Energy efficiency opportunities in the U.S. commercial baking industry

Journal of Food Engineering 130 (2014) 14–22

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Energy efficiency opportunities in the U.S. commercial baking industry Peter Therkelsen a,⇑, Eric Masanet b, Ernst Worrell c a

Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL, USA c Copernicus Institute of Sustainable Development, Utrecht University, Utrecht, The Netherlands b

a r t i c l e

i n f o

Article history: Received 18 July 2013 Received in revised form 29 December 2013 Accepted 3 January 2014 Available online 11 January 2014 Keywords: Commercial baking Energy efficiency Energy Food processing Energy management systems

a b s t r a c t Commercial bakery products in the United States such as breads, rolls, frozen cakes, pies, pastries, cookies, and crackers consume over $870 million of energy annually. Energy efficiency measures can reduce the energy costs of significant energy processes and increase earnings predictability. This article summarizes key energy efficiency measures relevant to industrial baking. Case study data from bakeries and related facilities worldwide are used to identify savings and cost metrics associated with efficiency measures. While the focus of this paper is on U.S. bakeries, findings can be generalized to bakeries internationally. A discussion of energy management systems is provided and how energy efficiency measures savings can be sustained. Energy and plant managers at bakeries can use this information to costeffectively reduce energy consumption while utilities and policy makers can apply the findings to energy efficiency program design. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Manufacturers worldwide are considering energy efficiency measures as a way to mitigate the impact of volatile energy markets, growing competition, and regulation of greenhouse gas (GHG) emissions. As part of an overall environmental strategy, implementation of energy efficiency measures often leads to reductions in GHG emissions and other air pollutants. Investments in energy efficiency are a sound and key business strategy in today’s manufacturing environment. Additionally, manufacturers are implementing energy management systems as a way to sustain gains realized from energy efficiency measures. Improvements in energy performance translate into continual increases in product quality, production, and process efficiency. Within the United States, commercial bakeries produce fresh and frozen breads, rolls, cakes, cookies, and other pastries. The North American Industry Classification System (NAICS) code system divides the commercial baking industry into three sectors. These sectors are listed in Table 1 along with the primary products produced by each, which highlights the diversity of products manufactured by the industry. Modern large commercial bakeries use highly automated processes during production. When operating at full capacity, a single large bread bakery may produce up to 136,000 kg of over 100 different varieties of bread and other bakery products per day. ⇑ Corresponding author. Address: One Cyclotron Road, MS 70-108B, Berkeley, CA 94720, USA. Tel.: +1 510 486 5645. E-mail address: [email protected] (P. Therkelsen). 0260-8774/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2014.01.004

All physical mixing and blending of ingredients, as well as the working and dividing of the dough, is performed mechanically. Most dough batches are mechanically conveyed through each step of the baking process, from the initial dividing through the final slicing and bagging, with minimal handling (U.S. EPA, 1992). Table 2 summarizes key economic and energy purchase data for the two major segments of the U.S. baking industry disaggregated by five digit NAICS code. Employment in 2010 exceeded 230,000 and the total value of product shipment exceeded $53 billion – numbers that underscore the significant contributions made to both U.S. employment and economic output. Over $870 million was spent on purchased fuels and electricity in 2010, or roughly 1.6% of the total value of product shipments. Electricity accounted for 63% of purchased energy and the remaining 37% was comprised mostly of natural gas. While energy costs represented only 4% of total cost of materials, the sheer amount spent on purchased energy suggests that energy efficiency can play a critical role in reducing operating costs. Energy efficiency measures can be used to reduce the cost and volatility of purchased electricity and fuels. This paper provides an overview of energy use and consumption in the U.S. baking industry and identifies the significant energy using processes and related systems. Specifically, proven energy efficiency measures are summarized for the following plant systems: boilers used to produce steam for fermentation, ovens used to bake product, coolers and freezers used to chill and freeze product, and cleaning systems used to wash pans and other equipment. Specific energy consumption can vary widely among different plants, depending on the types of product manufactured and the condition of

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P. Therkelsen et al. / Journal of Food Engineering 130 (2014) 14–22 Table 1 Key products of the U.S. commercial baking industry. Subsector description

NAICS code

Key products

Commercial bakeries Frozen bakery products Cookies and crackers

311812 311813

Bread, rolls, cakes, pies, other pastries Frozen cakes, pies, and other pastries

311821

Cookies and crackers

installed equipment. Additionally a brief description of energy management best practices is provided. Properly implemented, an energy management system can ensure savings from implemented energy efficiency measure are sustained. Energy and plant managers at U.S. baking facilities can use this information to reduce energy consumption in a cost-effective manner while maintaining the quality of products manufactured. Additionally, utilities and policy makers can use these results to target energy efficiency programs and incentives to the baking industry. 2. Methodology Energy use and consumption within the U.S. baking industry was determined by surveying academic literature and government reports, and conducting field visits. Available literature is limited in quantity but provides a clear picture of the significant energy uses and related energy consumption levels. Three commercial bakeries, which produce a range of products including rolls, frozen pies, biscuits, cookies, and crackers, were visited. Interviews with energy managers at these facilities were conducted, which focused on the use and consumption of energy in bakeries and confirmed identified energy use and consumption analysis data. Energy efficiency measures for systems related to the significant energy using processes were identified. When possible, quantified energy, energy cost, or payback data are presented. Data quantifying the savings and costs of energy efficiency measures have been drawn from multiple sources including case studies, government reports, trade journals, site visits, and the U.S. Department of Energy supported Industrial Assessment Center (IAC) database (U.S. DOE, 2012). Measures without savings values are presented as well. For these measures, absolute savings values are difficult to determine, as they are very dependent upon current bakery practices. While savings and cost values presented in this paper will vary plant to plant they provide indications of the practicality and cost-effectiveness of the identified measures in a realworld plant setting. 3. Energy use and consumption in U.S. commercial bakeries In their simplest form, most bakery products (bread, rolls, cookies, crackers, etc.) have similar ingredients consisting predominantly of flour, water, and salt. Minor ingredients are used to change attributes such as volume, crumb softness, grain uniformity, silkiness of texture, crust color, flavor and aroma, softness retention, shelf life, and nutrient value (U.S. EPA, 1992). Leavened

products contain yeast that is developed in various ways depending upon the production method and recipe. Products that require dairy ingredients require special sanitation practices. Fig. 1 shows generic production process diagrams for each of the three bakery subsectors disaggregated by NAICS code as identified in Table 1. NAICS code 311812 (commercial bakeries) is divided into two parts: (1) breads and rolls, and (2) cakes and pies. As previously described, the figure shows that most bakery products are produced with the same core processes: mixing, shaping or forming, baking, cooling or freezing, and packaging. Process steps specific to one of the different generic product types are also seen: fermentation, proofing, slicing, pan washing, and shortening storage. Yeast based products require time to develop, necessitating fermentation and proofing stages. Some bread and roll products are sliced before packaging. Other products require finishing work, adding decorative items or coatings, after baking and cooling. Frozen products are quickly frozen rather than cooled to room temperature (Sikirica et al., 2003). Table 3 lists energy intensity values (J/g of finished product) for processes used in each of the baking subsectors. The values listed are representative of the energy required to produce a typical product; however, product-to-product and plant-to-plant variations will occur. Additionally, each table lists the relative percentage of energy required for a given process step as a function of the total subsector energy requirement. Energy consumption is most concentrated in the fermenting, baking, freezing, and pan washing processes (greater than 10% of total subsector energy consumption each). The baking process typically consumes the largest amount of energy, ranging between 26% and 78% of total subsector energy. Only in the case of frozen products is baking not the largest portion of energy. Baking represents 78% of the energy requirement for cookies and crackers, a subsector that does not require the use of pan washing or a fermentation and proofing process. Breads and rolls as well as cookies and crackers – products that require significant baking times – require more energy per unit of production than frozen and non-frozen cakes. In the case of frozen products, the freezing process consumes the most energy, exceeding baking. When used, pan washing also consumes a consistently large amount of energy. The four processes identified typically employ one type of significant energy using system. The fermenting process typically utilizes steam from a steam system. The baking process is closely linked to the type and operation of oven installed, but typically direct-fired natural gas ovens are used. The freezing process is directly associated with a freezer and frozen storage system. The pan washing process is typically dependent upon the use of hot water though in this paper the process is included within the larger envelop of cleaning systems. 4. Energy efficiency measures Key energy efficiency measures are presented that can be implemented to improve energy performance for the major energy using systems described above: steam systems, ovens, cooling and freezing, and cleaning. These energy efficiency measures were

Table 2 Summary of economic and energy purchase data, 2010. NAICS

31181X 31182X Total

Description

Bread and bakery product manufacturing Cookie, cracker, and pasta manufacturing

Source: U.S. Census (2011).

Employees

187,309 45,453 232,762

Costs ($1,000,000)

Value of shipments ($1,000,000)

Materials

Purchased fuels

Purchased electricity

12,824 8762 21,587

224 95 319

384 169 553

33,138 20,282 53,420

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P. Therkelsen et al. / Journal of Food Engineering 130 (2014) 14–22

NAICS 311812 Breads and Rolls

NAICS 311812 Cakes and Pies

NAICS 311813 Frozen Cakes

Mix

Mix

Mix

NAICS 311821 Cookies and Crackers Shortening Storage

Ferment

Ferment Shape Proof

Mix

Form and Chill

Shape

Form and Chill Pan Washing

Pan Washing

Pan Washing

Proof

Bake

Bake

Bake

Bake

Cool

Cool

Freeze

Cool

Slice

Finish

Finish

Slice

Package

Package

Package

Package

Fig. 1. Bakery product production processes. Source: Adapted from Brown (1996), Sikirica et al. (2003).

Table 3 Process energy intensities of common bakery products. Significant energy processes for each subsector (>10%) are underlined. Italicized values are summations of either all energy sources or processes for a given bakery product. Product/process

Breads and Rolls (31182) Mix Ferment Shape Proof Bake Cool Slice Package Cakes (311812) Mix Form and chill Bake Cool Pan Washing Finish Package Frozen cakes, pies, and other pastries (311813) Mix Form and chill Bake Freeze Pan washing Finish Package Cookies and crackers (311821) Shortening storage Mix Shape Bake Cool Package

Energy intensity (J/g) Steam

Direct fuel

Electricity for refrigeration

All other electricity

Total

1354 – 668 – 371 314

3566 – – – – 3566

– – – – – –

1205 87 – 233 – 191

6125 87 668 233 371 4071

Percentage of total (%)

– – –

– – –

– – –

281 131 281

281 131 281

864 – 176 –

906 – – 906

– – – –

723 187 – 31

2493 187 176 937

– 688

– –

– –

– 131

– 819

– –

– –

– –

187 187

187 187

1233 – 251 –

1292 – – 1292

1680 – – –

1030 266 – 44

5235 266 251 1336

26

–

–

1680

–

1680

32

982

–

–

186

1168

– –

– –

– –

266 266

266 266

22 5 5

– – – – –

4187 21 – – 4167

– – – – –

1145 – 113 303 –

5332 21 113 303 4167

– –

– –

– –

364 364

364 364

1 4 6 66 5 2 5 7 7 37 – 33 8 8 5 5

– 0 2 6 78 7 7

Source: Adapted from Sikirica et al. (2003).

obtained from multiple sources including case studies, government reports, trade journals, site visits, and the IAC database (U.S. DOE, 2012). Significant energy using systems are disaggregated into sections focusing on major aspects of the system. Energy and cost savings values are presented for energy efficiency measures when available. In practice, energy savings values can vary widely and are dependent upon the plant, types of product manufactured, and operational practices. This paper is focused on U.S. plants but the findings presented here can be generalized to bakeries in any world region.

4.1. Steam system energy efficiency measures Most U.S. baking plants use significant amounts of hot water and steam. Steam is used in fermentation boxes and rooms to achieve optimal temperature and humidity levels. Steam systems are also used to produce process and domestic hot water, heat buildings, and clean process equipment. Most steam systems are natural gas fueled. Improved steam system efficiency can reduce susceptibility to volatile natural gas prices. Typical industrial steam system assessments result in the identification of annual

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energy cost savings opportunities of 10–15% (U.S. DOE, 2006a,b). Barriers to the implementation of steam system energy efficiency measures are most commonly associated with financial issues and can be managed by understanding the potential benefits and incentive programs such as those administered by utility companies (Therkelsen and McKane, 2013). Steam system energy efficiency measures are presented with savings and payback period information as available. 4.1.1. Boiler Boiler energy efficiency measures focus primarily on improved process control, reduced heat loss, and improved heat recovery. New boiler systems should be designed and installed in an optimal custom configuration for the bakery. Pre-designed boilers often cannot be fine-tuned to meet the unique steam generation and distribution system requirements of a specific plant in the most efficient manner (Ganapathy, 1994). 4.1.1.1. Properly size boiler systems. Boiler systems should be designed to operate at a pressure no greater than needed for process use. Minimizing boiler pressure can reduce flue temperature, piping radiation losses, and leaks in steam traps. A study of 30 boiler plants showed that this measure reduced total boiler fuel consumption between 3% and 8% (Griffin, 2000). Costs and savings for this measure will vary based upon individual plant current boiler system utilization. 4.1.1.2. Control boiler processes. Flue gas monitors can be used to maintain flame temperatures within optimal limits while monitoring carbon monoxide (CO), oxygen, and smoke levels. Oxygen measured in exhaust gas is a combination of deliberately added excess air and unintentional air infiltration. By combining an oxygen monitor with an intake airflow monitor, it is possible to detect even small levels of unwanted air infiltration. Elevated CO or smoke exhaust signals that either air/fuel ratios need to be adjusted or combustion burners need to be maintained or replaced. Using a combination of CO and oxygen readings, efficiency can be maximized and air pollutant emissions reduced through optimization of fuel/air mixture level. Case studies indicate an average payback period of 1.7 years for this measure and show that it is typically financially attractive only for large boilers (IAC, 2011). 4.1.1.3. Reduce flue gas quantities using visual inspection. Boiler or exhaust flue gas leaks can reduce heat transfer efficiency and increase parasitic energy loads. Repair of flue gas leaks are easily performed and should be incorporated in normal maintenance routines. These repairs can save 2–5% of boiler energy consumption (Galitsky et al., 2005). This measure is different than flue gas monitoring as it consists of a periodic repair based on visual inspection. Savings from this measure and from flue gas monitoring are not cumulative. 4.1.1.4. Reduce excess air. Excess air levels of approximately 15% are generally required to ensure complete combustion of boiler fuel. Complete combustion maximizes fuel energy efficiency and minimization of pollutant emissions (Zeitz, 1997). Excess air levels greater than this wastes fuel as this air passes through the boiler combustor and is heated without contributing to the release of chemical energy. Reducing excess air to an appropriate level can have a less than 1 year payback period (IAC, 2011). 4.1.1.5. Improve boiler insulation. New materials, such as ceramic fibers both insulate better and have a lower heat capacity than insulation materials originally installed on older boilers. Faster responding boiler process controls should be considered and installed if needed to maintain the desired boiler temperature

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range as legacy controllers may have been designed to account for greater heat losses. Savings of 6–26% can be achieved by improving insulation (Caffal, 1995). 4.1.1.6. Implement a boiler maintenance program. Establishing and implementing a simple maintenance program can ensure that boiler components are operating properly resulting in substantial savings. Without a maintenance system, burners and condensate returns can deteriorate, reducing steam system efficiency. A simple maintenance program can reduce boiler energy costs by up to 30% over two to three years (Galitsky et al., 2005). Fouling on the fireside of boiler tubes and scaling on the waterside of boilers should be controlled. Tests of various boilers show that a fireside soot layer of 0.8 mm reduces heat transfer by 9.5%, while a 4.5 mm soot layer reduces heat transfer by 69%. A 1 mm waterside scaling buildup increases fuel consumption by 2% (CIPEC, 2001). Fouling and scaling are a problem typically associated with coal-fed boilers rather than natural gas or oil-fed boilers. 4.1.1.7. Recover flue gas heat. Flue gas heat recovery typically offers the best potential for steam system heat recovery. Flue gas recovery is commonly used with large boilers where large levels of heat are available and can be used to preheat boiler feed water in an economizer. Systems that recover flue gas need to be designed so that acidic water does not condense on economizer walls. This is commonly accomplished by maintaining flue gas temperatures above the acid dew point. Economizer wall temperature is highly dependent on feed water temperature due to the high heat transfer coefficient of water. As a result, water should be preheated close to the acid dew point before entering the economizer allowing exiting flue gas to be above the acid dew point. Typically, 1% of fuel use is saved for every 25 °C reduction in exhaust gas temperature (Ganapathy, 1994). 4.1.1.8. Return condensate to the boiler. Fresh boiler feed water must be treated to remove solids that otherwise might accumulate and clog boiler tubes. By reusing hot condensate water the need for treated boiler feed water is reduced. Additionally energy will be reclaimed, up to 100 °C of sensible heat. Payback period will depend on the plant layout, but can vary between two and three years. 4.1.1.9. Recover blowdown steam. High-pressure boiler tank blowdown often produces substantial amounts of steam. This steam is typically low grade and can be used for space heating and feed water preheating. The recovery of blowdown steam can save about 1% of boiler fuel use in small boilers (Galitsky et al., 2005). In addition to energy savings, blowdown steam recovery may reduce the potential for corrosion damage in steam system piping. 4.1.2. Steam distribution system Steam and hot water distribution systems are often quite extensive and can be major sources of energy losses. Energy efficiency improvements to steam distribution systems focus on first reducing heat losses from the system and second recovering useful heat from the system wherever feasible. 4.1.2.1. Improve distribution system insulation. Use of more or better insulating material will save energy. When deciding upon which insulating materials to select the following factors should be considered: low thermal conductivity, dimensional stability under temperature change, resistance to water absorption, and resistance to combustion. Depending on the application other factors such as tolerance of large temperature variations, tolerance of system vibrations, and adequate compressive strength where the insulation is load bearing maybe important (Baen and Barth, 1994).

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Industrial plant case studies indicate that the payback period for improved insulation is typically less than one year (IAC, 2011). 4.1.2.2. Maintain distribution system insulation. Insulation removed when conducting repairs on steam systems is not always reinstalled. In addition, some types of insulation can become brittle or rot over time. A regular inspection and maintenance system for insulation can save energy. 4.1.2.3. Maintain steam traps. Steam traps should be checked to ensure that they are operating properly. This simple measure can save significant amounts of energy for very little money. It is common to find 15–20% of steam traps malfunctioning in a steam distribution system (Jaber, 2005). Energy savings for a regular system of steam trap checks and follow-up maintenance is conservatively estimated at 10% (Bloss et al., 1997; Jones, 1997). Though this measure has an attractive payback period, it is not often implemented because maintenance and energy costs are commonly separately budgeted. 4.1.2.4. Monitor steam traps. Steam trap inefficiencies or failures can be detected and reported by steam trap monitors. In conjunction with a maintenance program, information from automated monitors used can save even more energy without significantly adding costs. Use of automated steam traps can provide an additional 5% in energy savings compared to steam trap maintenance alone with payback periods of about one year (Galitsky et al., 2005). 4.1.2.5. Repair leaks. As with steam traps, steam distribution piping networks often have leaks that go undetected without a regular inspection and maintenance program. Repairing leaks in an industrial steam distribution system is estimated to provide energy savings of about 5–10% (U.S. DOE, 2006a,b). 4.1.2.6. Recover flash steam. When a steam trap purges condensate from a pressurized steam distribution system to ambient pressure flash steam is produced. As with boiler blowdown, flash steam can be recovered and used for low grade facility applications, such as space heating or feed water preheating (Johnston, 1995). The potential for this measure is plant dependent and its cost effectiveness depends on whether or not areas where low-grade heat is useful are located close to steam traps. 4.2. Oven energy efficiency measures Ovens typically consume five or more times as much energy as thermodynamically required to bake product. Most of this extra energy is lost as heat through the oven walls or out the flue in the form of oven exhaust. Beyond heat losses, the type of oven heating element greatly affects the efficiency of the oven system. Gas burners are typically 85–95% efficient while steam heat systems are 70–80% efficient. Due to losses at the power plant and transmission lines, delivered electricity is only about 30% efficient. Energy efficiency measures related to the selection and installation, operating conditions, air handling, burners, maintenance, and controls for ovens are presented. 4.2.1. Oven selection and installation 4.2.1.1. Oven selection. Modern ovens are significantly more energy efficient that they were even five years ago. Procurement of an oven that is energy efficiency is important as ovens typically have life spans greater than ten years. (McMullen, 2010a,b). Ovens come in many forms including rack and deck styles that bake product in batches as well as tunnel ovens that continuously convey product through the oven (Thompson, 2007). Heating elements too come in

many forms such as radiant, convective, or impingement (Rigik, 2009). Oven belts such as open mesh, closed mesh, or solid steel can be selected based upon production needs. Closed mesh belts typically offer greater flexibility as they can be used to bake cookie type products that need a more solid base to sit on and the ability for air to flow up through the belt to assist in drying cracker type products (Whitaker, 2012a,b). When selecting an oven to install or looking to upgrade an existing oven, systems that are flexible and are able to produce multiple products should be considered. Increasing the amount of one product made or diversifying the product line to include various baked goods reduces energy intensity and minimizes downtime. If an oven is purchased to produce only one type of product the oven should be designed to produce any foreseeable variations of that particular product. In order to design a flexible oven advanced controls and an appropriate mix of convection and direct-fired modules should be considered. Multilevel ovens offer the potential to bake multiple products at once, increasing the diversity of a bakery as well as throughput. These ovens can be controlled so that different levels within the oven have independent temperatures, airflows, and humidity levels. As production demands change the number of levels set to bake a single product can be adjusted appropriately. As ovens represent a potentially five or ten year investment for a bakery, maximizing product flexibility will help bakeries easily transition to new products in an efficient manner (Whitaker, 2012a,b). 4.2.1.2. Oven placement. Ovens should be installed in well-ventilated spaces away from temperature sensitive processes and equipment such as cooling racks, ingredient storage areas, and mixers. Ovens can be thermally isolated in a room that contains other processes and equipment by means of heat resistant curtains, proper ventilation. Alternatively, ovens can be placed in a different room. Insulation of oven and bare equipment such as exhaust stacks will improve system efficiency. Oven insulation typically has a payback period of about 1.5 years while insulation of other equipment have 0.5–1 year payback periods (IAC, 2011). 4.2.2. Operating conditions To maximize oven efficiency, equipment should be operated in a way that maximizes energy efficiency. Ovens should be operated for as short of a period as possible with the lowest possible exhaust temperature and flow rate. Baking should be scheduled to ensure that oven capacity is maximized to reduce standing losses. For ovens that use conveyors, product should be distributed across the full width of the belt and as uniformly as possible. By monitoring and recording production variables, bakers can understand how much energy is required to produce product. 4.2.2.1. Heat-up time. The amount of time to warm up the oven should be kept as short as possible. To minimize energy consumption, oven heat-up time should be determined. One bakery reduced heat-up time by 20–25 min for cabinet ovens and by 40–50 min for tunnel ovens. Minimum oven heat-up time can be determined by recording how long after the oven is turned on temperature sensors indicate a desired baking temperature has been reached. This value is the minimum heat-up time and is the absolute minimum time needed before product enters the oven (CoA, 2001). 4.2.3. Air handling The use of waste heat should be considered when designing an oven. Recovered waste heat can be used in a number of ways, including directly in the oven or in other bakery processes. Outside of the oven, the most common use of oven waste heat is in proofing processes. When a proofing process is not required the use of waste

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heat outside of the oven is typically limited to the production of hot water (Malovany, 2011). A portion of oven exhaust gas should be re-circulated in the oven or mixed with incoming combustion air. Exhaust gas recirculation transfers energy to a lower temperature section of the oven reducing fuel consumption. The amount of exhaust gas that can be recirculated will be dictated in part by product humidity requirements. Fresh combustion air can be pre heated by being mixed with exhaust gas. This action reduces fuel consumption though it may cost on the order of $8000 to implement with typical payback period of two to four years (CoA, 2000a,b). Canadian and Australian bakeries have installed heat recovery systems on rolls and bread ovens (NRCAN, 2010). The Australian bakery uses the recovered waste heat in its proofing oven. The project has an estimated payback period of 3.5 years. Additionally, the plant now requires less boiler maintenance and water treatment (EMIAA, 1998). Warm conditions inside an oven result in a natural convection of air, which draws conditioned air from the bakery into the oven and out the stack (Malovany, 2010). Ventilation doors that span multiple rooms need to be controlled to reduce drafts that can pass through the oven and affect temperature levels and gas usage. Air that leaks into an oven must be heated to maintain constant baking temperatures. Hot air leaking out of the oven heats the surrounding environment rather than the product. Both types of leaks waste fuel consumed by the oven and require additional air conditioning in warm climates. Air leaks can cause temperature imbalances, which decrease the quality of produced product. Air leaks can be detected by creating a temperature profile of the oven. An oven temperature profile will also indicate stages of the oven that are too hot or cool, potentially impacting product quality (CoA, 2000a,b). Repairing air leaks is cost effective and typically has a payback period of less than one year (IAC, 2011). 4.2.4. Burners Oven burners are a critical component of oven energy efficiency. Flue gas and temperature analysis can be used to determine burner operation and efficiency. As part of installation and commissioning, burners should be adjusted for efficient operation, which is an action with a five to ten month payback period (IAC, 2011). After completing burner commissioning, a reference flue gas and temperature sample should be taken. This reference measurement can be used to determine if burners are operating efficiently or not. When variations in flue gas and temperature are observed, the most common corrective action will be to adjust the burner air/fuel ratio. An oxygen trim control may be appropriate to manage combustion inefficiencies when fuel/air ratio adjustments cannot be made. Oxygen trim controllers cost between $6000 and $10,000 to install, but will reduce the time required to assess efficiency and maintain oven efficiency in the future. In some instances a burner will be damaged and need to be repaired or replaced. Burner repair commonly has a payback period of 1.5 years and can be performed during periods of routine maintenance (IAC, 2011). 4.2.5. Controls Controls can be used to increase oven efficiency. Active control of oven exhaust ducting can help ensure only combustion exhaust is expelled and that ambient air does not enter the bakery through the exhaust system. These types of control systems can result in 5– 20% energy savings. Removal of barometric dampers from forced draft ovens will also reduce the amount of conditioned bakery air being sucked out of the building through the oven while preventing cold air from being drawn in while the oven is not operating. Coupled with heat recovery devices, oven controls can be used to produce hot water as needed in the bakery (Exhausto, 2008).

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The Bakers Delight showcase bakery outside of Sydney, Australia utilizes an innovative oven hood design that includes adjustable speed drives on its oven air supply and exhaust fan motors. Use of a control system allows the motors to run at a low speed with reduced energy consumption, until an oven door is opened and switches the motors to run at high speed. Additionally, back draft dampers were installed in the ceiling to prevent unwanted infiltration when the fans are switched off. The oven hood controls reduce energy use by nearly 75% compared to standard designs (DITR, 2003). A U.S. bread plant utilizes adjustable speed drive controls on its thermal oxidizer blower. Based on the blower inlet pressure, the controllers ramp down the blowers when one or more of the plant’s ovens is not in use. The measure reduced plant energy costs by $7700 per year and cost $633 to implement, resulting in a payback period of less than two months (Base Co., 2012). Integrating controls across the whole bakery can greatly expand the efficiency of an oven. Controls can be used to minimize oven start up and shut down energy requirements (Whitaker, 2011). Centralized control allows bakeries to utilize a multiple zoned approach to baking that promotes product flexibility. Product can be tracked throughout the production process and oven settings can be adjusted based upon known product changes that will soon be ready for baking. 4.3. Cooling and freezing energy efficiency measures Energy and cost savings from cooling and freezing process changes will be dependent upon current bakery practices and the needs of the product being produced. 4.3.1. Cooling Unforced ambient air typically takes an unacceptably long time to cool bakery products, necessitating the use of energy consuming devices to speed up the cooling process. A balance between energy consumption and throughput must be made when considering options to cool bakery products. 4.3.1.1. Cooling air source. Compressed air is commonly employed in bakeries to cool hot products. Air compression, distribution, and use are energy intensive. Replacement of compressed air with air blowers has been shown to have a payback period of less than one year (IAC, 2011). The addition of controllable exhaust fans to air blowers may improve air circulation and create a more even distribution of cooling air. Placement of exhaust fans needs to be careful considered if they are to be effective (CoA, 2000a,b). 4.3.1.2. Placement. Cooling bakery products away from heat sources such as ovens can save energy. Spiral conveyor cooling saves space and can be enclosed with low cost materials to shield the cooling process from heat sources. These enclosures also aid in keeping product clean of foreign debris (McMullen, 2010a,b). 4.3.1.3. Controls. Installing automatic controls on a cooling line can improve energy efficiency and product quality. Control systems can be used to manage line speed, exhaust fans, and other cooling processes and components. Cooling system integration with controls has been shown to reduce maintenance costs and facilitate flexibility for easy modification to the cooling system. Such a control unit may cost about $7500 fully installed and result in a reduction in cooling time of about 25% (CoA, 2000a,b). 4.3.2. Freezing 4.3.2.1. Freezer process. Product line freezers should be selected with production process needs and energy efficiency in mind. Bakeries that freeze product typically chose either a batch or

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continuous line freezer. Shifting from continuous to batch production is normally not encouraged due to the generation of unnecessary production line bottlenecks. Such bottlenecks are not energy efficient as other equipment, such as an oven, may need to be left in an operational mode while waiting for the line to continue before product is able to pass through the freezer. 4.3.2.2. Spiral freezers. Spiral freezers are able to enclose a greater belt length into a smaller footprint as compared to linear freezers. This compact nature is suited for products that need to be frozen in a controlled manner. Spiral freezers can be arranged so that products progressively enter colder zones, preserving product features. 4.3.2.3. Cryogenic tunnel freezers. While cryogenic linear freezer tunnels do not share the flexibility of spiral freezers, they typically freeze products faster. Spiral freezers may require 45 min for a product to travel the length of its belt where as a cryogenic tunnel freezer could have the same product frozen in just 15 min. High velocity air in a freezing tunnel can alleviate some of the energy and operational costs associated with a cryogenic freezing (Atchley, 2011). 4.4. Cleaning energy efficiency measures Food safety standards and good operating practices require a bakery be kept clean. Energy efficiency considerations should keep food sanitation in mind. Baking pans and other equipment are typical cleaned with large amounts of hot water and compressed air. These cleaning agents require large amounts of energy to produce and use. Depending upon sanitation requirements, the equipment or area being cleaned, accessibility, and time allotted for cleaning, modifying cleaning practices can positively impact energy consumption (Thilmany, 2009).

4.4.1. Hot water To save energy bakeries should reduce the amount of hot water used for cleaning. Hot water can be expensive both due to the purchase water and energy required to heat it (Masanet and Walker, 2013). Additional costs may be incurred to process wastewater. Areas that can be dry-cleaned with chemicals or physical practices should be identified. Dry cleaning reduces energy and costs associated with hot water use and potential microbiological contamination. If hot water is absolutely necessary, roof mounted solar water heaters or waste heat recovery systems can be used to reduce or eliminate the need to purchase energy to heat water. Savings will be dependent upon current hot water consumption. 4.4.2. Compressed air Compressed air is often used in lieu of water to clean slicers, bagging machines, conveyors and floors. The use of compressed air is an energy intensive practice and should be avoided if possible. Compressed air cleaning may scatters debris from the area being cleaned to other locations in the bakery. Air pressure level should be maintained at as low of a pressure as possible. Reducing valves, air guns, and other equipment should be standardized and appropriate training conducted to instruct users to be conscious of compressed air use (CoA, 2000a,b). 5. Energy management Experience has shown that energy performance gains from various one-off energy efficiency projects may result in energy savings but that these savings are not necessarily sustained without continuous monitoring and adjustment (Jeli et al., 2010; Ates and Durakbasa, 2012). In order to ensure energy savings are sustained, energy should be managed and not treated as a fixed operational expense (Vikhorev et al., 2013). All industrial plants should make

Table 4 Energy efficiency opportunity measures presented with quantified savings or payback values. Measure

Quantified savings or payback values

Steam system (4.1.1) Properly size boiler systems Control boiler processes Repair flue gas leaks Reduce excess air Improve boiler insulation Recover flue gas heat Return condensate to the boiler Recover blowdown steam

3–8% reduction in fuel consumption (Griffin, 2000) 1.7 year average payback period for large boilers (IAC, 2011) 2–5% reduction in fuel consumption (Galitsky et al., 2005) 1 year or less payback period (IAC, 2011) 6–26% reduction in fuel consumption (Caffal, 1995) 1% reduction in fuel consumption for every 25 °C reduction in exhaust gas temperature (Ganapathy, 1994) 2–3 year payback period depending on plant layout 1% reduction in fuel consumption in small boilers (Galitsky et al., 2005)

Steam distribution system (4.1.2) Improve distribution system insulation Maintain steam traps Monitor steam traps Repair leaks

1 year or less payback period (IAC, 2011) 10% energy savings (Bloss et al., 1997, Jones, 1997) 5% energy savings and 1 year or less payback period (Galitsky et al., 2005) 5–10% energy savings (U.S. DOE, 2006a,b)

Oven selection and installation (4.1.4) Oven insulation Insulation of other oven equipment Air handling (4.2.3) Oven exhaust gas recirculation Repairing air leaks

1.5 year payback period (IAC, 2011) 0.5–1 year payback period (IAC, 2011) 2–4 years payback period (EMIAA, 1998, CoA, 2000a,b) 1 year or less payback period (IAC, 2011)

Burners (4.2.4) Burner adjustment Burner repair

1 year or less payback period (IAC, 2011) 1.5 year payback period (IAC, 2011)

Controls (4.2.5) Active oven exhaust control Oven hood controls Blower adjustable speed drive

5–20% energy savings 75% energy savings as compared to standard designs (DITR, 2003) 1 year or less payback period (Base Co., 2012)

Cooling (4.3.1) Replacement of compressed air with blowers Cooling line controllers

1 year or less payback period (IAC, 2011) 25% reduction in cooling time (CoA, 2000a,b)

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management of energy performance (comprised of energy use, consumption, and efficiency) a priority, and take action by implementing an organization-wide energy management system (EnMS). An EnMS should follow the Deming Plan-Do-Check-Act framework that has been successfully applied within manufacturing facilities for quality, environment and safety practices. A wide number of EnMS standards and best practices are avalible for use including the international EnMS framework ISO 50001 – Energy Management Systems (ISO, 2011). As an example of the impact EnMS can have at industrial plants, the cost and benefits of implementing a plant wide ISO 50001 EnMS and achieving energy performance targets were studied. Payback rates for implementing the ISO 50001 EnMS and achieving set energy performance improvement targets were found to be a function of plant baseline source energy consumption. Plants with baseline source energy consumption greater than 0.21 TJ can expect a less than two-year marginal payback (Therkelsen et al., 2013). To help companies assess the energy performance of their plants, ENERGY STAR develops Energy Performance Indicators (EPIs). ENERGY STAR EPIs are sector-specific benchmarking tools that compare a plant’s energy performance to the rest of the industry. EPIs have been published for Cookie and Cracker Bakeries and are planned for Commercial Bakeries that produce bread and rolls (U.S. EPA, 2012). ENERGY STAR EPIs provide an evaluation of how efficiently the whole plant is performing as a system. Plants that receive low energy performance scores should be prioritized for assessments to identify areas for energy saving measures. Plants that score high should be reviewed to identify potential best practices and strategies that can be shared across the organization. This tool can be used during an energy review to establish an understanding of current energy performance. Utility companies offer energy consumption data and consultation services that would support a plant wide EnMS. A growing number of utilities allow their customers access to real or near real time energy consumption data through the Green Button Initiative (Green Button, 2013). Participating utilities provide customers with instant access to their smart meter or monthly billing data. Some utilities provide their customers the ability to automatically complete the U.S. EPA benchmarking tools with data from their energy bills. A number of larger utilities also offer on-site audits by trained experts to guide food service utility customers about energy efficiency opportunities. These on-site auditors are well versed in utility rebate programs and can make efficient use of a customer’s time. The energy efficiency improvement measures detailed in this paper and the tools offered by the U.S. EPA and a growing number of utility companies can provide context to a plant wide EnMS. Use of this article can help plant management develop an energy policy, conduct an energy review, identify significant energy uses, and prioritize measures for energy performance improvement.

6. Conclusions The U.S. commercial baking industry produces a wide variety of products including breads, rolls, frozen cakes, pies, pastries, cookies, and crackers. In 2010 the sector spent over $870 million on purchased fuels and electricity. This paper examines current energy use and consumption in the U.S. industrial baking sector and identified cost-effective and proven energy efficiency measures that can be implemented to reduce costs for systems that consume signification amounts of energy. Energy use and consumption within the U.S. baking industry was determined by surveying academic literature and government reports. Available literature is limited in quantity but provides a

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clear picture of the significant energy uses and related energy consumption levels. Additionally, three commercial bakeries were visited to confirm energy use and consumption analysis data. Four major processes–fermentation, baking, cooling and freezing, and cleaning–consume the vast majority of purchased energy. Implementation of energy efficiency measures for these systems can reduce energy costs and lessen the impacts of volatile energy prices. These equipment and systems include boilers, ovens, chillers, freezers, and cleaning systems. A wide array of energy efficiency measures specific to these four systems can be implemented to reduce energy costs and increase predictable earnings. Quantified energy, energy cost, or payback data are presented when available. Some energy efficiency measures are presented without savings values when savings levels are dependent upon current bakery practices. While savings and cost values presented in this paper will vary plant to plant they provide a guide as to the practicality and cost-effectiveness of a given measure in a real-world plant setting. Further research on the economics of all measures—as well as on their applicability to different production practices—is needed to assess their cost effectiveness at individual plants. While the expected savings associated with some of the individual measures may be relatively small, the cumulative effect of these measures across an entire plant may potentially be quite large. Many of the measures have relatively short payback periods and are therefore attractive economic investments on their own merit. Measures with quantified savings or payback values listed in this paper are summarized in Table 4. Most measures can be implemented during routine maintenance periods or taken into consideration when new equipment is being procured. Individual plants should pursue further research on the economics of the measures, as well as on the applicability of different measures to their own unique production practices, in order to assess the feasibility of measure implementation. A brief discussion of how an energy management systems can provided a way to ensure savings realized from energy efficiency measure implementation are sustained was also provided. An effective energy management system will ensure energy performance improvements resulting from the implementation of energy efficiency measures are sustained. Additionally, a robust energy management system will result in the identification of energy efficiency improvement actions that are operational in nature, not just capitol upgrades. An energy management system will also create a framework for quantifying and communicating energy and energy cost savings resulting from the implementation of energy efficiency measures with plant management. Acknowledgements This work was supported by the Climate Protection Partnerships Division of the U.S. Environmental Protection Agency as part of its ENERGY STAR program through the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. References Atchley, C., 2011. Freeze Up. Baking industry News and Opinions Retrieved 28 December, 2012. and Packaging/2011/10/FreezeUp.aspx?cck=1. Ates, S.A., Durakbasa, N.M., 2012. Evaluation of corporate energy management practices of energy intensive industries in Turkey. Energy 45, 81–91. Baen, P.R., Barth, R.E., 1994. Insulate heat tracing systems correctly. Chem. Eng. Prog., 41–46. Base Co., 2012. Energy Savings for a Bread Plant. Retrieved 28 December, 2012. . Bloss, D., Bockwinkel, R., Rivers, N., 1997. Capturing energy savings with steam traps. In: Proceedings of the 1997 ACEEE Summer Study on Energy Efficiency in Industry, Washington, D.C., American Council for an Energy-Efficient Economy.

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