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San Francisco, CA 94102
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WATER HEATING SYSTEMS MEMORANDUM
Executive Summary
In accordance with Ordinance 155-21 that amends Article 12C of the San Francisco Health Code to require new construction of 100,000 gross square feet (gsf) or greater to install and operate an onsite non-potable water system, the San Francisco Public Utilities Commission (SFPUC) drafted the following report identifying the opportunities for the expansion of water heating systems including, but not limited to thermal solar hot water preheating systems, geothermal hot water preheating systems, graywater preheating systems, and wastewater heat recovery systems.
In this report, staff provide an overview of solar thermal water preheating systems, geothermal hot water preheating systems, wastewater heat recovery systems, including a description of the technology, considerations of the advantages and disadvantages of each technology in the context of San Francisco, considerations for different scales of implementation, and recommended opportunities for implementing these systems. The report also discusses an overview of the SFPUC’s new water conservation rebate program, which will offer a $100 rebate off the purchase cost of hot water recirculating pumps installed in single family and small multi-family properties served by a SFPUC water service account.
The report concludes that solar thermal water preheating systems are well suited for rooftop installations and the technology has a proven track record. Solar thermal systems generate high temperatures making them viable for commercial and residential installations. The report also found that solar thermal systems are affected by fog, so locations on the east side of San Francisco will be better suited for this technology. For facilities where the hot water heating loads are significant, solar thermal hot water systems are recommended when the solar thermal and solar PV systems are not competing for the same roof area.
Wherever roof area is limited, it is recommended to install solar PV due to its better cost economics and versatility.
Geothermal hot water preheating systems operate by using the earth as a source and sink for the thermal energy. These systems operate at high efficiency and have relatively low operation and maintenance cost. However, there are several challenges with geothermal systems when considering implementation in San Francisco. For example, there are high installation costs due to the required trenching for the thermal loop. Additionally, retrofitting existing buildings or in an urban setting with limited open space for an underground thermal loop is
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cost prohibitive and logistically difficult. Therefore, geothermal systems are not recommended in San Francisco due to the large footprint and mild climate resulting in a long payback period.
With wastewater heat recovery systems, thermal energy is extracted from warm wastewater and beneficially reused to offset existing energy requirements.
These systems have the potential to lower the energy use associated with onsite water reuse treatment, especially in residential buildings with large domestic hot water usage. Additionally, wastewater heat recovery systems require relatively constant flow, so coupling with an onsite water reuse system may allow for shared infrastructure. However, adding wastewater heat recovery to a building will add cost and take up additional building footprint. Building integration may also be challenging as close proximity to hot water boilers is preferred.
Furthermore, there are limited examples of the technology being successfully installed and operated in the United States. Therefore, it is recommended that the SFPUC continue requiring all Onsite Water Reuse Grant Program applicants to estimate the potential energy offset that can be achieved with wastewater heat recovery. In addition, it is also recommended that the SFPUC continue requiring mixed-use and multi-family buildings that are grant recipients to implement wastewater heat recovery. This recommendation supports the SFPUC’s goal to gather more information about the potential benefits of these systems and understand how implementation could occur successfully in San Francisco.
1. SFPUC Water and Power Enterprises
The SFPUC is committed to an innovative approach to water and energy resources management. The SFPUC is implementing several programs and projects that are diversifying the City’s water supply portfolio and ensuring resilient water sources for the future. The SFPUC’s water conservation program includes many efforts to help residences and business save water. One example is the Plumbing Fixture Replacement Program, a toilet and urinal direct installation program that connects residences and businesses to plumbers for retrofitting inefficient fixtures. Another example is the Leak Alert Program, which notifies customers of a potential leak by sending an email, text, letter, and phone call and advises to contact a plumber. In addition, the SFPUC is committed to developing local groundwater to enhance the City’s drinking water supply. More information on the SFPUC’s water conservation program is available at https://sfpuc.org/learning/conserve-water. The SFPUC also promotes the use of recycled water for irrigation at Harding Park Fleming Golf Courses, and Sharp Park Golf Course. Additionally, construction continues on the Westside Enhanced Water Recycling Project, which will begin providing recycled water in 2022 for irrigating Golden Gate Park, the Panhandle, Lincoln Park Golf Course, and the San Francisco Zoo. Also underway is a satellite treatment facility feasibility study to evaluate options to deliver recycled water to dual-plumbed buildings on the eastside of San Francisco. In addition, the SFPUC implements the City’s mandatory requirement for onsite water reuse in new large buildings. The SFPUC provides grant funding to encourage retail
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water users to install onsite water reuse systems, as well as encourage the installation of onsite treatment and reuse of brewery process water.
The SFPUC’s Power Enterprise provides numerous customer programs to both Hetch Hetchy Power and CleanPowerSF customers. Through rebates, specially designed rates and services, and other incentive structures, these programs help customers use energy as efficiently as possible, save money, and participate in San Francisco’s clean energy future. Details on these programs can be found at the links below.
• https://sfpuc.org/programs/clean-energy/hetch-hetchy- power/customer-programs
• https://www.cleanpowersf.org/customer-resources
2. Non-potable Water Ordinance
In September 2012, the City and County of San Francisco adopted the Non- potable Water Ordinance (NPO). It added Article 12C to the San Francisco Health Code allowing for the collection, treatment, and use of alternate water sources such as graywater, rainwater, stormwater, blackwater, and foundation drainage for non-potable applications such as toilet flushing and irrigation. The following alternate water sources are defined as:
• Graywater: untreated wastewater that includes, but is not limited to, wastewater from bathtubs, showers, bathroom sinks, clothes washing machines, and laundry tubs, but does not include wastewater from kitchen sinks or dishwashers.
• Blackwater: includes graywater and is defined as wastewater containing bodily or other biological wastes, as from toilets, dishwashers, kitchen sinks and utility sinks.
• Rainwater: precipitation collected from roof surfaces or other manmade, above ground collection surfaces.
• Stormwater: precipitation collected from at-grade or below grade surfaces or from any surface where hydrocarbon-based fuels, hazardous materials, or fertilizers are stored or used. • Foundation drainage: nuisance groundwater that is extracted to maintain a building's or facility’s structural integrity and would otherwise be discharged to the sanitary or combined sewer system. • Condensate: Water vapor collected from air conditioning systems.
A streamlined permitting process was developed through a joint collaboration with the SFPUC, San Francisco Department of Public Health (SFDPH), San Francisco Department of Building Inspection (SFDBI), and San Francisco Department of Public Works (SFPW). Water quality, monitoring, and reporting
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requirements were established, giving regulatory oversight and management over onsite non-potable water systems to the Department of Public Health.
In 2013, the Board of Supervisors amended the NPO to allow district-scale non- potable water systems consisting of two or more buildings sharing non-potable water. Subsequently, the Ordinance was amended in July 2015 to mandate the installation of onsite water systems in new developments of 250,000 gross square feet or more. And the Ordinance was amended again in 2016 to clarify implementation requirements for district-scale systems.
Most recently in October 2021, the Board of Supervisors amended the NPO to further increase opportunities for potable water savings from new developments and streamline the requirements. The ordinance amendments require new development projects that apply for a site permit after January 1, 2022 of 100,000 gross square feet or more to install and operate an onsite water reuse system. The required alternate water sources and required non-potable uses are based on development project type. For commercial buildings, the project must meet its toilet and urinal flushing and drain trap priming demands through the collection, treatment, and use of available blackwater and condensate. For residential and mixed-use buildings, the project must meet its toilet and urinal flushing, irrigation, clothes washing, and drain trap priming demands through the collection, treatment, and use of available graywater and condensate. The requirements apply to both development projects consisting of a single building or multiple buildings.
3. Onsite Water Reuse Grant Program
The SFPUC’s Onsite Water Reuse Grant Program provides grant funding to encourage retail water users to reduce SFPUC water supply usage by collecting, treating, and using alternate water sources including rainwater, stormwater, graywater, foundation drainage, air conditioning condensate, and blackwater for non-potable uses such as toilet flushing, irrigation, and cooling tower makeup.
Grant applicants must demonstrate the ability to achieve at least one of the following thresholds to eligible for grant funding:
• Projects that replace at least 450,000 gallons of SFPUC water per year are eligible for grant funding up to $200,000; or
• Projects that replace at least 1,000,000 gallons of SFPUC water per year are eligible for grant funding up to $500,000; or
• Projects that replace at least 3,000,000 gallons of SFPUC water per year are eligible for grant funding up to $1,000,000.
For context, the SFPUC lowered the threshold of eligibility to 450,000 gallons of water per year offset to incentivize existing dual-plumbed buildings with no current recycled water source to install onsite water reuse systems. Recognizing
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that these buildings would likely otherwise not install onsite water reuse systems, the SFPUC is offering financial assistance to encourage additional potable water savings.
Additionally, projects that are installing an onsite water reuse system on a mandatory basis in compliance with the Non-potable Water Ordinance can apply for a grant if water savings can be demonstrated above and beyond the requirements. As an example, a mandatory project consisting of a commercial building reusing blackwater and condensate for toilet and urinal flushing and drain trap priming could elect to use excess available blackwater to meet cooling tower make-up water demands.
Furthermore through the Onsite Water Reuse Program, the SFPUC is incentivizing the implementation of wastewater heat recovery systems within development projects installing onsite water reuse systems. The Grant Program requires all projects applying for a grant to estimate the potential energy offset that can be achieved with wastewater heat recovery. In addition, mixed-used and multi-family buildings that are grant recipients are required to implement wastewater heat recovery. SFPUC’s goal in integrating wastewater heat recovery into the Grant Program is to gather more information about the potential benefits of these systems and understand how implementation could occur successfully in San Francisco.
4. Solar Thermal Water Heating Systems
Solar thermal technology is a well-established means of heating domestic hot water. Over the last 40 years the technology has been refined and improved upon. Today’s solar thermal products follow established codes, certifications (Solar Rating and Certification Corporation (SRCC)), and make use of modern engineering practices.
For the purposes of this document, Flat Plate Collector (FPC) and Evacuated Tube collectors are considered. These two technologies are commercially available, have proven track records, and meet commercial and residential water heating needs. FPC and Evacuated Tube collectors rely on standard solar principles such as good sun exposure and southern orientation. These collectors are manufactured in various sizes; 4’ x 10’ is a common size for commercial installations. Both collector types are comprised of copper tubing (which contains the fluid that is heated by the sun) inside an insulated glass enclosure.
These systems are very modular; collectors can be added together in groups to increase the system size depending on the energy production requirements for a given site. These characteristics make FPC and Evacuated Tube collectors ideal for roof top installations.
In San Francisco’s climate, FPC and Evacuated Tube collectors are capable of heating water to 140 degrees Fahrenheit. Using industry standards, an estimate can be made of the energy production of a solar thermal system in San Francisco. A FPC and Evacuated Tube collector will generate about 1,000
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BTU’s per square foot of collector on a sunny day (850-1,000 watts/meter squared). Therefore, a standard sized 4’ x 10’ collector will produce about 40,000 BTU/day (which is equivalent to 12 kWh). A solar thermal system will need a conventional electric or natural gas-based back-up system to supplement it during low solar irradiation days or if a higher water temperature is needed.
Pricing for commercial systems is based on a price per square foot of collector.
In San Francisco, prices range from $190 to $250 per square foot of collector, depending on site conditions and project complexity. For example, in 2020, a commercial solar thermal system (with 16 collectors) was installed at a low- income multifamily housing development. The total project cost was $152,885, or $237 per square foot of installed collector. The cost of a fully installed residential system in San Francisco is typically between $6,500 and $14,000.
Solar thermal systems are more efficient, require smaller roof area, and cost lesser than solar photovoltaics. However, hot water loads in commercial and residential buildings are typically a smaller fraction of the total building load and the utility cost reduction resulting from solar thermal systems is not considerable. Also, solar thermal systems have a shorter lifetime when compared to PV systems due to mechanical component failures and fouling of the heat exchangers.
System sizing and cost savings can be calculated based on the energy needs of a business or residence and current utility rates. There are several online calculation tools that are free to users. Links to two examples are shown below. • https://www.eere.energy.gov/femp/solar_hotwater_system/ • http://www.freehotwater.com/solar-calculators/solar-thermal- calculator/
Below are some considerations for San Francisco businesses to factor into their evaluation process when looking into FPC and Evacuated Tube solar thermal systems.
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Table 1. Considerations for evaluating solar thermal systems in San Francisco Pros Cons
• Solar thermal systems are very well
suited for rooftop installation.
• These systems generate high
temperatures making them viable for
many commercial and residential needs.
• The products have a proven track record
and are commercially available.
• Energy production can be estimated with
a high degree of certainty allowing
businesses and home owners to better
understand what their investment can do
to reduce energy demand and increase
savings.
• Solar thermal systems are affected by
fog, so locations on the east side of San
Francisco will be better suited for this
technology.
• Solar professionals that are qualified to
work on solar thermal systems are
limited in the Bay Area. Finding the right
company to do the installation will take
some research.
• Solar Thermal systems will not eliminate
100% of the annual domestic hot water
heating needs. These systems need a
backup source of energy in times of the
year when there is not enough sun to
cover the water heating needs.
• The proposed site for a solar thermal
installation should have minimal shading
and a southern exposure.
Recommendations for Solar Thermal Systems
• For facilities where the hot water heating loads are significant, solar thermal hot water systems are recommended when the solar thermal and solar PV systems are not competing for the same roof area.
Wherever roof area is limited, it is recommended to install solar PV due to its better cost economics and versatility.
• It is recommended that solar thermal systems may be encouraged but not required at commercial or residential sites.
5. Geothermal Systems
Geothermal energy is the third largest source of renewable energy in the world, behind hydropower and biomass. The term “geothermal systems” encompass both indirect (power generation) and direct (heat transfer) applications.
Geothermal power generation requires a high temperature resource which is typically found in tectonically active areas, such as the Geysers near Santa Rosa. The scope of this document is limited to low temperature heat transfer applications for use in heating and cooling of buildings.
The term ‘geothermal heat pump’ (GHP) has become an all-inclusive term to describe a heat pump system coupled with a groundwater, surface water, or other earth-based heat exchange loop that uses the relatively constant
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temperature of the earth as a heat source and/or sink, to provide heating, air conditioning and, in many cases, hot water.
The type of GHP is characterized by how the system exchanges heat with the earth or water body. In all systems, a fluid is circulated between the heat pump and the earth or water body to exchange heat. Open loop systems directly utilize well or surface water. Closed loop systems recirculate refrigerant or a water mixture in an isolated loop with a heat exchanger that does not mix with ground or surface waters. Closed loop systems can be constructed as horizontal, vertical, or submerged in a body of water. Closed loop systems would be most applicable to applications in San Francisco.
Figure 1. Example configurations of geothermal heat pump systems applicable to San Francisco
Vertical Closed Loop Geothermal System Surface Water Closed Loop Geothermal System
GHP systems are known for their higher energy efficiency, lower maintenance, extreme durability, and minimal environmental impact when compared to conventional HVAC systems. The notable drawbacks of geothermal systems are the large installation cost and footprint required. Additionally, GHP systems operate at peak efficiencies where the heating and cooling loads are large and balanced. San Francisco’s mild climate as well as the typical load imbalance in commercial and residential building types require a higher level of engineering to design and install efficient and sustainable systems.
A significant part of the installation cost is from the ground loop since it involves drilling. A US-DOE sponsored study, under the GHPsRUS project, published “Measuring the Costs and Benefits of Nationwide Geothermal Heat Pump Deployment” that examined the relationship of geographic location to installation costs across 30 Metropolitan areas in the US. Western states (all
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states to the west of the Mason-Dixon line) accounted only for 14% of the market. Median price for residential systems in the West was the highest among the four census groups considered at $6,116 per ton when compared to the national average of $5,663. There were no data reported for commercial installations hinting at weak regional market adoption for these systems. Federal Investment Tax Credits (ITC) are available but other than that no other state or utility incentives exist for geothermal systems in California except at Plumas- Sierra Rural Coop where they offer a rebate of $600 per ton.
Present day geothermal systems make use of a secondary heat exchanger called the desuperheater on the refrigerant loop that makes use of waste heat to produce 130F+ temperature hot water whenever the heat pump is operating. A GHP can produce between 4 and 8 gallons of hot water per ton of cooling during peak cooling season (summer), but its hot water production will be low or minimal during peak heating (winter), and shoulder seasons (fall and spring).
As such, such a system needs to be complemented by conventional water heating system. The desuperheater does not affect the primary function or performance of the GHP in anyway but can reduce water heating costs by about 50-60%.
According to the International Ground Source Heat Pump Association (IGSHPA), GSHPs can offer between 25 and 50 percent savings on heating and cooling costs compared to conventional fossil fuel systems. According to the Department of Energy the payback period for a typical geothermal heat pump system, is between 5 and 10 years, depending on the cost of energy and available incentives but the payback periods tend to be longer for systems in California (30+ years). System life is estimated at up to 25 years for the heat pump and 50+ years for the ground loop.
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Table 2. Considerations for evaluating geothermal systems in San Francisco Pros Cons
• GSHPs operate at very high efficiency.
• These systems have relatively low operation
and maintenance cost due to very few
mechanical components.
• GSHP’s design life is very long. The
underground thermal exchange loop uses
extremely durable materials which have 50+
years while most heat pumps have 25-30
years of life.
• The system operates by using the earth as a
source and sink for the thermal energy which
is a very stable resource.
• Free domestic hot water (with desuperheater)
is produced as a by-product by GSHP when
the system is space-conditioning.
• GSHPs incur high installation costs due
to the required trenching and drilling
for the thermal loop.
• Depending on the system choice of
horizontal or vertical loops, large
footprint may be required.
• One of the important aspects for
sustainable operation of GSHPs over
the long term is reservoir thermal
maintenance. This requires complicated
simulation and design studies with
sophisticated tools.
• Geothermal system design is a niche
expertise (design engineers and sub
surface drillers).
• Retrofitting existing buildings or in an
urban setting where there is limited
open space for the underground
thermal loop will be cost prohibitive
and logistically difficult.
• While the buried infrastructure is
typically made of very durable
materials, repairing these would be
costly if something goes wrong with
them.
• GSHP systems will have long payback
in mild climates.
Recommendations for Geothermal Systems
• Geothermal systems for heating, cooling, and hot water needs at residential and commercial facilities will be challenging and are not recommended in San Francisco due to the large footprint requirement and mild climate.
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6. Wastewater Heat Recovery Heating Systems (Graywater Or Blackwater)
There are a number of uses of hot water in buildings, including showers, tubs, sinks, dishwashers, and clothes washers where the wastewater retains a significant portion of its initial energy. Wastewater heat recovery refers to the extraction of thermal energy from warm wastewater, and subsequent beneficial use of this energy to offset existing energy requirements. There are several ways to transfer this heat from wastewater to domestic hot water. Heat from wastewater can be removed from the wastewater drain system or from an onsite water reuse system, if available, and used to pre-heat or maintain heat for domestic hot water systems. This type of technology is typically installed in facilities with large domestic hot water (DHW) usage, such as residential buildings and hotels. Incorporating wastewater heat recovery into mechanical systems to serve a building’s space heating and/or cooling demands expands the potential viability of this technology into other building types with lower DHW demands.
Table 3. Average Hot Water Use by Building Type
More broadly, decentralized wastewater heat recovery using heat pumps is relatively new in the U.S. with limited application at both the building and district scale. Typically, these systems have been implemented as demonstration projects and, to date, have not resulted in any known formal regulatory processes being developed or widespread adoption. Emerging research out of the USEPA analyzes the environmental and economic suitability of onsite non- potable reuse for large buildings, with specific emphasis on graywater membrane bio-reactors (MBR) and wastewater MBRs. The research suggests that incorporating thermal heat recovery in these onsite wastewater systems can reduce life-cycle costs of the system by up to 10%. Additionally, the research suggests that thermal recovery can reduce the environmental impacts of MBR systems across the board, with savings increasing incrementally based on the carbon intensity of the electrical grid (the power source being offset). The SFPUC serves over 70 percent of the electricity consumed in San Francisco between its two power programs, Hetch Hetchy Power and CleanPowerSF with minimal to no greenhouse gas emissions. Therefore, the environmental impact reductions may be less than a geography with more carbon intensive grids.
Given the relative maturity and specific use cases it is not recommended to require wastewater heat recovery systems. Longer term, high performing
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buildings may utilize the water coming in to a building, circulating through a reuse system, or exiting the building as part of a comprehensive strategy to minimize energy use. Wastewater heat recovery and rejection makes the most sense when incorporated into a holistic assessment of a building’s energy demands and design, not being limited to domestic hot water heating. It is recommended the NPO require new development projects that apply for a site permit after January 1, 2022 of 100,000 gross square feet to evaluate the potential of water or wastewater heat recovery as part of the permitting process.
With the considerations outlined above including space constraints and use case limitations San Francisco can begin requiring architects and Mechanical, Electrical, and Plumbing Engineers (MEPs) to conduct a limited review to evaluate wastewater heat recovery or rejection potential as part of a building’s energy analysis for larger projects during the design phase. Using this limited evaluation approach creates a potential path to more widespread commercial adoption through awareness in the design community to include the most effective technologies and approaches to achieve community and program decarbonization goals.
Single-Family Residential Applications: Drain-Water Heat Exchanger When installed in a single-family home application, typically heat is recovered from hot water used in showers, bathtubs, sinks, dishwashers, and clothes washers to preheat water going to the hot water tank. This drain-water heat exchanger (or gravity-film heat exchanger) usually has a copper heat exchanger that replaces a vertical section of a main waste drain. For a non-storage system, as warm water flows down the waste drain, incoming cold water flows through a spiral copper tube wrapped tightly around the copper section of the waste drain. For storage-type systems, the drain water flows through a spiral tube at the bottom of the clean water tank to warm the tank water. By preheating cold water, drain-water heat recovery systems help increase water heating capacity.
Figure 2: Drain Water Copper Heat Exchanger Schematic
The best opportunity to install a drain-heat recovery system in a single-family home application would be during new construction or a major renovation. This is a minimal market since there are limited new single family home development opportunities in San Francisco. Application of this technology in
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existing homes would most likely be limited due to technical or integration challenges around physical space constraints, relative complexity integrating this type of installation into construction and limited total energy available for recovery. There are commercially available simple heat exchangers that are available to retrofit for less than $1,000; however, given the space and retrofit constraints and using a multiple of equipment cost estimating approach, it is possible that total installed costs are 2-5X the cost of the heat exchanger itself.
It is not recommended to require installation of drain-heat recovery systems in San Francisco single-family homes.
Building Scale Applications (Multi-Family Residential, Mixed Use, and Commercial): Heat Exchanger Coupled with Heat Pump:
At the individual, larger building scale, the heat recovered from a building’s wastewater (blackwater or graywater) typically involves a heat exchanger and a specialized water-to-water heat pump that recovers energy from the warm wastewater. The recovered heat can then be used to either pre-heat potable water that is then sent through a hot water boiler, to maintain the temperature of water that has already been heated and is being stored in a hot water storage tank, or for space heating and/or cooling demands.
Common components of wastewater heat recovery systems include:
• Consistent Wastewater Source: Typically a holding or equalization tank used to collect the building’s raw wastewater upstr eam of a wastewater heat recovery system for the purpose of supplying a consistent flow.
There are also emerging technologies that utilize the adjacent municipal sewer system as the wastewater source.
• Solid-Liquid Separation: an initial process step whereby solids are removed from the liquid portion of the wastewater prior to the liquid being sent to the heat exchanger and/or heat pump.
• Heat Exchanger: a device for passively transferring heat between two or more fluids. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. In a heat exchanger, heat will move spontaneously from the hotter fluid into the colder fluid. • Heat Pump: a device that actively transfers heat from a colder fluid to a hotter fluid, i.e. in the opposite direction of spontaneous heat transfer. In a heat pump, an additional energy source (typically electricity) is needed to facilitate the heat transfer. Heat pump efficiency is defined by a coefficient of performance (COP), which describes the ratio of energy recovered to external energy used. Generally, a COP above 3 is deemed fairly efficient.
In general, these types of systems experience the highest efficiency when the wastewater storage tank, the heat exchanger, and the hot water boiler are located in close proximity to one another and are integrated into comprehensive system design that likely has to happen early in the design process.
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Figure 3. Wastewater Heat Recovery Schematic
Research shows that multi-family and mixed-use buildings present the best opportunity for wastewater heat recovery because they are most likely to have centralized hot water boilers. There can be a significant volume of graywater and thus more energy to recover via wastewater heat recovery. Commercial buildings are not optimal for the installation of wastewater heat recovery because they do not tend to have centralized hot water systems, and so using the heat recovered from wastewater becomes more challenging because it would require integration into the building’s mechanical system. When heat exchangers are coupled with a hydronic loop, the wastewater to be used as either a thermal source or a thermal sink.
The following example provides context for the potential energy savings that could be achieved in a multi-family residential building with separate graywater and blackwater collection systems, based on the following assumptions:
• Assumed graywater as source water, as is required by the NPO • Assumed 15 gpy/sf of graywater generation for primarily residential buildings, based on SFPUC’s Single-building Water Use Calculator • Assumed a 15°F drop in graywater temperature through the heat recovery system
Under these conditions, a theoretical 250,000 square foot residential building could recover 300 kWh/day using wastewater heat recovery. This type of building is likely to use in total somewhere in the range of 10,000 kWh/day (Energy Star Portfolio Manager, 2018). Although the recovered energy is a small percentage of the overall building energy demand, it is likely to be a significant portion of the energy used for onsite water reuse treatment. The
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assumed potential temperature differential would be smaller for a blackwater system due to a large portion of the building wastewater coming from toilet flushing, which is cold or temperate water, therefore energy recovery would be lower.
District-Scale Application
Alternatively, district-scale wastewater heat recovery systems can provide domestic water heating, space heating, and space cooling to entire neighborhoods. The district-scale model is based around a centralized treatment facility, which acts as the energy and wastewater hub for a community and distinguishes district-scale systems from those installed in individual buildings.
Incorporating wastewater heat recovery into district-scale water reuse system would require projects to include a district energy system that could be used to distribute the recovered heat to individual buildings. This would provide a larger thermal energy opportunity while introducing a significant level of complexity, requiring a central energy plant, as well as additional infrastructure in the streets and likely in each building.
Wastewater Heat Recovery Application in San Francisco’s Onsite Water Reuse Grant Program
In an effort to consider ways to reduce energy footprint of onsite water reuse systems, SFPUC updated its existing Onsite Water Reuse Grant Program in 2019 to incentivize the implementation of wastewater heat recovery systems within development projects installing onsite water reuse systems. The grant program requires all projects applying for a grant to estimate the potential energy offset that can be achieved with wastewater heat recovery. In addition, mixed-use and multi-family buildings that are grant recipients are required to implement wastewater heat recovery. SFPUC’s goal in integrating wastewater heat recovery into the grant program is to gather more information about the potential benefits of these systems and understand how implementation could occur successfully in San Francisco.
There may be a synergy to integrating wastewater heat recovery at buildings with onsite water reuse systems. Equalization tanks upstream or downstream of the onsite water treatment system may be leveraged for wastewater heat recovery to provide a consistent flow to the wastewater heat recovery system.
Treated water storage tanks downstream of the treatment system may be preferred to enable heat recovery from a much cleaner stream (thereby reducing challenges for the equipment because of solids, oils, grease, etc) however, the thermal heat differential may be higher upstream of the onsite water treatment system. In addition to energy recovery, cooling down the treated water has several benefits, including improved efficiency in cooling towers and improved control of Legionella growth in premise plumbing. When considered holistically in design, onsite water reuse systems may provide a heat source or sink that can be used in heating and cooling.
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Emerging Technologies and Case Studies
There are a few emerging companies that offer packaged wastewater heat recovery systems that can be compact and self-contained. For example, SHARC Energy Systems Inc. offers the Piranha packaged system, which is intended for use in buildings with 50 – 200 units (International Wastewater Systems, 2014).
The PIRANHA Wastewater Heat Recovery & Cooling System is a specialized water-to-water heat pump that recovers energy directly from wastewater and uses this source energy to produce hot water. The hot water produced through the unit is typically used for potable Domestic Hot Water (DHW) purposes, however it may also be used for hydronic heating or alternate hot water demand purposes.
Figure 4. Example Building Wastewater Heat Recovery System Diagram
Another packaged system offering out of Germany is the HUBER ThermWin system. The system uses a heat exchanger to extract the energy contained within pre-screened wastewater paired with a heat pump and can be integrated with a building’s domestic hot water system or coupled to the HVAC system (HUBER Technology, 2018). Using wastewater from the sewer (that would have higher thermal availability due to the flow), the HUBER ThermaWin supplies a building’s heating and cooling.
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Figure 5. Example Municipal Sewer Wastewater Heat Recovery System Diagram
Table 4. Examples of Wastewater Heat Recovery Installations Project Summary Technology
Lake Louise Inn
Banff National Park
Heat is recovered from hotel’s laundry
wastewater to offset energy required by hotel to
heat water for laundry
SHARC
Wall Center
Vancouver, Canada
Heat is recovered from multi-family residential
building’s wastewater to offset energy required
for domestic hot water
SHARC
The Sail Project
University of B.C.,
Canada
Heat is recovered from multi-family residential
building’s wastewater to offset energy required
for domestic hot water
SHARC
False Creek
Neighborhood Energy
Utility (NEU)
Vancouver, Canada
Heat is recovered from municipal wastewater to
support a large-scale district heating network for
building heating and hot water
SHARC
Toronto Western Hospital
Toronto, Canada
Heat is recovered from building wastewater and
the sewer system to cover approximately 85% of
hospital’s heating and cooling needs
HUBER
New retirement and
nursing home Hofmatt
Heat is recovered from wastewater from multiple
buildings and used for water heating and hot
HUBER
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Münchenstein,
Switzerland
water supply for same buildings
AGU Headquarters
Washington D.C.
First "net-zero" office building in Washington,
D.C. will utilize wastewater from the municipal
sewer as a heat source and heat sink for the
building’s heating and cooling plant
HUBER
Table 5. Considerations for evaluating wastewater heat recovery systems in San Francisco
Pros Cons
• There’s potential to utilize thermal
energy in wastewater as a heat source or
heat sink.
• Wastewater heat recovery systems
require relatively constant flow, so
coupling with an onsite water reuse
system with equalization may allow for
shared infrastructure.
• If reviewed or incorporated early in the
design process, wastewater heat recovery
may support efficient thermal transfer in
a building for cooling and heating or a
comprehensive energy minimization
approach.
• Wastewater heat recovery has the
potential to lower the energy use
associated with onsite water reuse
treatment. The biggest opportunity is in
multi-family residential and mixed-use
buildings with large domestic hot water
(DHW) usage rates.
• Adding wastewater heat recovery to a
building will add cost and take up
additional building footprint.
• Building integration may be challenging
as close proximity to hot water boilers or
building HVAC is preferred.
• Implementation may require
comprehensive evaluation of mechanical
systems.
• Standalone heat recovery systems (i.e.
heat only) may have smaller impacts on
energy footprint and wastewater heat
recovery will not eliminate the domestic
hot water heating needs. Wastewater heat
recovery would work best when
integrated with a hydronic loop.
• System capabilities might require
additional infrastructure to meet building
loads (i.e. potential need for redundant
systems on peak days).
• There are limited examples of the
technology being successfully installed
and operated in the United States. Full
building integration may have limited
use cases.
• Including wastewater heat recovery in a
district-scale system can be highly
complex, as it would require a district
energy system to recover and distribute
the energy.
19 January 4, 2022
Recommendations for Further Integrating Wastewater Heat Recovery Benefits
• It is recommended that the SFPUC continue requiring all Onsite Water Reuse Grant Program applicants to estimate the potential energy offset that can be achieved with wastewater heat recovery. In addition, it is also recommended that the SFPUC continue requiring mixed-use and multi-family buildings that are grant recipients to implement wastewater heat recovery. This recommendation supports the SFPUC’s goal to gather more information about the potential benefits of these systems and understand how implementation could occur successfully in San Francisco.
• It is recommended the NPO require new development projects that apply for a site permit after January 1, 2022 of 100,000 gross square feet to evaluate the potential of water or wastewater heat recovery as part of the permitting process to create a potential path to more widespread commercial adoption through awareness in the design community to include the most effective technologies and approaches to achieve community and program decarbonization goals.
• It is not recommended to require installation of drain-heat recovery systems in San Francisco single-family homes because there are limited opportunities for new single family home development in San Francisco, the technical and integration challenges around physical space constraints, the relative complexity integrating this type of installation into construction, and the limited total energy available for recovery. • It is not recommended to require commercial buildings to install wastewater heat recovery systems because they do not tend to have centralized hot water systems; therefore, using the heat recovered from wastewater becomes more challenging because it would require integration into the building’s mechanical system.
• It is not recommended to require district-scale wastewater heat recovery as it would require a district energy system to recover and distribute the energy, which would be highly complex.
7. On-Demand Hot Water Heater Rebates
The SFPUC is launching a new water conservation rebate program offering a $100 rebate off the purchase cost of hot water recirculating pumps installed in single family and small multi-family properties served by a SFPUC water service account. Customers must meet all eligibility requirements, submit a complete and accurate application, and provide proof of purchase and installation to be approved for a rebate. Hot water recirculating pumps reduce wait times for hot water to arrive at showerheads and taps and may help customers save water otherwise wasted. Pumps are either installed at water
20 January 4, 2022
heaters or under kitchen or bathroom sinks that have electrical outlets. Once activated, they quickly draw hot water from the water heater, while simultaneously sending cool water from the hot water lines back to the water heater.
8. Summary of Recommendations
SFPUC staff have identified the following recommendations for implementation of solar thermal water preheating systems, wastewater heat recovery systems, and geothermal hot water preheating systems in San Francisco:
• It is recommended to install solar thermal hot water systems when solar thermal and solar PV systems are not competing for the same roof area.
When roof area is limited, it is recommended to install solar PV due to its better cost economics and versatility in facilities where the hot water heating loads are significant.
• It is recommended that solar thermal systems be encouraged, but not required at commercial or residential sites.
• It is recommended that the SFPUC continue requiring all Onsite Water Reuse Grant Program applicants to estimate the potential energy offset that can be achieved with wastewater heat recovery. In addition, it is also recommended that the SFPUC continue requiring mixed-use and multi-family buildings that are grant recipients to implement wastewater heat recovery. This recommendation supports the SFPUC’s goal to gather more information about the potential benefits of these systems and understand how implementation could occur successfully in San Francisco.
21 January 4, 2022
References:
Air Conditioning, Heating, and Refrigeration Institute. (2021) Home.
Arden, S., B. Morelli, S. Cashman, X.C. Ma, M. Jahne, J. Garland. (2020) Onsite Non-Potable Reuse for Large Buildings: Environmental and Economic Sustainability as a Function of Building Characteristics and Location. Water Research, not yet published.
Battocletti, E., and W. Glassley. (2013) “Measuring the Costs & Benefits of Nationwide Geothermal Heat Deployment.” United States: N. p., 2013. Web. doi:10.2172/1186828.
California Energy Commission. (2012) Assessment of California’s Low Temperature Geothermal Resources: Geothermal Heat Pump Efficiencies By Region.
California Geothermal Heat Pump Association. (2021) Geo Basics.
Carmichael, J., and Y. Suda. (2015) "Heat-Seeking Sewer Model: Finding Waste Heat in Sewers and Matching It to Opportunities." District Energy: 2015 4th Quarter, 29-33.
Dandelion. (2021) Understanding the Federal and State Incentives for Geothermal Heat Pumps.
Dodge, D., and D. Kinney. (2014). “Waste Heat: How Vancouver Mined Its Sewage to Heat an Entire Neighborhood.” Green Energy Futures.
Energy Star Portfolio Manager. (2018) U.S. Energy Use Intensity by Property Type.
Go Solar California. (2021) Welcome to California Solar Thermal Statistics.
HUBER Technology. (2018) Heat Recovery from Wastewater HUBER ThermWin.
HUBER Technology. (2021) HUBER ThermWin Utilizes Wastewater Heat.
International Ground Source Heat Pump Association. (2021) Geothermal.
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Kaufman, Rachel. (2012) “Waste Wattage: Cities Aim to Flush Heat Energy Out of Sewers.” National Geographic News 11 December 2012.
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Natural Systems Utilities. (2019) NSU Thermal Energy Recovery, Water/Energy Nexus.
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Sharc Energy. (2021) Piranha Wastewater Heat Recovery System.
U.S. Department of Energy. (2020) Drain-Water Heat Recovery.
U.S. Department of Energy. (2021) Geothermal Heat Pumps.
U.S. Department of Energy. (2021) Geothermal Technologies Office.
Wallin, J. (2021). Case Studies of Four Installed Wastewater Heat Recovery Systems in Sweden. Case Studies in Thermal Engineering, 26(101108). https://doi.org/10.1016/j.csite.2021.101108.