Reducing fossil fuel use is key to meeting the dual goal of carbon and energy cost reduction. A Full Heat Recovery Engagement (FHRE) approach can dramatically reduce both, through applying simple principles and using existing technology. Simple measures can help focus the design of both the buildings served and the systems used to achieve these goals.
The Integration of Heating and Cooling Systems
At one time, a common approach to heating systems was to run steam throughout a building and convert locally to hot water. Generating and distributing low-temperature Heating Hot Water (HHW or LTHW) cuts distribution losses by a factor of two or three. Generating at 130°F (54.4°C) can change boiler efficiency from 75 to 92 percent, saving more than 20% of gas use.
So, what does heat system efficiency have to do with cooling best practices? While condensing boilers were once the foundations of sustainable conversions, they are increasingly being considered a transition step to a fully renewable energy portfolio.
Integration is one of the most effective ways to achieve the aggressive sustainability goals that have been adopted across the country. In this case, integration means the coordination of heating and cooling production and distribution to reduce overall energy use. And reductions can be very significant. The key is the vapor compression cycle — heat pumps and heat recovery chillers. Many have recognized this for some time. The New York State Energy Research & Development Agency (NYSERDA), for example, has recognized that heat pump technology is the key to achieving their sustainability goals.
A quick note on water source heat pumps and Heat Recovery Chillers (HRCHs). The primary difference between heat pumps and HRCHs is that heat pumps use reversing valves on the refrigerant side and keep the same water connections to a heat sink and source. HRCHs use fixed refrigeration circuit and evaporator while the condenser may reject heat to a HHW system, geothermal wellfield or cooling tower. Heat pumps can be used in many configurations. When the building is large enough, HRCHs provide improved reliability and efficiency. We will focus this discussion on HRCHs.
Conventional HRCH applications offset baseloads. The application of HRCHs requires an instantaneous balance of heating and cooling load, or a source/sink acting as a storage system to handle load mismatches. A geothermal wellfield is an example of long-term storage that can also improve the efficiency of the HRCHs. If we restricted operation to native cooling loads only, few buildings would see significant improvements in energy efficiency and meaningful reductions in gas use.
This may be why so many view HRCHs as a luxury that gets value-engineered out of a project. But we have available opportunities to go well beyond these limitations using FHRE. Two projects HGA recently designed include a Midwestern hospital that will see an 85% reduction in gas use, and a West Coast college with the potential to shift 100% of the heating load away from fossil fuels with little building modification.
The Principles Behind FHRE
The most fundamental design factor for chillers is the Condenser Ratio (CR). This is the ratio of energy rejected in the condenser to the energy absorbed in the evaporator during the vapor-compression cycle. CR must be understood to effectively design and control the system. It is even more important when working with HRCHs. Table 1 shows the relationships of several key terms.
Table 1: Basic relationships of terms.
For a centrifugal chiller at typical design conditions CR is about 1.15, while a HRCH operating at 130°F (54.4°C) Leaving Condenser Water Temperature (LCWT) and 42°F (5.5°C) Evaporator out temperature (Tchws) with R-134A is about 1.36. Thus, for this HRCH example, a load match means Heating output, QH = Cooling output, QC * 1.36.
To successfully control this match we need to know how much heating we can derive from a given cooling load and vice versa. We call the degree of correspondence between QH and QC*CR its coincidentality. Creating and maintaining this balance between the heating and cooling loads across the HRCH is the means to achieving energy use improvements we are outlining.
On its own, coincidentality is a subjective term, but we can quantify it using the objective measure: Engageable Load Ratio (ELR). This term was developed to explain varying plant performance as we expanded a CUP at the University of Virginia. While defined for cooling and heating performance, we will only show heating ELR.
|Heating Engageable Load Ratio ELRH =
Engageable Heating Load
ELRH is a function of the building envelope and HVAC system. To calculate ELRH using a peak load, total annual cooling and heating loads or even a bin analysis is meaningless because these energy estimating methods cannot capture the coincidentality of the loads. ELRH must be calculated in an 8,760-hour analysis for the building or system served. When this is done, it becomes clear that we can manipulate ELR to the advantage of more recovered energy. The example of a hospital load profiles helps describe this concept.
Figure 1 is an 8,760-hour plot of the heating and cooling load profiles in kBtu/hr. The cooling load is reduced to a baseload of about 170 tons in the winter and spikes on warm days. The base cooling load can serve hot water heating loads of about:
170 Tons x 12 kBtu/ton x 1.36 = 2,770 kBtu/hr.
Figure 1: Native chilled water and heating hot water load profiles.
Many would size the HRCH heating capacity as the lesser of the base cooling load * CR, or the summer heating load. This would result in a 170-ton HRCH in the heat recovery mode with 2770 kBtu/hr. heating output. For these profiles, ELRH = 0.50 and ELRC = 0.18.
If we stopped here, we would be missing an opportunity to have an even more significant impact on building energy use. Since ELR is a function of the building and HVAC systems, ELRH can be improved if we reconfigure the systems.
Figure 2 shows a few options at the Air Handling Units (AHUs) that can be used to improve the ELRH and take the first step toward a Fully Engaged Heat Recovery System. Examples include use of energy recovery at the AHU, adding a cooling coil in the exhaust and/or relief airstream and incorporating Mixed Air Temperature (MAT) reset to shift from airside economizing to water side economizing. Shifting the load from centrifugal chillers to HRCHs reduces plant condenser water pump and cooling tower fan energy and water use. Bypass dampers can be used to eliminate added pressure drop when the coil is not in use.
Figure 2: FHRE system configuration.
In Figure 1, we can see at present we are limited by the small cooling load below 50°F (10°C). At first, ELRH looks reasonable and could result in a 30% reduction in heating hot water energy use. The ELR, however, cannot stand on its own. The system is limited by HRCH turndown, capacity, temperature limits and reliability, all reducing the savings that can be achieved. We need to know how the equipment will perform in the system. This leads to the concept of Achievable Load Ratio (ALR).
|Heating Achievable Load Ratio ALRH)=
Achievable Engaged Heating Load
If we take the ratio of the two measures, we get the ELR Efficiency — the ratio of Achievable Load to Engageable Load.
ηELRH = ALRH / ELRH
This is a machine-dependent measure of how effective the selected equipment will be in capturing the potential for thermal energy recovery for a given building.
When presented with lifecycle costs, the ηELRH provides a full picture of the cost to achieve a given degree of energy recovery. It also reveals the incremental costs of drawing closer to the site goals.
Figure 3 compares ηELRH for several equipment options that were considered for the University of Virginia. The scroll chillers showed poorly because of reliability problems experienced by the University. The centrifugal options were too large for the application.
Figure 3: ELR efficiency of various HRCH applications.
Calculating ηELRH allows the engineer to compare equipment and understand the source and magnitude of different load components. Taking short cuts reduces the potential for the system and blinds the designer to the insight needed to find potential innovations presented by the project.
In calculating the ηELRH, we can envision the interactions between many different energy savings strategies and see how they would work in aggregate - we can see the impact on the whole system expressed as one factor. Calculating ηELRH will reveal the benefit of a fully sized airside economizer, while providing for waterside economizing at the same time. It will show that adding a cooling coil and bypass to a centralized exhaust system has economic benefit. And we now have a tool to analyze the energy impact of decreasing outside air while increasing minimum volumetric airflow rate or increasing ventilation in response to COVID or the next concern, which could be an external threat.
The steps taken to calculate ηELRH may reveal that driving one factor to its minimum energy use may result in an overall higher energy use. This may be counter-intuitive and seems to violate both energy code and a principle of net zero buildings. When presenting this concept, an objection often raised is that energy code says we cannot use mechanical cooling where we could airside economize. So, why use energy (waterside economizing) when we can do it for free (airside economizing)?
The response is “environmental prudence.” If the right thing to do is to reduce overall energy use, then judiciously creating a cooling load where there was none is the right thing. Using the relationships defined above, the HRCH CR corresponds to a COPH of 3.78 and COPC of 2.78. When serving native loads, the net COP is 6.56. When using waterside economizing instead of airside, the benefit is 3.78/0.92, the COPH/the boiler efficiency. This results in a 75% reduction in energy use, including parasitic losses.
To obtain this benefit, AHU MATs must be coordinated with plant operations. When called for by the control system, a measured amount of Energy Recovery (ER) can be added to the chilled water system load by adjusting the MAT, enabling a response in heating output.
Figure 4 shows two AHUs, both with 33% OA at rated airflow. Outdoor air in the AHU without ER (blue diamonds) ramps down between 55°F (12.7°C)and 35°F (1.6°C)dry bulb. Because the supply airflow is reduced as load drops off, this unit requires 44% OA at reduced airflows and this limits our ability to recover energy from the installed cooling coil when outdoor air temperature is below about 35°F (1.6°C). The AHU with ER (green triangles) will have higher MATs if we don’t bypass the ER device. This allows waterside economizing through the entire heating range. This cooling load can be engaged with native heating loads and this improves the ELRH , ALRH and ηELRH. Reducing the OA recirculates humidified air back into the space, reducing humidification steam requirements.
Figure 4: Outside air control strategies.
One final comment on the implementation: many sites charge buildings for the heating and cooling energy used. Allowances or incentives may be needed to encourage building operators to provide the cooling load as a means of energy recovery when airside economizing is available.
When all opportunities incorporated into the design are modeled, we can see the impact of FHRE on the cooling loads served by the centrifugal chillers and the heating loads served by boilers.
Figure 5 is the Midwestern hospital example showing the peak cooling loads dropping and base and winter loads increasing (due to FHRE). Notice the increase in winter cooling loads with FHRE that become the sizing criteria for the HRCHs. Modeling will reveal the benefit of adding HRCHs that can be weighed against goals and added cost.
Figure 5: Cooling load profile manipulation.
Figure 6 is the heating loads corresponding to Figure 5. It shows the dramatic drop in heating load served by the boilers that can be achieved.
Figure 6: Boiler load profile after FHRE.
Figure 7 shows the resultant energy savings and electrical energy increase for the FHRE. Each energy category is compared using the base case as the 100% value.
Figure 7: Energy and water use reduction. Click here to enlarge.
Simple Measures Generate Results
On your next project, when considering HRCH and heat pumps, show the team what you can accomplish for FHRE by taking simple measures that can help focus the building and system design to reduce carbon and energy costs.
About the Authors
Joe Witchger, PE, is a Senior Mechanical Engineer at HGA, where he specializes in high-efficiency central plants, energy management systems, and mechanical systems optimization.
Brendan Huss, PE, WELL AP, is a mechanical engineer at HGA, where he specializes in analysis and design of central boiler and chiller plants and district heating and cooling distribution systems.
HGA is a national multidisciplinary design firm rooted in architecture and engineering. Ranked in the top ten healthcare firms in the country, HGA strives to deliver innovative, value-based solutions that meet the triple aim of quality in healthcare planning and design: operational efficiency, human experience, and clinical outcomes. More than 850 people in 11 offices from coast-to-coast work to make a positive, lasting impact for clients in healthcare, arts and culture, community, corporate, education, government, science and technology, and energy markets. For more information, visit www.hga.com.
All images courtesy of HGA.
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