A LITERATURE REVIEW ON LOW QUALITY WASTE HEAT RECOVERY IN RESIDENTIAL AND COMMERCIAL SETTINGS EMMANUEL OMERE THESIS

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1 A LITERATURE REVIEW ON LOW QUALITY WASTE HEAT RECOVERY IN RESIDENTIAL AND COMMERCIAL SETTINGS BY EMMANUEL OMERE THESIS Submitted in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2016 Adviser: Professor Anthony Jacobi Urbana, Illinois

2 Abstract Increasing demand for energy saving technologies has led to substantial interest in waste heat recovery systems. This interest is reflected in rapid growth in the rate of publication of related technical articles. The purpose of this literature review is to evaluate proposed low quality waste heat recovery systems in residential and commercial settings. The basics and limitations of these systems are outlined with attention directed towards identifying new systems with promise. ii

3 Table of Contents Introduction Definition of Concept. 2 Conventional Heat Exchanger Heat Recovery... 2 Novel Heat Exchanger Heat Recovery... 3 Conventional Heat Pump Heat Recovery... 6 Novel Heat Pump Heat Recovery Conventional Hybrid Heat Recovery Novel Hybrid Heat Recovery Conclusions References iii

4 Introduction Economic growth has led to expansion of building sectors, increasing building energy consumption in the process. About 48% of consumable energy was utilized by the residential and commercial sector of the U.S. in 2014 [29]. Most of this energy is used for space heating, water heating, and air conditioning. In 2009, The U.S. Energy Information Administration stated that these needs consume 65% of energy use in residences [30]. The remaining 35% was expended by appliances, electronics, and lighting. Increased energy costs and concerns with environmental impacts drive continued efforts to improve end-use energy efficiency. Waste heat recovery may be an important part of these efforts to reduce primary energy consumption. However, the initial cost of heat recovery systems, skilled labor required for installation, and space limitations within residential and commercial settings hinder the advancement and expansion of waste heat recovery. For residential and commercial systems, it is practical to purchase systems that are very dependable, hence the acquisition of separate components for water heating and space temperature conditioning is favored over integrated heat recovery systems that could have an impact on every building in the residential and commercial sector. However, with novel ways to recover some of the energy, benefits will be significant. The main objective of this study is to conduct a thorough and critical review of lowtemperature waste heat recovery in residential and commercial settings. Current applications of waste heat recovery will be identified, and the potential of emerging systems assessed. 1

5 Definition of Concept In residential and commercial settings, most waste-heat recovery efforts focus on discarded heat from drains, the condenser of heat pumps, and a few key appliances. Heat pumps and heat exchangers can be used to recover heat from wastewater. Using a heat pump, wastewater could provide a heat source for the evaporator. Alternatively, using a heat exchanger, wastewater could be used to preheat tap water. In appliances like clothes dryers or dishwashers, instead of discarding heated fluids (air or water) to the environment, a heat exchanger could be used to recover some of the waste heat. Recovery of condenser waste heat is carried out in a heat pump when the operating function of the heat pump is space cooling. The three forms of waste heat recovery in residential and commercial settings are heat exchanger, heat pump, and hybrid waste heat recovery. Hybrid waste heat recovery is a combination of heat exchanger and heat pump heat recovery. Conventional Heat Exchanger Heat Recovery Heat exchanger heat recovery is an obvious form of waste heat recovery. It is generally inexpensive, and requires the least space. Not functioning as a component of a refrigeration cycle, these systems have been designed to recover waste heat exclusively from drains. They preheat water using single pass counter flow heat exchangers [1][7]. Wong et al concluded that a horizontal heat exchanger in a shower drain could save between 4% and 15% of energy consumed on hot water showers annually [1]. Leidl et al 2

6 stated that vertical heat exchangers could recover between 52% and 72% of the heat from shower wastewater [7]. These values are believed to be relative to the temperature of tap water, so could be interpreted as heat exchanger efficiency. A reason behind the vertical counter flow heat exchanger s superior efficiency to its horizontal counterpart is discussed on page 8. Novel heat exchanger waste heat recovery systems comprise heat exchangers designs deviating from the canonical, with the resolution to optimize cost or effectiveness. There are other novel heat exchanger heat recovery systems with applications within appliances. Novel Heat Exchanger Heat Recovery The main objective of Paepe et al was to develop a prototype to recover warm wastewater out of one dishwasher cycle and use it as a heat source for tap water to be used in the subsequent cycle that requires hot water [3]. A quasi-steady calculation model was developed for the proposed swirl inlet system to be used for heat recovery. Experimental results validated the developed model. Using a tube length of 15 m inside a 5.5-liter tank, heat recovery of 25% was achieved. With payback on initial investment of 6 years, the swirl inlet filling system was determined to be an economical means to exchange heat between tap water and wastewater for a dishwasher. Figure 1 shows the proposed prototype. 3

7 Figure 1: Principal design of the recovery system for a dishwasher and the swirl inlet system [3] The main objective of Guo et al was to determine the energy savings potential of a heat exchanger particularly designed to recover waste heat from the shower drainage [9]. Unlike other drain heat exchangers, in this device, wastewater preheats tap water twice before it reaches the water heater. The device performance was obtained through experimentation. This system recovers more than 50% of waste heat. Figure 2 shows the designed heat exchanger. Figure 2: Schematic diagram of waste heat recovery device [9] 4

8 A lot of residences are space constrained, therefore a convenient location for the placement of a waste heat recovery heat exchanger is beneath the shower stall / bath tub. Unless the apartment is on the ground floor, a horizontally oriented counter flow heat exchanger is a more suitable fit than its vertical counterpart. The authors of this paper proposed a design that optimizes horizontal counter flow heat exchangers in order to make them just as effective as vertical counter flow heat exchangers [14]. McNabola et al improved the horizontal counter flow heat exchanger by optimizing film flow heat transfer, which is the fundamental characteristic behind high efficiency heat transfer in vertical counter flow heat exchangers. Furthermore, this system is cost effective since plastic pipes were used for the wastewater tubing as opposed to copper in vertical counter flow heat exchangers. CFD simulations were proven to accurately predict experimental results before system optimization was carried out using CFD. CFD optimization shows that this system can recover waste heat at efficiencies above 50%. Figure 3 shows the cross section of the proposed heat exchanger. Figure 3: Cross section of the proposed drain waste heat recovery device [14] 5

9 Conventional Heat Pump Heat Recovery Heat recovery heat pumps have been designed to recover condenser waste heat by using water-cooled condensers to produce hot water. Jiang, and Jie et al designed systems for all combinations of space heating, space cooling, and water heating [4][25]. These heat pumps have five functions: space cooling, space cooling and water heating, space heating, space heating and water heating, and water heating. Because space heating and water heating at the same time requires a heating capacity that may overwhelm the heat pump in the winter, due to low ambient temperature heat sources, some heat pumps were designed to perform only four of the five combinations previously mentioned [6][11][18]; the space heating and water heating function being excluded. Other heat pumps have been designed solely for water heating [13][19]. Baek et al used the wastewater from saunas and public baths in a hotel as a heat source for a heat pump water heater that maintains a coefficient of performance between 4.5 and 5 throughout the year [13]. For regular heat pump waste heat recovery systems, the coefficient of performance of the heat pump rises, and then falls, during any operating mode that includes water heating. The coefficient of performance rises initially because condenser waste heat is being used to heat water. However, the temperature differential between water and condensing refrigerant eventually decreases such that the heating capacity and cooling capacity of the heat pump drop as well. Consequently, the coefficient of performance of the system drops. The heat pump performance is not stable. Another issue that arises with regular air-source heat pump heat recovery systems is the low speed of production of hot water, especially in the winter when ambient temperatures are low. 6

10 Even when hot water is produced and stored, because there is no heater in the water tank to maintain water temperature, heat is lost when the need for hot water is not urgent. Heat pump waste heat recovery systems could be novel for design modifications that result cost-effectiveness, steady performance, water heating speed increase, hot water storage enhancement, multiple heat source incorporation, or condenser waste heat utilization for purposes other than water heating. Heat pump water heaters are stable when they employ air-cooled condensers that are operated as the heating capacity of the water-cooled condenser drops. Using wastewater-source evaporators solely, in parallel with, or in series with air-source evaporators could enhance the performance of the heat pump in the winter, speeding up water heating. Besides ensuring that energy expended on heating is not wasted, optimizing hot water storage will ensure hot water is always be readily available. Novel Heat Pump Heat Recovery Deng et al investigated the energy saving potential of split-type residential air conditioners used for clothes drying through condenser waste heat in tropical and sub tropical regions [5]. The system performance was obtained through experimentation. This energy consumption by this unit is 1.2 % that of an electric tumbler dryer. This energy consumed is used to increase the condenser fan speed so that the original condenser airflow rate, without with loaded drying rack, is maintained. Deng only provided a schematic of the experimental set up for the residential air conditioner (RAC), which is shown in figure 4. 7

11 Figure 4: Schematic diagram of the laboratory experiment rig [5] Devised for year round operation, Shao et al proposed a heat pump that has been designed for various combinations of space heating, space cooling, and water heating [12]. This system comprises a reheater that absorbs heat into domestic water from superheated refrigerant, and a preheater that absorbs heat into domestic water from subcooled refrigerant. The performance of this system is stable because reheaters and preheaters are used for water heating, as opposed to the system condenser. Additionally, in order to prevent the problem of insufficient hot water in the winter, water can be pumped from the tank and stored in the reheater, keeping it heated and available when the need arises. The system performance was determined using a simulation model of an inverter air conditioner. Working purely as a water heater, a COP of 4 is attained compared to 1 for an electric water heater. This system has 31.1% energy saving compared to a heat pump plus electric water heating system per year. It has an annual 8

12 performance factor of compared to of a heat pump plus electric water heater. Figure 5 shows the proposed heat pump system. Figure 5: Schematic diagram of the heat pump system [12] Liu et al discussed the use of wastewater source only or both wastewater and air sources in order to optimize heat pump heat recovery for heating in residences [15]. During winter, due to low ambient temperatures, wastewater source alone is best for the heat pump. Otherwise, engaging wastewater and air sources in parallel yields the best heating capacity and coefficient of performance of the system compared to using both heat sources in series or alone. Unlike in the hot water supply only mode where two heat exchangers are used to heat up water, in the space heating plus hot water supply mode, the smaller of the two heat exchangers is used for water heating, and the remaining heat from the refrigerant is used for space heating. This is an alternative to increasing compressor speed during peak load periods, which increases power input. In hot water supply mode only, the coefficient of performance and heating capacity of the system steadily drops with increase in water temperature. In space heating plus hot water supply mode, the system performance is much more stable despite water temperature rise because refrigerant heat is released to the space, though a drop in coefficient of 9

13 performance and heating capacity is noticeable still. Figure 6 shows the proposed heat pump system. Figure 6: Schematic diagram of the multifunctional heat pump system [15] Liu et al discussed the use of wastewater source only or both wastewater and air sources in order to optimize heat pump heat recovery for cooling in residences [16]. An air heat exchanger is placed in series after the water heat exchanger for hot water production because it is required in order to produce water at up to 49 C. This air heat exchanger was run in both forced and natural convection. The natural convection mode produced hot water faster. The measured system performance was obtained through experimentation. Figure 7 shows the proposed heat pump system. 10

14 Figure 7: Schematic diagram of the multifunctional heat pump system [16] In this paper, the transient operating characteristics of a heat pump water heater in single stage and cascade mode was investigated through dynamic experiments and analysis [17]. In the water heater, Wu et al have placed phase change materials within the storage tank of the heat pump. With phase change materials in the hot water storage tank, water can stay at higher temperatures for longer. Hence, there is less heat loss when hot water is not being used and off electricity peak periods when the heat pump may not be in use. Cascade mode has a higher coefficient of performance in cold ambient temperatures and single stage mode performs better in warmer ambient temperatures. The critical point to switch between both modes depends on ambient temperature and hot water temperature. The coefficient of performance for the single stage mode ranges between 1.5 and 3.05 for ambient air conditions of -7 C and 20 C respectively. For the cascade mode, 11

15 and the same ambient temperatures, coefficient of performance ranges between 1.74 and Figure 8 shows the proposed heat pump system. Figure 8: Schematic diagram of the multifunctional heat pump system [17] A heat pump water heater designed by Huang et al to produce hot water rapidly by using a mechanical heat-sensitive device made from shape memory alloy was introduced [20]. In this system, a supply tank comprising a water-cooled condenser and a holding tank for storing hot water was proposed in order to separate water into smaller portions so it can be heated to desirable temperatures much quicker. The shape memory alloy valve connecting both tanks opens when water temperature in the supply tank exceeds 50 C. The measured system performance was obtained through experimentation. This system does not need an auxiliary water heater in cold ambient conditions since is it has fast temperature response. For ambient temperatures between 6 C and 38 C, the 12

16 temperature response speed is C/min and C/min in the supply and holding tanks respectively, before the shape memory alloy valve opens. After it opens, the temperature response speed increases to C /min and 1.1 C /min for the supply and holding tanks respectively. This heat pump water heater is also energy efficient; using a maximum of 0.016KW/l, while a regular electric system uses 0.06kW/l of energy to heat water up to 55 C. Figure 9 shows the proposed heat pump system. Figure 9: Schematic diagram of the multifunctional heat pump system [20] Because hot water demand is not stable, Gu et al proposed a heat pump water heater enhanced with phase change materials of lower and higher melting points designed to store latent and sensible heat respectively [22]. These materials store condenser waste heat that is utilized when there is demand for hot water. The phase change material commonly used is paraffin wax, and it has been applied to this system. Typically, paraffin wax has low resistance when absorbing heat, thus it experiences large 13

17 temperature rise. On the other hand, when releasing heat, paraffin has high heat transfer resistance. It does not transfer as much heat to water as is required. Furthermore, it experiences a sharp decline in temperature when heat is being transferred to water. Experiments were carried out to determine how much additive to be added to technical grade paraffin wax such that it can satisfy household temperature demand. Paraffin wax with 40% liquid paraffin as the additive was used for heat recovery because it is an economical option. The performance of this system is stable since a cooling tower is used when the accumulators cannot take all the heat out of the refrigerant. When hot water is required, tap water flows through the accumulators containing phase change material and can reach between 40 C and 45 C on exiting the accumulators. An auxiliary electric heater is used if further heating is required. Figure 10 shows the proposed heat pump system. Figure 10: Schematic diagram of the air conditioning system with thermal energy recovery devices [22] 14

18 Bertsch et al proposed a heat pump water heater designed to perform efficiently in extremely cold weather conditions, supplying water at 50 C in ambient conditions as low as -30 C [28]. In warmer ambient temperatures, single stage mode is used, and the twostage mode is used in cold ambient temperatures due to the resulting increase in heating capacity. An economizer within the heat pump cycle is used to prevent the compressor temperature from rising too high and to improve coefficient of performance by subcooling refrigerant exiting the condenser before it enters the expansion valve. As opposed to a conventional heat pump, which has a coefficient of performance less than 2 in the winter, this system has a coefficient of performance of at least 2.3 in extremely cold weather. Figure 11 shows the proposed heat pump system. Figure 11: Two-stage cycle with closed economizer [28] 15

19 Conventional Hybrid Heat Recovery Utilizing wastewater as a heat source, a heat exchanger comprising wastewater can be used to preheat tap water, and after exiting the heat exchanger, the remaining heat in the wastewater can serve as the heat source for a heat pump designed for further tap water heating through condenser waste heat [10][23][24][26]. Sun et al applied this form of waste heat recovery to a barber s shop and reduced the yearly cost of water heating by 93.7% [10]. Liu et al proposed this system for water heating in public shower facilities with a gas boiler used for auxiliary heating in the event that the hot water produced by the heat pump is insufficient [23]. The gas boiler replaced the ubiquitous auxiliary electric heater because it produced fewer emissions. In the event that wastewater is not available, Zeshao et al designed a system that uses either or both wastewater and air heat sources in the heat pump [24]. Hybrid systems provide heated water at higher temperatures and have higher heat pump coefficient of performance [26]. Novel hybrid waste heat recovery systems comprise designs with resolution to optimize heat recovery in a manner similar to heat exchanger and heat pump waste heat recovery systems. Novel Hybrid Heat Recovery Because ambient conditions are unstable (due to varying seasonal weather conditions), Wei et al have proposed shower wastewater as the heat source for a heat pump water heater [8]. A shell and tube heat exchanger for transferring heat from wastewater to tap water is also part of the design. This system is unique because tap 16

20 water is split into two streams to be heated separately by the heat pump condenser and the heat exchanger. Both streams are eventually mixed. No water storage tank is required; hence this system is likely to satisfy space requirements. Furthermore, the performance of this system is stable because tap water constantly runs across the condenser; therefore the heating capacity of the condenser is optimum, and varies slightly depending on the effects of ambient conditions on tap water. A mathematical set of equations developed to model the physics of every component of the system was solved using MATLAB. The optimized coefficient of performance of the heat pump is Figure 12 shows the proposed hybrid system. Figure 12: Schematic of the heat pump bath water heating system [8] Wang et al discussed the use of wastewater and air heat sources for a multifunction heat pump designed for space cooling and water heating [27]. Wastewater is a reliable heat source in the winter when ambient air temperatures are low. Besides having no valves, hence reducing cost, this system can produce hot water above 36 C just 13 seconds after being turned on in the summer. This work is novel because it has an optimized cycle. A heat exchanger has been placed after the water evaporator to further heat up refrigerant entering the compressor by using refrigerant exiting the condenser. 17

21 Consequently, refrigerant exiting the condenser is sub-cooled, improving system performance. The measured system performance was obtained through experimentation. The coefficient of performance of this system ranges between 3.69 and The payback time on initial investment is 5.5 years. Figure 13 shows the proposed hybrid system. Figure 13: Schematic diagram of the multifunctional heat pump [27] 18

22 Conclusions A literature review of low temperature waste heat recovery in residential and commercial settings was conducted. Heat discarded via drains and the condenser of heat pumps were identified as the main sources of waste heat, and the potential of emerging systems that have been proposed for heat recovery was assessed. In order to support future research, the main conclusions that can be drawn from this study are listed as follows: - The most efficient form of waste heat recovery is hybrid waste heat recovery. Heat exchanger heat recovery is the cheapest and requires the least space. - For drains, heat exchangers in the horizontal orientation better satisfy space constraints. They can be designed and optimized to perform just as well as heat exchangers in the vertical orientation. - Unless an economizer or air-cooled condenser is included in a heat pump cycle to further cool refrigerant exiting the water-cooled condenser being used to heat water, which recirculates from a tank, the performance of the heat pump will not be stable. - Wastewater as the heat source for a heat pump can ensure reliable year round performance. For cascade air-source heat pumps, the single stage mode is used in warmer ambient temperatures, and switched to the twostage mode in the winter due to the resulting increasing in heat capacity. 19

23 - The use of phase change materials to store thermal energy ensures the availability of water at high temperatures for longer periods of time after hot water has been produced. 20

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