating system. It is possible, in fact, for qualifying projects to earn many points within each of these categories.
The first way to achieve LEED points with renewables is under Energy & Atmosphere Credit 1 by reducing the total annual energy cost of a project building. This credit awards up to 19 points to projects that are able to demonstrate a percentage of energy savings, as measured in terms of a building’s total annual energy costs, compared with the minimum energy requirements for the same building as outlined in ASHRAE 90.1—Appendix G.
The LEED rating system, however, assumes that the input fuel costs of all renewables is zero. This is true for solar and wind, but rarely the case for waste products such as wood chips, oat hulls or poultry waste. As a result, the assumed "free" cost of renewables increases the amount of points that can be achieved under Credit 1.
Second, under Energy & Atmosphere Credit 2, projects can achieve up to seven points by using renewable energy. To do so, they must consume what the USGBC considers to be renewable, and the amount used must reach certain thresholds as defined by the percentage of total annual building energy use.
The relatively high thresholds required to achieve points for renewables make it difficult for a project building to capture any points under Credit 2 with on-site renewables. For example, the amount of photovoltaics required to achieve just 1 point would cover most buildings.
In addition, the operational and fuel-handling challenges of burning biomass within a single project building are substantial. On the other hand, the size and scale of district energy often provides good opportunities for burning renewables.
Originally, the USGBC did not include provisions that would enable renewables consumed in district energy systems to be applied to customer buildings. This mandate was changed in the first district energy guide published for new construction and has been improved in the second version of that guide. As such, district energy often is the best and sometimes the only option for obtaining credit for renewables in the LEED rating system.
Renewables On Campus
Moving forward, the USGBC and federal and state governments want to continue to create incentives for the use of renewables. Although there are challenges associated with using renewables, many of these are reduced when applied to district energy:
•Combined heat and power. When electricity is generated at a traditional fossil fuel powered plant, 33 to 35 percent system efficiencies are typical. When produced using a combined heat and power (CHP) process, total system efficiencies can exceed 80 percent.
Why? In a traditional process, energy in the form of heat is rejected to the atmosphere or to a body of water as part of the condensing cycle, and thus this energy is wasted. In a CHP process, this same wasted energy instead is put to use to heat a building or its domestic hot water, provide heat for a manufacturing process, or even drive absorption or turbine driven chillers for cooling.
Why then don’t traditional electric generators cogenerate? Normally they do not have a large enough use for this waste energy near their generating site, and it becomes impractical. District energy on school campuses by its very nature has these loads close by and often chooses to cogenerate as part of the operational model.
More than doubling the efficiency of electric generation is certainly in line with USGBC goals. However, onsite building cogeneration systems rarely are practical. The USGBC, however, does allow credit for CHP in a district energy system to apply to buildings seeking LEED certification that receive waste heat from the plant.
In fact, the buildings that receive this waste heat get credit for the corresponding electricity generated even if that specific electricity was not sent directly to the building. This normally is the case as CHP systems deliver waste steam directly to customers, but the electricity generated often is sold to the utility company. Therefore, proposed buildings normally may receive additional energy efficiency points in the LEED program by receiving waste heat from district energy systems.
•Thermal energy storage (TES). Electric power cannot be stored easily; instead, it must be generated as it is needed. Therefore, the peak system electric load sets the size of generation and transmission required to keep systems humming along.
Also, as would be expected, electric utilities base load their most efficient generation equipment and use the least efficient equipment for peaking purposes only. The combination of these issues are powerful incentives to drive down overall peak loads and are in line with USGBC goals.
Thermal energy storage is the ability to shift cooling production, which requires significant electric energy, from peak times to off-peak times. As a result, TES is a powerful peak lowering tool.
Sidebar: Longwood University, Farmville, Va.
Longwood University, which was founded in 1839, recently commissioned a wood-burning heating plant that takes in waste wood chips from the surrounding community that are combusted to heat the campus. This asset helps increase the percentage of renewable energy delivered to all of the buildings on campus.
Sidebar: University of Virginia, Charlottesville
The University of Virginia’s campus in Charlottesville includes a chilled- water storage tank capable of holding 16,200 ton-hours of chilled water that can be released during peak load periods. This feature increases the LEED points a proposed building can obtain under the energy efficiency category by connecting into UVA’s chilled-water system. Often, in these cases ice is used as the storage medium in lieu of water.
Griffin, PE, CEM, LEED AP, is a principal and branch manager at RMF Engineering, Baltimore, Md., a full-service engineering firm with a specialization in the design and commissioning of district energy. He can be reached at firstname.lastname@example.org.