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Consulting - Specifying Engineer | How all-electric design achieves energy efficiency

Consulting - Specifying Engineer | How all-electric design achieves energy efficiency

The electrification of new buildings is a practical and impactful solution to address carbon emissions in the building sector. Predicated on the “greening of the grid,” all-electric building design yields the promise of a carbon-neutral operation. The electric utility grid is changing from carbon emissions and heavy fossil fuel power production to increased production of power by wind and solar (eGRID). Driven by market demand, utilities across the country now offer customers the opportunity to buy carbon-free electricity. Designing for a truly all-electric facility leverages utility-scale investment in renewable energy production to accomplish building energy decarbonization goals.

Driven by codes, standards and sustainability rating systems such as the U.S. Green Building Council’s LEED program, energy-efficiency in buildings continues to steadily improve. ASHRAE advanced energy design guides provide a cost-effective framework to achieve zero energy in K-12 school buildings and small to medium office buildings. For many facilities, all-electric design bridges the current emissions reduction gap between energy-efficiency and zero energy on-site. Natural gas and electricity are the dominant energy sources consumed in the U.S. commercial building sector. According to data collected and published in the 2012 Commercial Buildings Energy Consumption Survey, electricity accounts for 61% of building energy use, with steady growth in electricity use trending since 2003. However, natural gas still accounts for 32% of commercial building energy usage in the United States. Coupled with a renewable grid resource, replacing natural gas use with electricity is a prime opportunity to cut emissions. The challenge ahead is how to fully electrify a facility while maintaining energy-efficient design, moderating the associated peak demand growth placed on the grid and using currently available solutions (see Figure 1).

It is insightful to know which building energy end uses are currently primarily gas. Data from the 2012 CBECS summary reports provide an overall picture of the building energy end use categories across commercial buildings in the United States. 2018 CBECS data is expected to be released soon. This data may show a shift in end uses recognizing the uptick in LEDs reducing lighting energy and improvements to compressor efficiency impacting refrigeration and cooling; but through 2018, major shifts in gas end uses are unlikely. More detailed end-use estimates for a particular facility can be established using energy simulation tools, benchmark data or survey data (see Figure 2).

Specific building type and climatic location influence the absolute percentages, but the significant end uses of gas are space heating, water heating, cooking and “other,” as shown with the CBECS survey data. Feasible options now exist for serving these loads with electricity-generated heat rather than gas-generated heat.

Projects looking to decarbonize via all electric use must have clear goals of overall project success to guide decisions. For example, a project seeking to use all electric for carbon savings while maximizing the highest energy efficiency to achieve zero energy on-site would prioritize different space heating solutions compared to a project with lowest construction cost all-electric design as a goal. The design must strike a balance of several factors for the proposed system solutions: energy efficiency, system simplicity to operate and maintain, equipment availability and construction cost dollar efficiency are all under consideration.

Space heating is accomplished most simply via electric resistance heating, but resistance heat is the least energy-efficient means of providing electric heat. Electric resistance heating has a coefficient of performance of 1.0 and is the baseline for efficiency consideration. Alternatives to electric resistance include solar hot water systems, transpired solar air heating, waste heat recovery opportunities or heat pumps in various system solutions.

As illustrated in Figure 3, the relative efficiency of these alternatives is examined compared with heat source temperature. At the high end, space heating produced directly from renewable sources, such as transpired solar systems or solar thermal hot water systems, is very energy efficient — taking the small energy input needed to move air or fluid across the collectors and extract usable heat.

Technologies reliant on vapor compression cycles and heat pumps have a direct relationship with heating efficiency and source temperature. Solutions such as air source heat pumps operate over a wide range of source temperatures, but as ambient air source temperature increases, so do COP values and efficiency.

The greater efficiency of a ground-coupled system versus air source is directly attributable to the source temperature differences. Heat sources such as wastewater or exhaust air, when available, can be of higher efficiency than either air or ground-coupled heat pumps as the lift between source and space heat temperatures is narrowed. These waste sources are rejected at or near space heating temperatures and should be leveraged when a sufficient volume of flow is available.

In addition to energy efficiency, there are many factors to consider when using electricity as a fuel source for space heating. As an example, let’s explore space heating electrification of a higher education academic facility in a cold climate such as Detroit or Chicago, ASHRAE climate zone 5. Electric resistance heating is a simple baseline solution with a relatively low installation cost for heating equipment. Maintenance is low, but with a COP of 1.0, operational efficiency is also low, resulting in high operational energy cost. The physical space required for the baseline electric resistance solution is minimal.

However, the electric resistance option has the largest connected electrical demand and requires a large electric service size, which should be accounted for as part of construction costs. An additional factor to consider is also the local energy code, such as California’s Title 24 Energy Code. While electric resistance heating may be a simple solution for parts of California with low heating loads, the Title 24 restricts the use of electric resistance heating for space heating and alternate technologies must be used.

Seeking improved energy efficiency and lower operating cost alternatives, the design engineer can propose an air source heat pump system. In cold climates, the air source heat pump faces capacity and efficiency issues as the ambient temperatures drop, with capacity significantly reduced at conditions below 0°F in the traditional vapor compression refrigerant cycle. Advances in heat pump technology are improving low ambient options, with notable improvements in recent years as manufacturers introduce refrigerant vapor-injection to boost capacity. The low ambient range of air source heat pumps has expanded into -10°F to -15°F or even lower. At the new low ambient conditions, the ASHP frequently needs supplemental heat or alternative backup heat sources. The heat pump COP at these low design conditions can be around 1.5, still greater than electric resistance heat, but not nearly as good as the Air-Conditioning, Heating, & Refrigeration Institute/Air Conditioning and Refrigeration Institute ratings would suggest.

However, for most hours in this climate, the ASHP design alternative benefits from a COP of 3 to 4. For mild climates such as San Francisco, ASHRAE climate zone 3, the ASHP can perform at this higher COP year-round and does not encounter the efficiency penalties at low ambient conditions. The connected electrical load is likewise reduced compared to electric resistance alone, but a portion of the peak heating as electric resistance must be accounted for.

The engineer must also account for a typical defrost cycle in the outdoor unit. In climate zones colder than ASHRAE climate zone 5, an ASHP likely will not meet peak design conditions without supplemental heat.

In addition to low ambient concerns, ASHPs are a relatively space-intensive solution. The condenser units must be located on the roof or at grade on-site and have sufficient clearance for service and airflow to achieve the required heat transfer. A design engineer can provide a mix of ASHP and electric resistance heat to deliver peak design capacity, serve most of the expected load-hours with heat pumps and minimize space and first cost impact.

In cold climates, there are significant benefits in designing for a mix of electric resistance and ASHP or including thermal storage compared to designing for only ASHP. This balance is climate and building use type-dependent and best informed by engineering analysis or energy simulation models, which account for typical weather patterns and expected building use.

An alternative to using an ASHP is using the ground to provide a stable temperature for the source of heating rather than ambient air. In cold climates, extracting heat from the ground with heat pumps is viable when the air source heat pumps cannot produce sufficient heat. Using the ground as a heat source alleviates the low ambient issues that air source heat pumps suffer.

Ground-coupled heat pumps have a significant efficiency advantage. With long-term seasonal storage capability an added benefit, summer heat rejection is stored in the ground and later extracted. A properly designed ground-coupled system will never see the low source temperatures air source systems will at ~40°F versus winter air temperatures. Facilities with balanced heating and cooling work best. Without a balanced load, supplemental heating or cooling is required to balance the load.

Design challenges include physical space and cost associated with installing a ground-coupled exchanger field, well or pond. Building designs targeting zero energy on-site often leverage the efficiency gains of ground-coupled heat pump systems, accepting the energy improvement and the first cost premium. The ground-coupled design can be applied regardless of climate and is reliable in providing high COP performance.

In a mild climate, air source heat pump systems supply the necessary heat without requiring resistance heat to meet peak design capacity. System selection then becomes a matter of choice, making decisions based on system operational complexity, maintenance requirements, energy efficiency and construction costs. Resilience should also be factored in regardless of how mild the heating climate, while considering if reserve resistance capacity is prudent planning regardless of typical peak design, given the increase in climate disruptions and recent extreme swings beyond what was once typical observed in many locations.

In buildings with year-round simultaneous heating and cooling demands, (such as laboratories or health care facilities), water source heat pumps, variable refrigerant flow and heat recovery chillers provide heating as a byproduct of cooling. Rather than rejecting condenser heat to a cooling tower or ambient air, it is used to serve building space heating demands. For large laboratories or hospitals where hydronic systems are common, a heat recovery chiller is typical.

A heat recovery chiller operates at a higher condenser water temperature when compared to a typical water-cooled chiller, thus the chilled water system efficiency is decreased. However, the combined performance of heating and cooling is greater than typical chilled water and heating water systems, at a COP of 7 to 9. The main limitation with heat recovery systems is the requirement for simultaneous heating and cooling demands and most buildings do not have continuous simultaneous loads year-round.

To fully electrify the building space heating demands, the heat recovery chiller system would likely need to be paired with thermal energy storage. Thermal storage allows for storing excess chilled or heating water when loads are not completely simultaneous. The system could also be paired with an ASHP or other means of electric heating to meet an excess heating demand when the simultaneous cooling load does not meet the full heating load (see Figure 4).

The air exhausted from the building itself is a very efficient source of heat recovery, with building exhaust air remaining 65°F to 75°F year-round, regardless of climatic region. Similar in concept to the ground-coupled heat pump, heat pump chillers coupled with exhaust air energy recovery as the source “field” may allow for full electrification of building space heating should the building have adequate volumes of exhaust air.

By leveraging the stable temperature of the exhaust air instead of the ground, significant installation costs are avoided and the “simultaneous” operation of the heat pump or heat recovery chiller increases (see Figure 5).

The design team must analyze the annual heating load, the simultaneous cooling load and the coincident exhaust airflow to ensure the building exhaust provides adequate heat recovery during all hours of the year. A well-designed system balances the building’s simultaneous cooling/heating demand (which can be met with the standard operation of a heat pump chiller), with the amount of heat that must be recovered from the exhaust airstream to meet the remaining heating demand.

Should the exhaust air energy recovery not meet the full building heating load, the load reduction simplifies the electrification of the remaining heating load. For all electric projects also pursuing the highest energy efficiency, it should be noted that this solution includes fan and pump energy penalties when compared to many other all electric design solutions.

In contrast to space heating loads, domestic water heating loads are typically much smaller, allowing for ease of electrification. Domestic water heating uses the same technologies discussed for space heating, with either point-of-use resistance heaters or the ASHP as the most commonly used technologies for domestic water heating.

With typically smaller loads and peak capacity than space heating loads, ASHPs used for domestic water heating can be located indoors within a mechanical room, kitchen or penthouse having compact spatial requirements. A potentially added benefit of locating domestic hot water ASHPs indoors is free cooling provided by the byproduct of compression, cold air to the room. The ASHPs can use warmer ambient conditions of a room with equipment generating heat (75°F to 85°F), allowing for increased COP performance.

For facilities with a large domestic hot water load, the alternative systems discussed under space heating may apply to minimize electrical system impact, energy use and demand costs associated with all-electric design.

Electrification of cooking is a necessary piece for cutting gas use and emissions in buildings. As identified in New Buildings Institute’s Building Electrification Technology Roadmap, the challenge of electrified cooking is less an issue of available technology and more an issue of outreach and acceptance. Electric equipment is available for all commonly used kitchen appliances. Gas appliances are publicized in real estate and cooking shows for the ability to deliver superior cooking performance.

However, the performance claim is not supported with evidence-based reviews or results for the majority of cooking processes, especially considering the availability of induction ranges and convection ovens. Training in commercial cooking techniques using electric appliances is a primary need, which can result in an increase in cooking with electric appliances. To further reduce emissions, appliances can be selected for efficiency versus lowest purchase cost; for example, induction technology consumes less energy compared to resistance heating, but often comes at a first-cost premium.

In laboratories or health care facilities, gas is commonly used for washing, sterilizing and humidification among other process equipment. In some regions of the U.S., hospitals, multifamily or dorms use gas clothing dryers. These process loads are the last remaining significant driver of gas used in buildings after space heating, water heating and cooking have been addressed. Several of these process uses require high-temperature heat and are only available electrically as resistance heating.

Selecting process equipment with electric alternatives to traditional gas-fired units needs to be carefully evaluated. The design engineer must consider a likely increase in electrical service and annual utility cost when planning for this equipment to be all electric.

Major hurdles have been discussed to electrify building energy use, but the design challenges are still unresolved. Designing a building for all electric instead of using natural gas has significant ramifications for the necessary electrical service capacity to and in the building. The service capacity required for an all-electric facility can easily double or be even higher compared to past benchmark data. This may require the quantity of electrical service entrances into the building to grow, where it may have required just one in the past, the facility with all electric design may require two services.

In mild climates where the peak cooling demand is higher than the peak heating demand and in buildings that leverage energy recovery strategies, the electrical service increase may be small for the electrification of heating as the service must already be sized for the building’s cooling electrical demand. The potential increase in electrical capacity needed must be carefully considered when evaluating alternatives for traditional gas end uses.

When in the early design stages of a new building project, electrical engineers may commonly rely on benchmark data or experience-based rules of thumb to estimate the necessary electric service according to the type of building and climate.

However, with gas so commonly used, benchmark data of all-electric facilities is currently limited. Expanding the size of the main and emergency electrical rooms and the equipment within should be part of early programming discussions around electrical room space requirements in a new facility design. Further, as the design team evaluates alternative solutions to replace gas in systems design, the construction cost of proposed alternates should also account for the cost impact of connected electric load on the electric infrastructure and potential infrastructure increase within the facility for distributed electric heating alternatives.

With the cold climate academic building example, the choice of using electric resistance versus heat pumps can be optimized to lower connected electrical load. Thermal storage could be considered as one tool to lower the peak connected load, peak load demand shaving or potentially even reduce the required number of ASHP outdoor units.

Thermal storage sized to load shift capacity during daily peaks and facilitate the staging of heat pump units in the system can be accomplished with moderate storage volumes. This is an effective strategy to trim a morning warmup peak, reducing the most common peak electrical demand introduced when fully electrifying a facility. Using thermal storage requires building systems designed for hot water rather than direct air to air source heat pumps, a cost consideration where smaller facilities will not typically invest. For these facilities, battery storage systems may be the more effective demand management strategy.

For facilities with hot water heating systems, low-temperature hot water maximizes ASHP efficiency, but are less likely to realize physical space savings using thermal storage. The opposite is true of high-temperature heating and domestic hot water systems, where storage can be highly effective at reducing required unit capacity.

As building loads are moved from natural gas to electric, two discussion points on backup power must be addressed — the possible capacity increase of required backup power and if the backup power will also be all electric. Depending on the climate, the heating electrical demand may be greater than the cooling electrical demand. When sizing the backup power traditionally, the heating demand does not impact the sizing of the generator as it is served by natural gas. As heating transitions from natural gas to electric, the design team must evaluate how to properly size the backup power capacity. This is likely more critical for laboratories, health care and residential facilities as compared to a standard commercial building, but the design team should discuss with the owner and facilities team what functions of the building must remain operational in the event of a power failure.

Second, the design team must evaluate if the backup power generation source is to remain fuel combustion or also be all electric. Battery energy storage systems may be used as a backup power source. However, depending on the capacity needed, they could be very space-intensive when compared to a typical diesel generator. In addition to space concerns, the total carbon impact over the life cycle should be considered when evaluating battery alternatives.

The impact of significant electric equipment increases at the facility level and is magnified as the scale expands to a campus or development of multiple facilities. Electrical infrastructure at the grid level is impacted by the ever-increasing demand loads.

Further, integration of renewable power production from distributed photovoltaic systems and demand management with battery energy storage systems complicates the electrical system infrastructure and management. Opportunities for microgrid installations and storage solutions benefit a community or campus by intelligently managing and dispatching electric resources. With multiple facilities, shifting investment from electrification of heating on a building-by-building basis to a campus or district-scale solution can leverage increased diversity of building loads.

The simultaneous heating and cooling loads generally have increased overlap. When coupled with large-scale thermal energy storage, campus systems designed as fourth-generation heating and cooling plants offer the benefit of reduced installed equipment, efficient electric heating leveraging heat recovery chillers and reduced peak electric demand compared to multiple stand-alone all-electric buildings.

Electrification of building energy use provides the opportunity to reduce carbon emissions using a renewable energy grid. Designing for zero energy on-site where feasible and cost effective is the priority. For designs where zero energy on-site is prohibitive due to efficiency limits of current technology, site constraints or exorbitant construction costs, building electrification is a way to deliver carbon neutrality.

There are benefits and challenges with electrification of building energy end uses. A fully electric design can be achieved across various climates for all building types. End uses commonly served by gas are space and water heating, cooking and process loads. Space and water heating all-electric design solutions are most impacted by the climate. Solutions such as ASHP, ground-coupled heat pumps and heat recovery can be used to deliver a fully electrified building.

System choice must weigh factors such as energy efficiency, installed cost, operational complexity and physical space requirements to deliver on stated project goals. Electric resistance heat is possible but suffers from poor energy efficiency. This often results in a substantial increase in the electrical service and energy costs.

Mild climates should consider using an ASHP and where available, use a system designed to take advantage of simultaneous heating and cooling. In cold climates where ASHPs capacity and performance is affected by the low ambient conditions, ground-coupled systems have proven reliable, but costly to install. Alternative system designs such as heat recovery via exhaust air or other waste heat sources can be accomplished with marginal construction cost increases, while delivering greater efficiency than the ASHP.

The electric service size impact of electrification of end-uses can be significant. If backup generator power is needed, the increase in demand loads must be considered as a factor in heating system choice. The design should use demand management and load shifting solutions like thermal storage or battery energy storage systems where appropriate to minimize the impact to electrical service.

All-electric design across building sectors has tremendous potential to reduce carbon emissions or set up for a future when the available grid power has been decarbonized. Thoughtful consideration of the climate and building loads can lead to an informed design that is energy efficient, mindful of both construction and operational cost and delivers on the stated emissions reduction goals.