Making informed decisions about system selections can be challenging. While rules of thumb and prior experience can be helpful in pointing to a direction, they do not inform the specific energy use of a given application. Obtaining usable input and data is an all-hands task requiring a wholistic approach from the design team on how best to proceed in creating an energy-efficient solution.

We have come a long way with the development of technology and programs, which, when used individually or combined, become very powerful tools. However, are we taking full advantage of the resources available to us? Are we providing our clients with the right data to aid them in making the decision as to whether a system is worth their investment?

Building construction and ongoing system operations contribute nearly 40% of all greenhouse gas (GHG) emissions. Implementing energy-efficient designs can substantially reduce fossil-fuel energy usage of buildings while, at the same time, reusing energy that might otherwise be wasted. 

For example, instead of efficiently heating with gas and throwing the remaining waste heat away, there are approaches that can provide beneficial cooling to other systems while simultaneously providing beneficial heating. This reduction in energy consumption, in heating and cooling, directly leads to decreased carbon emissions, aiding in the mitigation of climate change’s impacts.

Energy Model: Where to Begin?

Depending on the type of building and the system complexity, multiple energy modeling tools are available, such as IES VE and EnergyPlus, to validate the system’s performance. Energy models can be system-specific or more comprehensive in nature. 

The use of sustainable heat pump technologies, whether air- or water-source (for energy rejection), requires a more detailed analysis to determine energy efficiency and carbon reduction over the course of a year. The use of water-source heat pumps, where applicable, has the potential for energy reuse, provided complementary systems and demands exist within the building. 

Understanding those potential savings and impacts often requires a more comprehensive, overall building energy model.

When constructing an overall building energy model, step one is to gather the building’s geometry from the architectural team. The complexity of the received geometry determines whether it can be directly imported into energy modeling tools or if it needs to be simplified (and redrawn) using other software such as SketchUp, Rhino, Revit or other similar tools. 

The next step defines the type of spaces together with the location and size of the windows in the geometry. Once the geometry is ready, thermal properties can be added to these components. Project location and orientation is then added, followed by construction materials for surfaces, internal heat gains, temperature setpoints and ventilation requirements. This then allows the energy model to predict the cooling and heating loads required over the year. 

With the building elements and characteristics in place, the energy modeler then begins to inform the building mechanical systems information, which includes the proposed system types and distribution, equipment efficiency and controls logic, and other details. If complementary building systems exist, they can be added at this time. 

The energy model then consolidates all these inputs to calculate overall building energy usage, such as building envelope, mechanical systems, lighting and receptacles that can be broken out per hour, per month or per year. These results are then compared to a minimally code-compliant building to highlight energy performance and identify further opportunities for improvement.

Using the Power of Analytical Tools

Often, an Energy Usage by End Use Donut Chart is used to represent energy model outputs. This chart and others like it include different colors representing various energy uses, graphically illustrating the relative portion of energy usage for each one. Taken as a whole, this then represents the energy use for the entire building. 

For instance, the chart in Figure 1, located in a heating-dominated climate, highlights that the building heating, electrical receptacle (internal equipment) and fan energy are the top energy consumers, with cooling and lighting having less of an impact.

While we often cannot change the internal equipment energy use, this highlights opportunities to improve building energy efficiency. For this example, our resources should be focused on reducing fan and heating energy rather than cooling energy. A focus on waste heat recovery, lowering HVAC face velocities and improved fan efficiency can significantly impact building performance. 

The next step is to model some proposed alternatives to determine their impact on building performance.

Comparing Systems: Water-Heating Solutions

For decades, the natural gas-fired water heater has been the go-to solution for energy-efficient water heating. While the process of heating via natural gas rejects excess heat to the environment, it is nonetheless an efficient and reliable means of water heating. With a bigger emphasis on reducing greenhouse gas emissions and fossil fuels, this once staple of water heating is now being compared to more advanced heat pump technologies.

Heat pumps for water heating use self-contained refrigeration cycles to extract heat from the environment or other energy sources. Compared to traditional natural gas boilers, heat pumps have a higher coefficient of performance (energy efficiency) and inherent environmental advantages.

Heat pump water heaters are provided in the following configurations:

• Air-source heat pump. As the name suggests, this equipment extracts heat from the environment via an outdoor unit to provide beneficial heating with no natural gas. Its efficiency varies directly with the outside air temperature, and, if it becomes cold enough outside, can use even more energy than a traditional electric water heater until the point that it no longer continues to operate (typically below 0 F outdoors). 

As these units are independent, however, their operation is simpler as compared to water-source versions.

• Water-source heat pump. This is an option for facilities with access to a complementary water source for heat extraction, such as a geothermal system or continuous cooling demand. These units are often smaller and more energy efficient as there is no fan to reject heat to the air and the compressor does not need to work as hard.

• Hybrid systems. These use air- or water-source heat pumps based on resources available at the time.

Each of these heat pump configurations contribute to the reduction of greenhouse gas emissions by using electricity, which has the potential to be sourced from renewable energy instead of burning fossil fuels. This is consistent with efforts to transition to more sustainable and environmentally friendly energy systems. The use of next-generation heat pumps with low global-warming-potential refrigerants or carbon dioxide further enhances their environmental benefits.

Traditional boilers can use gas or electricity power as a fuel type. Like heat pumps, electric boilers have the potential to be sourced from renewable energy sources, but require an increased size of electrical service, transformers and panels for a building. Natural gas boilers often take advantage of lower natural gas prices and are available in larger capacities than electric. The gas boilers also have a faster reaction time and are not impacted by power cuts. 

No matter electric or gas, a boiler’s efficiency is limited and restricts the potential for greenhouse gas emission reductions to some degree.

Energy Model Example

Consider a potential project located on a campus with a geothermal water loop, in a mild climate zone, and in an area with low fuel prices. Both traditional and heat pump technologies are viable, so which is the best option to proceed with? 

Answering that question definitively depends on the local climate, property type, system size and usage, first cost to integrate systems, and more. Energy modeling coupled with lifecycle cost and GHG emissions analysis can answer those questions. The following is an energy model-informed lifecycle cost analysis we did for this potential project. 

The first step was the generation of an energy model for this project, comparing the relative efficiencies of an air-source heat pump, water-source heat pump and a traditional natural gas boiler. Figure 2 highlights the annual energy cost in thousands of dollars per year for the three approaches.

Working with a contractor, we collected the first cost for each of these systems to highlight the relative payback period for these approaches in years as shown in Figure 3. While the gas boiler had the lowest first cost, its increased energy use did not result in a payback.

While the water-source heat pump has the lowest energy usage, it requires more infrastructure to support. The air-source heat pump, while a higher first cost than a gas boiler, is more compact and requires less infrastructure to support, giving it the shortest payback period.

This example highlights the relative energy use of the heating system in isolation. Of those three approaches, only the water-source heat pump was integrated into another system (geothermal in this case) and did not directly impact the energy use of another system. Configured properly, however, water-source heat pumps can provide beneficial energy savings to other systems within the building. 

A heat recovery chiller (HRC) is a form of water-source heat pump that provides heating to one system while simultaneously cooling the other, improving the efficiency of both. And the closer the temperatures of these loops are to each other, the higher the energy efficiency.

Data centers require continuous cooling year-round, while laboratories in heating-dominated climates can require significant amounts of heat. With proximity of these complementary building programs, the integration of a an HRC can result in significant energy savings. This then leads to the larger question of how using the waste heat can influence the heating and cooling load balance, and the overall efficiency of the HRC system.

Figure 4 compares building heating and cooling loads over the year for a standalone laboratory in a heating-dominated climate. The figure reads from January to December (left to right), with blue lines above the axis representing building cooling and red lines below the axis representing building heating. The further a line extends away from the axis, the larger the load. The heating and cooling loads are independent in this case, representing a gas boiler or air-source heat pump solution. 

Figure 5 adds a large, consistent process cooling load into the building cooling load profile. In this case, the magnitude of process cooling is such that the load profile moves from heating dominated to cooling dominated. If all these loads are maintained independently, then the building energy use noticeably increases. Even if the associated systems are efficient, there remains wasted energy due to the nature of simultaneous heating and cooling.

Figure 6 explores the integration of an HRC to offset process cooling while simultaneously offsetting building heating. The yellow areas represent periods of time where these loads are aligned. By combining these systems, we significantly reduce the energy use of both systems. As process cooling normally relies on the evaporation of water for cooling, this approach results in a dramatic reduction in water usage as well.

It is not guaranteed that the integration of process cooling load into a heating system will always benefit the HRC efficiency. A number of factors influence it and requires a wholistic approach to realize potential savings. Even with proper integration, the programs needed to sustain this project should be sufficiently long enough to realize the savings and opportunities.

Stay tuned! Part 2 of this article will cover whole-building performance modeling, including façade optimization, solar radiation study schemes (including opportunities for solar thermal integration) and other topics that complete an energy-efficient design study.

Ting Cai is an energy modeler with SmithGroup’s Phoenix office. She has experience with laboratories, hospitals and health-care facilities, higher education classrooms and office buildings.

Lowell Manalo is the plumbing discipline leader for the western region at SmithGroup. He is a member of the American Society of Plumbing Engineers and has more than 20 years of experience designing plumbing systems for a variety of building types.