On a square foot basis, the food store category, which includes
supermarkets, grocery stores and convenience stores, has the second
highest energy consumption of major building types in California,
with electricity representing over 80% of this. Supermarkets are
open for long hours, and the energy-intensive refrigeration system
must operate continuously in any case.
This newsletter will focus on the refrigeration system as it
consumes the greatest fraction of supermarket electricity use, with
lighting and HVAC each about one-third as large. Also mentioned
briefly are applicable utility incentives, proposed refrigeration
requirements in Title 24, and the analysis of efficiency measures
in energy modeling software.
Supermarket refrigeration systems and how they
consume energy
Numerous components consume energy in a refrigeration system. To
keep product cases cool, fans move air across an evaporator coil
which causes the refrigerant inside it to boil, absorbing heat from
the air circulating in the case. Heat is introduced to the cases
from the warmer store environment, lights, defrost and anti-sweat
heaters, and the fans themselves. The greater these heat gains, the
greater the amount of work the entire system must do. In addition,
three of four of these heat gains are caused by components which
consume electricity themselves. This refrigerant is piped to
compressor(s) which raise its pressure and temperature (consuming
electricity). The hot gas from the compressor is piped to an
outdoor condenser coil where a fan (consuming electricity) moves
outdoor air across coil, cooling the refrigerant back to liquid so
it can return to the cases to repeat this cycle. A typical
supermarket will have many cases connected to a refrigerant
"circuit" and each circuit will usually have multiple compressors.
Three to six circuits will serve each market, each dedicated to
different temperature cases or areas of the store.
| Figure 1: Energy Common Large Supermarket Refrigeration
Diagram |
Source: Supermarket Refrigeration Codes and Standards Enhancement
Initiative (CASE), Pacific Gas and Electric Company, March 17, 2011
pg. 71 |
Alternative refrigeration systems
Recently some variations on this design have emerged in response
to concerns about the global warming contribution of the inevitable
leakage of conventional refrigerants. These involve the use of CO2
or glycol "brines" in place of typical refrigerants in some
components of the systems. As these systems are still rare, they
are not detailed here. Recent analyses have shown that energy use
of these variations range is equivalent or somewhat greater than
conventional systems, but have savings in greenhouse gas emission
(due to reduced refrigerant leakage) that more than offset the GGHG
emissions associated with increased energy use. Many of the
applicable EEMs for typical systems also apply to these variations.
(E.g. reducing case heat gain will reduce energy use in any
system.)
Energy Saving Opportunities
Given the interacting energy consuming components in
supermarkets, many opportunities exist for both energy waste and
savings.
Store Lighting and HVAC
The ambient store lighting is not covered in this E-Newsletter,
as the applicable EEMs are generally the same as those used in
other large retailers. For restaurant operations within
supermarkets, the EEMs are similar to those for any restaurant
operation.) Special HVAC systems are often used to dehumidify air
for supermarkets, but simulation analysis has shown limited savings
for conventional methods of providing this extra dehumidification
in California.
Refrigerated Cases
The entire refrigeration system must be designed to remove heat
that enters or is generated within the cases, so this is the first
place to look for energy savings. Federal standards for new
supermarket refrigerated cases will take effect in 2012, and some
features which have been optional will become standard to meet the
new standard.
Doors and Open Cases: Many case types
never have doors, some may have doors, and others always have
doors. On average, where the choice exists, choosing a case with
doors will save energy, but care must be taken to minimize lighting
and anti-sweat heater energy consumption in cases with doors. The
thermal gains through doors can be reduced by utilizing better
glazing and frames. Condensation on the inside of these doors may
increase, but anti-fog films are available to minimize this. Night
curtains will effectively reduce the loads on open cases during
unoccupied periods, if any. Door gaskets deteriorate with time and
should be replaced when this is apparent.
Anti-Sweat Heaters: In CA, these
heaters, which prevent exterior condensation on doors and cases,
should be needed only occasionally. Controls to make sure these
heaters stay off as long as possible are typically installed but
may not be commissioned or maintained properly. Some newer case
doors are designed with low-conduction glass and frames,
eliminating the need for these heaters, however the cases
themselves will still typically have ASHs, and the store will still
need to control these ASHs.
Case Lighting: Standard lighting in
cases has been florescent, but both LED lighting and fiber-optic
lighting systems are now available with comparatively less energy
consumption. The fiber-optic lighting appears to reduce overall
energy use to the greatest degree, in part because more of its heat
is left outside the case, where it often beneficially warms the
store, instead of adding to case heat gains. Any lighting can be
switched off by a timeclock. LED lighting can be controlled by a
motion sensor and be turned off when the area in front of a case is
vacant, and this may be required in non-24 hr stores under the 2013
Title 24.
Figure 2 - Luminance Image Comparing the Same Case Lighted by
Different Sources
Source: SCE Design & Engineering
Services
Fans, Coils and Defrost: For cases
with similar designs, lower design power for evaporators will
generally minimize energy consumption. Exact determination of fan
power for cases is hindered by lack of consistent ratings. EC
("Electronically Commutated") motors are readily available and are
significantly more efficient than older PSC ("Permanent Split
Capacitor") or shaded-pole motors. These motors can be retrofit to
existing case fans and can be used with variable speed controls in
walk-ins. Floating suction pressure control can raise the
temperature of refrigerant in the coils when the load is light,
saving compressor energy. This requires the use of electronic
expansion controls on the case evaporators, connected to a computer
control system. This has become relatively common in new stores and
may be required in non-24 hr stores under the 2013 Title 24.
Compressors
Centralized supermarket systems are typically served by
reciprocating compressors, with smaller distributed systems often
served by scroll compressors. In all cases, multiple staged
compressors typically serve each circuit. Compressors of similar
sizes are typically of similar design efficiencies for a given
temperature range. Part-load efficiency of various types of
unloading may be examined by simulation, but simulations may not
completely capture the systemic effects of staging.
Condensers
Condensers are available which cool refrigerant with either
outside air alone or outside air and the evaporation of water. For
recent new supermarkets in CA the market is about evenly split
between the two. Under some utility programs, incentives are
available for replacement of air-cooled condensers with evaporative
condensers in dry climates. Particularly during hot, dry weather,
evaporative condensers decrease compressor power demand by
decreasing the temperature and pressure which the compressor must
achieve. They should reduce annual energy consumption, but may not
perform as well as expected at part loads. Condenser fan energy use
is reduced by lower specific fan power (rejected heat/fan power)
and by variable speed fans, and the proposed 2013 Title 24 will
regulate both. Two less common condenser types are water-loop,
where refrigerant condensers are attached to water loop cooled by
central fluid cooler(s) and hybrid, where a condenser which can
switch between evaporative cooling and air cooling.
A tradeoff is always made between the energy use of the
condenser and the compressor, because the condenser fans must run
harder to produce the cooler refrigerant condensing temperatures
and pressures that allow compressors to run more efficiently.
Because the compressors use more energy than the condenser, the
most efficient systems use controls which adjust the pressure the
condenser is trying to achieve in response to outside drybulb or
wetbulb temperature, particularly when the fans are operated by
VSDs.
Heat recovery is the use of additional refrigerant heat
exchangers to reject heat to coils in the store's heating system or
domestic hot water system. These typically save considerable
heating energy, more than offsetting the small compressor power
penalty which will occur while in this heating mode.
System and Piping
While most of the EEMs discussed above relate to individual
components, the overall design of the store and systems will
contribute to the energy consumption. The maximum achievable
efficiency decreases with the temperature of the lowest temperature
case on a system. So in general energy consumption will be
minimized by maximizing the number of circuits as long as this is
done so that the temperatures on each circuit are similar. Greater
pressure drops through components will increase compressor energy
consumption. Some piping designs may decrease efficiency of
condenser operation. Systems are now being installed which
distribute smaller compressor/condenser units around a store roof,
closer to the case groups served, reducing piping runs and
refrigerant inventory.
Liquid-to-suction heat exchangers (LSHXs) cool the hot
refrigerant before it enters a case through heat exchange with cool
refrigerant leaving the case. Simple LSHXs are common, but
high-performance LSHXs significantly improve the efficiency of low
temperature circuits, and some medium temperature circuits.
Mechanical subcooling (use of medium-temperature circuits to
reduce the load on low-temperature circuits) is a common
cost-effective feature in new stores and may be required by the
2013 T24.
Some components in refrigeration systems can be de-activated on
a call for load shedding, qualifying for further incentives.
Defrost can be deferred, and some case lighting can be switched
off.
Retrofits
Energy cost savings can justify the replacement of some
components of the refrigeration system. These are typically either
case components or controls. Published results from a program in
San Diego reported savings averaging 309,000 kWh per market. The
distribution of those savings is shown in the following chart.
| Figure 3 - Distribution of Savings from Retrofit Program* |
|
Source: PECI, Supermarket Controls and Commissioning:
Uncovering Hidden Opportunities, Diane Levin and Lawrence
Paulsen, Portland Energy Conservation, Inc. 2006 ACEEE Summer Study
on Energy Efficiency in Buildings
* ECMs = Electronically Commutated
Motors; ASHC = Anti-Sweat Heater
Control; FHPC = Floating Head Pressure Control; FSPC = Floating Suction
Pressure Control
|
Annual Energy Simulation of Supermarket Systems
In CA, supermarkets have been modeled using the DOE-2.2R program
to determine incentives under the Savings By Design program.
Because of the complexity of refrigeration systems, this modeling
is performed by a few select consultants.
The DOE-2.2R program has also been used to analyze measures for
inclusion into Title 24. The DOE-2.2R program requires numerous
specific inputs for refrigeration components. For instance, the
representation of a fixture or case includes 130 input values,
though not all would be needed for any given case. Some of these
inputs are not clearly evident from manufacturers' literature. The
program also makes assumptions about operation of controls that may
not be always be achieved in actual operation.) All programs share
these issues.
Other Programs
The EnergyPlus program has been updated to include detailed
models of refrigeration system components, libraries of American
components, secondary loops and new refrigerants. The EnergyPlus
representation of components generally requires fewer inputs than
the DOE-2.2R; for instance a case is represented by a maximum of 35
inputs. This program is being used at NREL to model prototype
markets to develop Advanced Energy Design Guidelines for
supermarkets and to model energy efficiency measures for specific
supermarket design projects.
CyberMart is a Swedish program with simplified inputs and
libraries of European components.
| Training Highlights |
|
California utilities offer outstanding educational opportunities
that focus on the design, construction and operation of
energy-efficient buildings. Listed here are a few of the many
upcoming classes and events; for complete schedules, visit each
utility's website.
Energy Auditing Techniques for Small & Medium
Commercial Facilities
The intention of this three-day workshop is to develop the energy
auditing skills of individuals associated with small-and
mediumsized (under 500 kW) commercial facilities. We will provide
an overview of building technologies with an emphasis on
|
distinguishing older, inefficient equipment from newer efficient
systems. In-class exercises and a mock-audit at the end of the last
day will allow the attendees to apply what they have learned.
Though geared to the non-experts, attendees should have a
fundamental understanding of building components and energy
concepts.
January 9-11 (Monday, Tuesday, Wednesday), 8:30 am to 4:30
pm
San Francisco - PEC
register
>
|
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e-News is published by Energy Design
Resources (www.energydesignresources.com),
an online resource center for information on energy efficiency design practices in
California.
Savings By Design (www.savingsbydesign.com)
offers design assistance and incentives to design teams and building owners in California
to encourage high-performance nonresidential building design and construction.
Energy Design Resources and Savings By Design are funded by California utility customers
and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility
District, San Diego Gas and Electric, Southern California Edison and Southern California
Gas Company, under the auspices of the California Public Utilities Commission.
