Utilization of Waste Heat from Sugar Mills in Cuba for Thermally Driven Cooling
Sofia Feychting Marina Vitez
Bachelor of Science Thesis
KTH School of Industrial Engineering and Management Energy Technology EGI-‐2014
SE-‐100 44 STOCKHOLM
II
Bachelor of Science Thesis EGI-‐2014
Utilization of Waste Heat from Sugar Mills in Cuba for Thermally Driven Cooling
Sofia Feychting
Marina Vitez Approved
2014-‐06-‐10
Examiner Catharina Erlich
Supervisor Eyerusalem Birru
Commissioner
Contact person
III
Abstract
The demand for air conditioning keeps rising, especially in developing countries where the standard of living has improved. This results in an increased consumption of electricity and puts further pressure on the power grid. In Cuba, electricity is a scarce commodity and the electricity production relies heavily on fossil fuels, which causes high emissions. An alternative technology for producing cooling is thermally driven cooling where the installment of an absorption chiller could utilize waste heat from existing industries to provide cooling to buildings. Therefore, there are possibilities of lowering the amount of electricity needed for air conditioning.
In this thesis, the potential of using waste heat from sugar mills in Cuba was investigated. The sugar industry is high water consuming and often produces large amounts of heated waste water that is rarely utilized. To collect the data needed for the investigation, a study visit was conducted at the sugar mill Carlos Baliño in Villa Clara, Cuba. Since the factory did not track water mass flows, calculations based on sugar concentrations and energy balances were used to determine the different water outlets. The identified excess water has a mass flow of 10 kg/s and a temperature of 96 °C, which is enough to supply the factory with cooling. The result of the investigation also showed that the mill could invest in thermally driven cooling with a payback time of between three to six seasons depending on the cost of the selected equipment. The energy savings per crushing season would be nearly 140 000 kWh which equals to financial savings of above 40 000 dollar per season.
If the sugar mill Carlos Baliño would invest in an absorption chiller, the cooling supply would be unreliable because of the high number of production shutdowns. Before any possible implementation, the causes for the stops in production need to be further examined. The supply of cooling would otherwise have to rely on thermal energy storage of chilled water, which in such large quantities would be costly. The factory only produces waste heat during the crushing season, which lasts from December throughout April, but there is a cooling demand during the whole year, which means that alternative cooling methods for an off-‐season cooling supply would have to be investigated. The study concludes that thermally driven cooling would be very suitable for similar industries that also produce large amounts of heated excess water, but are operating all year around and have a more even production rate, both on a daily and seasonal
basis.
IV
Sammanfattning
Efterfrågan på luftkonditionering fortsätter att öka, speciellt i utvecklingsländer där levnadsstandarden har förbättrats. En ökad efterfrågan på luftkonditionering resulterar i en ökad användning av elektricitet, vilket i sin tur leder till en ökad belastning på elnätet. På Kuba är elektricitet en bristvara och elproduktionen är starkt beroende av fossila bränslen vilket leder till stora utsläpp. En alternativ teknologi för att producera kyla är värmedriven kyla där en absorptionkylmaskin kan utnyttja spillvärme från redan existerande industrier för att leverera kyla till byggnader. Därav finns det möjlighet att minska användandet av den elektricitet som behövs för att driva luftkonditioneringsapparater.
I denna uppsats undersöks potentialen för att använda spillvärme från sockerfabriker på Kuba.
Sockerindustrin konsumerar stora mängder vatten och producerar ofta betydande kvantiteter av uppvärmt eller förångat spillvatten som sällan utnyttjas. För att samla in de data som krävs för undersökningen genomfördes studiebesök på fabriken Carlos Baliño i Villa Clara, Kuba.
Eftersom vattenflöden inte mättes i fabriken baserades beräkningarna på sockerkoncentrationer och energibalanser för att fastställa utloppsflöden av vatten. Det identifierade spillvattnet har ett massflöde på 10 kg/s och en temperatur på 96 °C, vilket är tillräckligt för att förse fabriken med kyla. Resultatet av undersökningen visade också att fabriken skulle kunna investera i värmedriven kyla med en återbetalningstid på mellan tre till sex säsonger beroende på kostnaden för vald utrustning. Energibesparingarna per produktionssäsong skulle bli närmare 140 000 kWh vilket motsvaras av en ekonomisk besparing på drygt 40 000 dollar per säsong.
Om en absorptionskylmaskin skulle implementeras på Carlos Baliño skulle leveransen av kyla vara osäker på grund av det höga antalet produktionsstopp i fabriken. Före en eventuell implementation måste orsakerna till stoppen undersökas, annars skulle kylningsmöjligheterna bero starkt på termiska energilager av kallt vatten vilket i stora volymer kan bli kostsamt.
Fabriken producerar endast spillvärme under produktionssäsong vilket pågår från december till och med april men kylbehovet existerar under hela året. Det betyder att alternativa kylmetoder behöver undersökas för att kylbehovet ska kunna tillgodoses året runt. Slutsatsen av studien är att värmedriven kyla är en ytterst passande lösning för liknande industrier som också ger upphov till stora mängder av varmt spillvatten men som producerar hela året och har en
jämnare produktion, både på daglig basis och säsongsbasis.
V
Acknowledgement
We are thankful for the guidance from Pablo Romelio Roque and Manuel Alejandro Rubio Rodríguez on location in Cuba, who enabled the study visit at the sugar mill Carlos Baliño in Villa Clara. We also want to direct a special thanks to our Swedish supervisors, Eyerusalem Birru and Catharina Erlich, who made this project possible and also the Department of Energy, CETER, at CUJAE University in Havana.
We are very grateful for the scholarship from Din Els Miljöfond, which enabled our field trip to Cuba. We also want to thank ITM for the support in writing the thesis abroad.
Finally, we want to mention the wonderful employees at the sugar mill Carlos Baliño in Villa Clara in Cuba, who showed us the factory and help us gather the information needed.
Marina Vitez and Sofia Feychting Havana, Cuba 2014-‐04-‐24
VI
Table of Content
Abstract ... III Sammanfattning ... IV Acknowledgement ... V Table of Figures ... IX Table of Tables ... X Nomenclature ... XI
1. Introduction ... 1
2. Objectives ... 2
2.1. Problem Formulation ... 2
2.2. Aim of Study ... 2
2.3. Methodology ... 2
3. Literature Study ... 4
3.1. Global Energy Usage ... 4
3.2. Global Air Conditioning Usage ... 4
3.2.1. Negative Effects of Air Conditioning ... 5
3.2.2. Positive Effects of Air Conditioning ... 5
3.3. Energy Situation in Cuba ... 5
3.3.1. Historical Background ... 5
3.3.2. Cuban Energy System Today ... 6
3.4. Air Conditioning Usage in Cuba ... 7
3.5. Cooling Methods ... 8
3.5.1. Vapor Compression Cooling ... 8
3.5.2. Absorption Cooling ... 9
3.6. The Sugar Industry ... 13
3.6.1. Global Sugar Industry ... 13
3.6.2. Sugar Production ... 13
3.6.3. Cogeneration in Sugar Mills ... 16
3.6.4. Water Usage in Sugar Mills ... 17
3.7. Previous Work on the Subject ... 18
4. Model ... 19
4.1. System Boundaries ... 19
4.1.1. Steady State Modeling ... 20
4.1.2. Consideration of Losses in the Models ... 20
4.2. The Process ... 20
4.3. Modeling of the Mass Balances in the Sugar Mill ... 21
4.3.2. Numerical Calculations of the Mass Flow ... 22
4.3.3. Assumptions Made when Calculating the Mass Flow ... 30
4.3.4. Sensitivity Analysis ... 31
4.4. Model of Energy Supply and Demand ... 32
4.4.1. Modeling of the Energy Demand ... 32
4.4.2. Modeling of the Energy Supply ... 33
4.4.3. Model of the Energy Savings ... 35
4.4.4. Sensitivity Analysis of the Energy Supply and Energy Savings ... 36
4.5. Modeling of Financial Aspects ... 37
4.5.1. Numerical Calculations ... 37
4.5.2. Restrictions ... 37
VII
4.5.3. Assumptions ... 38
4.5.4. Sensitivity Analysis ... 38
5. Results and Discussion ... 40
5.1. Results from Mass Balances ... 40
5.2. Results from the Energy Demand and Supply Models ... 42
5.2.1. Cooling Demand of the Factory ... 42
5.2.2. Cooling Demand of the Village ... 42
5.2.3. Maximum Cooling Supply ... 43
5.2.4. The Cooling Supply to the Factory ... 43
5.2.5. The Thermal Energy Storage ... 44
5.2.6. Energy Savings ... 44
5.3. Results from Financial Calculations ... 44
5.4. Results of the Sensitivity Analysis ... 45
5.4.1. The Sugar Balance ... 45
5.4.2. Mass Flow and Temperature of the Contaminated Condensate ... 46
5.4.3. Energy Supply and Energy Savings ... 47
5.4.4. Financial Calculations ... 48
5.5. General Discussion ... 49
6. Conclusions and Recommended Future Work ... 53
6.1. Conclusions ... 53
6.2. Recommended Future Work ... 53
References ... 54
Appendix A: Sugar Balance ... 57
Appendix B: Specific Heat Capacity of Juice ... 59
Appendix C: Mass Flows through Juice Heaters ... 60
Appendix D: Specification of the Juices Heaters ... 63
Appendix E: Calculations of Mass Flows from the Evaporators ... 64
Appendix F: Results of the Mass Flow of Evaporated Water from the Evaporators ... 65
Appendix G: Condensate from Evaporators ... 66
Appendix H: Mass Flow through Crystallizer ... 67
Appendix I: Mass Flows through the Evaporators and the Crystallizers ... 68
Appendix J: Temperature and Mass Flows of the Contaminated Condensate Tank ... 69
Appendix K: Sensitivity Analysis of Sugar Balance ... 70
Appendix L: Sensitivity Analysis of Contaminated Condensate Tank ... 71
Appendix M: Cooling Demand of the Factory -‐ Results from Interview Study ... 72
Appendix N: Cooling Demand of the Village -‐ Results from Interview Study ... 73
Appendix O: Questionnaire for the Residents in the Nearby Village ... 74
Appendix P: Maximum Cooling Effect from the Absorption Chiller ... 75
Appendix Q: Maximum Theoretical Cooling Supply ... 76
Appendix R: Dimensioning of the Thermal Energy Storage ... 77
Appendix S: Capacity of Absorption Chiller ... 79
VIII
Appendix T: Energy Supply per Day -‐ When Considering Production Shutdowns ... 81
Appendix U: Energy Savings ... 82
Appendix V: Sensitivity Analysis of the Energy supply and Energy Savings ... 83
Appendix W: Implementation Costs ... 84
Appendix X: Financial Calculations ... 86
Appendix Y: Sensitivity Analysis of Implementation Costs ... 88
Appendix Z: Sensitivity Analysis of the Financial Calculations ... 90
IX
Table of Figures
Figure 3.1. World energy demand, exajoules (The Economist, 2013) ... 4
Figure 3.2. Amount of electricity not served (IAEA, 2008) ... 6
Figure 3.3. The fuel shares of the total primary energy supply (IAEA, 2008) ... 7
Figure 3.4. Fuel share of electricity generation (IAEA, 2008) ... 7
Figure 3.5. Vapor compression cycle (Hundy et al., 2008) ... 8
Figure 3.6. Absorption cycle (Hundy et al., 2008) ... 9
Figure 3.7. Mass flows in sugar production process (Erlich, 2009) ... 13
Figure 3.8. Cogeneration plant in sugar mill (Erlich, 2009) ... 17
Figure 4.1. Model depicting system boundaries ... 19
Figure 4.2. The process of determining energy savings and cost reductions ... 20
Figure 4.4. Processes and mapped flows in the sugar mill ... 22
Figure 4.5. Mass balance of the cane crushers and milling tandem ... 23
Figure 4.6. Mass balance of the clarifier and filtration unit ... 24
Figure 4.7. Mass balance of the evaporators ... 25
Figure 4.8. Mass balance of the crystallizers ... 26
Figure 4.9. Illustration of the juice heaters ... 27
Figure 4.10. Illustration of the evaporators ... 28
Figure 4.11. Illustration of the mass flows in and out of the contaminated condensate tank ... 30
Figure 4.12. Production and distribution of cooling ... 32
Figure 5.1. Results of the mass flows of the sugar balance ... 40
Figure 5.2. Mass flows of contaminated condensate ... 42
Figure 5.3. Energy supply and demand during 24 hours ... 44
X
Table of Tables
Table 3.1. Energy per 100 kW cooling, at 3 °C evaporation, 42 °C condensation (Hundy et al., 2008) ... 11
Table 3.2. Approximate sugar cane content by type (de Camargo et al.,1990) ... 14
Table 4.1. Equations used for calculating water content of cane and the milling efficiency ... 23
Table 4.2. Equations used to determine the mass balance of the cane crushers and milling tandem ... 24
Table 4.3. Equations used to determine the mass balance of the clarification and filtration process ... 25
Table 4.4 Equations used to determine the mass balance of the evaporators ... 25
Table 4.5. Equations used to determine the mass balance of the crystallizers ... 26
Table 4.6. Equations used for calculating the mass flows through the juice heaters ... 27
Table 4.7. Equation used for calculating the mass flow of the first effect evaporator ... 29
Table 4.8. Equation used for calculating the mass flow through the crystallizers ... 29
Table 4.9. Equations used for calculating the mass flow at atmospheric pressure ... 29
Table 4.10. Equations used for calculating the temperature and mass flow of the contaminated condensate tank ... 30
Table 4.11. Results investigated in the sensitivity analysis ... 31
Table 4.12. Results investigated in the sensitivity analysis ... 32
Table 4.13. Equations used for determining the possible energy supply ... 33
Table 4.14. Equations used for dimensioning water storage ... 34
Table 4.15. Equations used for determining size of the absorption chiller ... 35
Table 4.16. Equations used for determining the possible energy savings ... 35
Table 4.17. Results investigated in the sensitivity analysis ... 36
Table 4.18. Equations for financial calculations ... 37
Table 5.1. Mass flows to the contaminated condensate tank ... 41
Table 5.2. Mass flows from the contaminated condensate tank ... 41
Table 5.3. Cooling demand of the factory ... 42
Table 5.4. Rough estimation of village cooling demand ... 43
Table 5.5. Maximum cooling supply ... 43
Table 5.6. Specification for absorption chiller needed ... 43
Table 5.7. Thermal energy storage ... 44
Table 5.8. Results of financial calculations ... 45
Table 5.9. Ranges of the result of the sugar mass balance ... 45
Table 5.10. Ranges of the result of the excess water ... 46
Table 5.11. Ranges of the result of the energy supply ... 47
Table 5.12. Ranges of the financial results ... 48
XI
Nomenclature
Symbol Denomination Unit
Peso Convertible CUC
Peso Cubano CUP
a Surface area m2
C Cost $
cp Specific heat capacity J/kg°C
COP Coefficient of cooling performance -‐
COPav Average coefficient of cooling performance for vapor -‐
compression chillers on the Cuban market
d Number of days -‐
e! Electricity price $
e! Saved electricity after installing thermally driven cooling kWh
h Enthalpy J/kg
I!"! Total initial investment $
m Mass kg
𝑚 Mass flow kg/s
𝑚in Incoming mass flow kg/s
n Economical lifetime of an investment season
Pr Percentage of target production rate achieved during season % PV Percentage of cooling produced out of the village cooling demand %
p Pressure bar
p! Purity of the cane before milling %
p!" Purity of filter cake %
PVF Present value factor -‐
PVSF Present value sum factor -‐
Q Energy kJ
Q Effect kW
Qin Incoming effect kW
Qout Outgoing effect kW
R!"#$% Residual value $
r discount rate %
REV Revenues $
T Temperature °C
∆T Temperature difference °C
tstop Consecutive hours with production shutdown hours
tpeak Hours per day with peak demand hours
V Volume m3
x Content %
y Factor for calculating mass flow of evaporated water from -‐
evaporators
η!"## Milling efficiency or pol recovered after milling tandem %
XII
Symbol Subscript
24h 24 hours
abs Absorption chiller ah After heater 1, 2, 3 or 4
atm Atmospheric pressure
av Average
b Bagasse
bh Before heater 1, 2, 3 or 4 br Brix content
c Cane
cap Capacity
cctank Contaminated condensate tank ch Chilled water from absorption chiller
cj Concentrated juice
cl Clear juice cond Condensate cool Cooling central
cry Crystallizers
d Demand
dr Driving temperature
e Evaporator 1, 2, 3, 4 or all ed Existing cooling demand
el Electricity consumption per season
ev Evaporated water
excess Excess water from contaminated condensate tank
pre Pre-‐evaporators
f Fiber
fan Fan coil fc Filter cake
g Generator in the absorption chiller
h Heater (in evaporator, pre-‐evaporator or in crystallizer) h1-‐h4 Heater 1, 2, 3 or 4
i Index for the mass flows in to the contaminated condensate tank imb Imbibition water
in Water into given process or equipment inst Installation of equipment
j Juice
l Liquid
lat Latent heat of vaporization
m Molasses
main Maintenance
max Maximum
min Minimum
p Pol content
pip Piping needed for ventilation system rd Return temperature after distribution
rg Return temperature after generator in the absorption chiller
XIII
s Sugar
st Storage
st,f Days needed to fill storage unit
tot Total
v Vapor form
vc Vapor compression chiller
w Water
1. Introduction
The standard of living has improved in many countries all over the world, which has resulted in a higher consumption of electricity. In developing countries, the expansion of the power grid cannot keep up with the increase in electricity consumption, leading to an unreliable energy supply and recurring blackouts in many areas. The demand for air conditioning is also rising which puts further pressure on power generation and distribution. The use of air conditioning enhances productivity and wellbeing but the main cooling device on the market, the vapor compression chiller, is powered by electricity. There are, however, alternative ways of producing cooling, for example thermally driven absorption technology that can use waste heat from already existing industries, instead of electricity.
In Cuba, the energy usage has also increased over the years but electricity is in short supply.
Fossil fuels are the main source of energy, causing more emissions of carbon dioxide. Since 2005, the Cuban government has initiated several projects to improve energy efficiency, which has successfully reduced the risk and number of blackouts (Carpio, 2010). Still, there are not enough investments being made into renewable resources and the possibility of increasing the utilization of existing resources needs to be further investigated.
One possible solution that would lower the electricity consumption caused by air conditioning would be to implement thermally driven cooling in the sugar mills in Cuba. The sugar industry uses large quantities of water in the production process and also produce sizeable amounts of waste water which often contain heat that is not utilized. Absorption chillers can use the low-‐
grade heat in the waste water to produce a cooling effect, which is then distributed in the form of chilled water to the nearby buildings that need cooling. If the vapor compression chillers could be replaced with an absorption chiller that is powered with waste heat, the energy savings could be significant. The aim of this project is to investigate these possibilities by identifying the amount of waste heat produced by a sugar mill, examine the necessary absorption technology and determine the investment costs.
2. Objectives
The objectives express the aim of the project and explain the problems which were be investigated. There is a short background to the purpose that point out the most important issues that the project focuses on.
2.1. Problem Formulation
Electricity is a scarce commodity in Cuba, and the consumption of electricity increases even further as air conditioning ownership becomes more widespread, which causes a higher pressure on the power grid. In Cuba, electricity is mainly produced from fossil fuels and the carbon dioxide emissions are substantial.
The sugar mills in Cuba produce a considerable amount of waste water that is rarely utilized. To lower the electricity consumption, the low-‐grade heat from the waste water could be used to produce cooling, but the necessary absorption technology does not exist in Cuba. This study will investigate the potential of using waste heat from the sugar mills in Cuba to implement thermally driven cooling.
2.2. Aim of Study
The aim of the study is to investigate the possibility of utilizing waste heat from excess water produced by the sugar mills in Cuba. The potential energy savings and financial benefits from utilizing the waste heat by distributing thermally driven cooling to the factory and nearby houses are determined. The amount of energy that can be obtained from the waste heat from the factory is calculated and compared to the estimated amount of energy needed to power the existing electricity-‐driven air conditioning units. The data needed is retrieved from one sugar mill, Carlos Baliño in Villa Clara in Cuba, but most models will be applicable to other similar sugar mills.
The study will determine:
• The mass flow and the temperature of the excess water produced by the sugar mill.
• The amount of cooling that can be supplied when utilizing the excess water containing low-‐grade heat.
• The air conditioning demand and the amount of electricity used by the existing air conditioning units in the offices and nearby houses.
• The capacity of the absorption chiller needed to supply the cooling.
• Needed storage possibilities.
• The total energy savings as a result of implementing the project.
• The financial benefits of implementing the project.
• The technical and financial feasibility of utilizing waste heat and implementing absorption cooling.
o If the energy supply matches the energy demand o Investment costs
o Payback time
2.3. Methodology
The study started with an extensive literature study, focusing on the technology of thermally driven cooling, specifically absorption chillers, as well as the operational processes of a sugar mill and the effects of an increasing electricity and air conditioning usage, worldwide and in Cuba.
During the project, a study visit was conducted at the sugar mill Carlos Baliño in Villa Clara, Cuba. Compared to other sugar mill in Cuba, Carlos Baliño is quite small and is the only ecological sugar producer in Cuba, but the mill also produces conventional non-‐organic sugar.
The crushing season, when the sugar mills produce sugar, usually lasts from the middle of December throughout April.
The mass flow and the temperature of the heated waste water from the sugar mill determine the amount of energy that can be retrieved and used for thermally driven cooling. The sugar concentrations between each process in the production were used to calculate the water mass flows. In order to identify the excess water, separate investigations into certain processes were conducted. The pressure, temperature and mass flow of the water and the sugar juice were used to calculate the mass flow of the excess water which made it possible to determine the cooling supply that can be produced by using the waste heat from the excess water in an absorption chiller.
The existing air conditioning units on the Cuban market were investigated and the capacity, and the amount of electricity needed to power these units, was determined. The cooling demand of the factory and the nearby village was estimated by conducting an interview study with the employees at the mill and the nearby residents. A model based on the calculated possible cooling supply and the estimated cooling demand was used to determine the amount of energy saved and the financial benefits of implementing the project. Thereafter, the results were discussed and conclusions were made based on the results. Recommended future work is also presented in
the report.
3. Literature Study
In the literature study, relevant collected information is presented both as a background into the technology and the objective of the thesis. The information was gathered from relevant books, scientific articles, web pages and from personal contacts. The references used are considered to be reliable since corresponding information has been collected from different sources, both from literature and from personal contacts. The collected information has been complemented with the knowledge obtained from the study visit at the sugar mill Carlos Baliño.
3.1. Global Energy Usage
As economies grow and the standard of living increases there is an upgrade in living comfort.
This has occurred in many countries in the developing world, where rising income levels has led to an increased usage of electrical devices. By 2004, the global electricity consumption over the last 30 years had tripled and the consumption is expected to double over the following 20 years (Rydstrand et al., 2004). The international Energy Agency, IEA, estimates that two thirds of the increase would consist of energy production from oil and natural gas (Udomsri, 2011), leading to more emissions of carbon dioxide. The largest increase in energy consumption, 67 % of the total increase, will occur in developing countries (Udomsri, 2011). Today the total electricity consumption worldwide is approximately 19 000 TWh (Index Mundi, 2013), of which more than 1 000 TWh is consumed by air conditioning usage (Dahl, 2013; Udomsri, 2011).
3.2. Global Air Conditioning Usage
Over the last two decades there has been a surge in air conditioning usage in developed countries. In recent years the consumption of cooling has also increased in developing countries.
In 2011, the world sales of air conditioning were up 13 % compared to 2010 and the sales are expected to grow even more rapidly in the future due to improved living conditions in warmer countries (Cox, 2012). By 2050, the electricity consumed by air conditioning units is expected to expand more than tenfold (Dahl, 2013), as seen in Figure 3.1.
Figure 3.1. World energy demand, exajoules (The Economist, 2013)
The usage of air conditioning is not only affected by climate, but also by income level, though there are statistics showing that the use of air conditioning is increasing even faster than the economic growth in countries with a high daily average temperatures (McNeil and Letschert, 2008). A middle class with an increasing wealth is a particularly strong indicator of a rising future demand for electrical devices (Sivak, 2013).
3.2.1. Negative Effects of Air Conditioning
As the ownership of air conditioning units continues to spread over the world, the electricity consumption will also increase. For developing countries where a household generally consumes a low amount of electricity, the installment of air conditioning would significantly increase the total energy demand. This jump in electricity consumption causes a higher pressure on the power grid in these countries, since the grid capacity is not adapted to a widespread air conditioning usage (McNeil and Letschert, 2008). In developing countries this results in chronic shortages of power. In order to increase the capacity and improve the existing distribution system, a lot of resources and capital are needed, which is not possible to obtain in financially struggling countries. Therefore, the advancement of the power grid will always be one step behind the expansion of cooling, which can be very disruptive for the society.
The electricity consumed by air conditioning is produced mainly by fossil fuels in most countries. The predominant source is coal, which causes a lot of emissions of carbon dioxide to the atmosphere, having a negative impact on the environment (Cox, 2012). Renewable energy sources supply only around 14 % of the world’s primary energy demand (Perera, 2012). The use of renewable resources is growing, but currently air conditioning usage is causing a lot of emissions. The emissions will only increase as the demand for cooling continues to rise. This is especially a problem in developing countries where investments into green technology have not reached the same levels as countries with greater resources. Since every prognosis made over the demand for cooling is projecting a great surge in air conditioning usage, there is an even greater incentive to invest in green energy technology.
3.2.2. Positive Effects of Air Conditioning
Air conditioning is a major investment for households in developing countries and is deemed a luxurious product more than a necessity. There are however a lot of positive effects resulting from the use of cooling in households and businesses. There is a correlation between productivity and the average temperature of the environment. A study at Yale University found that the economic output by people in colder areas was 12 times that of the output in warmer areas. In the United States, the number of people falling ill or dying during heat waves has drastically decreased mainly due to an expanding air conditioning usage. Developing countries stricken by malaria, dengue fever or other diseases would profit enormously from a widespread utilization of cooling (The Economist, 2013).
3.3. Energy Situation in Cuba
The Cuban energy sector has been very influenced by the politics within Cuba, and by the social relationships with other countries for several years, instead of economic and environmental influences (IAEA, 2008)
3.3.1. Historical Background
Between 1958 and 1990, Cuba and the former Soviet Union became allies after the United States posted a trade blockade against Cuba (Gustafsson, 2011). Cuba imported fuel oil at favorable prices from the Soviet Union and the Cuban energy system was adjusted and structured based on the imported oil (Belt, 2008). At this time, 72 % of the installed capacity was based on fuel oil-‐driven thermal power plants and a dependency of imported oil had arisen (IAEA, 2008)
During the special period, from 1990 until 1997, Cuba was suffering from an economic crisis due to the dissolution of the former Soviet Union. Cuba did not longer have the possibility to import the necessary fuel oil or machines for the energy generation needed to supply the demand for electricity (IAEA, 2008). The Cuban government started to encourage an increased usage of
other energy sources in order to reduce the fuel oil dependency without significant results (Cherni and Hill, 2009).
The power plants could not supply all the electricity needed, therefore Cuba suffered many electricity blackouts, from 1997 and forward, due to the former economic crisis and the lack of investments made (Benjamin-‐Alvarado, 2010). In 2005 there were blackouts during 224 days that, each day, lasted for more than one hour. Some of the blackouts lasted for as long as seven to twelve hours and could be greater than 100 MW (Ekeström, 2012). As shown in Figure 3.2, there have been several periods in Cuba with large quantity of electricity not served.
Figure 3.2. Amount of electricity not served (IAEA, 2008)
In 2006, the Cuban government launched a program called The Energy Revolution (La Revolución Energética) in order to secure the electricity supply by reducing the consumption and develop the generation capacity (Suárez et al., 2012). The aim of these major investments was primarily to limit the number of blackouts and it was partly done by decreasing the demand for electricity on a daily peak basis by replacing the majority of energy inefficient appliances, like air conditioners, fans, light bulbs and refrigerators which significantly decreased the demand (Belt, 2008).
3.3.2. Cuban Energy System Today
The Cuban energy sector is controlled by the state. One of the largest companies in Cuba, La Unión Eléctrica, is responsible for generation, transmission and distribution of the electrical energy. Oil is the far most used energy source, but some of the energy supply is obtained from renewable resources (hydro, wind and biomass), diesel and gas. The domestic oil is not enough to supply the Cuban energy demand, thus creating a dependency on oil imports from Venezuela (Benjamin-‐Alvarado, 2010). In 2003, the total primary energy supply was 76% oil, 16% biomass and 5% from gas (IAEA, 2008). Figure 3.3 shows the fuel shares of the total primary energy supply over the years.
Figure 3.3. The fuel shares of the total primary energy supply (IAEA, 2008)
As seen in Figure 3.4, the share of biomass of the total amount of fuel for electricity generation has decreased between 1975 and 2003. Bagasse, a byproduct from sugar production, is the most commonly used biomass in Cuba. Since the sugar industry is one of the largest industries in Cuba, the bagasse has the potential to become a significant part of the Cuban energy supply by increasing the utilization of bagasse as fuel (Suárez et al., 2012).
Figure 3.4. Fuel share of electricity generation (IAEA, 2008)
Today, Cuba has rather well developed national electrical grid and 97% of the population has access to it (Suárez et al., 2012). In 2009, the electricity consumption in Cuba was 17 802 million kWh; this equals to 1584 kWh per person. This represents an increase by 20% since the year 2000 (Strömdahl, 2010). There have been lots of changes in the Cuban energy sector and the number of electricity blackouts has decreased. However, further investments are necessary as higher economic stability leads to higher electricity demand and the energy system is not sustainable when it relies on oil as the main supply for energy.
3.4. Air Conditioning Usage in Cuba
Since 2005 it was prohibited for individuals to bring air conditioning units and other larger electrical appliances in to the country, because of the existing energy shortages. By 2013 the Cuban government had lifted the ban on private importation of air conditioning devices (Chumley, 2013). Since many electrical appliances are in short supply in Cuba, it is now very
common for Cubans with the possibility to go abroad to import these appliances from other countries (Garcia, 2013).
During Cuba’s rational energy use programs, starting in 2005 and financed by the government, 265 505 air conditioning units were replaced with newer, more efficient ones. These units made up 88 % of the total amount of air conditioning units in the country (Carpio, 2010).
3.5. Cooling Methods
The most common product on the air conditioning market is the vapor compression chiller.
There are alternative methods of cooling, for example the absorption chiller. This technology can use waste heat as a driving force.
3.5.1. Vapor Compression Cooling
The most common cooling unit on the market is vapor compression chillers. Electricity is needed to run the mechanical compression process in the unit, to produce cooling. When a vapor compression chiller is installed in a room, the process is as follows (see Figure 3.5); the evaporator absorbs heat from the air as the refrigerant evaporates at a low pressure, thus lowering the temperature of the room. The refrigerant is transported to the compressor at a high pressure and high temperature. In the condenser the temperature is lowered as heat is emitted to the outside. The refrigerant is then transported to the expansion valve and the process starts all over again (Larsson and Nilsson, 2009; Persson, 2012).
Figure 3.5. Vapor compression cycle (Hundy et al., 2008)
Energy Efficiency
The energy efficiency of air conditioning units has more than doubled over the last 30 years (The Economist, 2013). However, this only applies to newer units in developed countries. In countries with less economic development, older units are still in use and the regulations on new imports are not as strict. In wealthier countries, the manufacturers are forced to abide by standards of minimum efficiency in order to optimize the equipment and reduce the electricity consumption (ASME, 2011).
The efficiency of air conditioning units installed in the United States increased by almost 30 % from 1993 to 2005 – at the same time the electricity consumed by household air conditioning units doubled because of a more widespread and intensive usage (Cox, 2012). This further confirms the need for a sustainable production of cooling.
COP
The coefficient of performance (COP) is one way to measure the energy efficiency of the device by using a ratio of the cooling produced and the energy needed to produce the cooling. A modern compression chiller can have a COP of 7 but it is more common for COP to be around 5.
Depending on the method used to cool the waste heat from the condenser, the COP can be as low as 2-‐3 (Larsson and Nilsson, 2009).
If you count the efficiency of the electricity production, the total efficiency of the process is much lower. It is more efficient to produce cooling directly than to first produce electricity that is in turn used to produce cooling (Rydstrand et al., 2004).
3.5.2. Absorption Cooling
Since the growing consumption of air conditioning leads to an increased electricity production and more emissions of carbon dioxide, it is pressing to find a better solution in order to achieve a sustainable energy system. An efficient solution would be to produce the energy by using renewable resources or waste products, and to produce cooling directly instead of first generating electricity. Absorption cooling is such a solution – by utilizing thermally driven cooling where low-‐grade heat in water can be used to generate cold water, which is distributed to buildings through district cooling. The technology is not new but is still improving and becoming more efficient. There are now devices on the market that can use heat with a driving temperature of as low as 75 °C (Jardeby and Nordman, 2009) to produce cooling of 6 °C (Zinko et al., 2004).
Like vapor compression cooling, the refrigerant in an absorption cooling cycle is evaporated at a low pressure as heat is absorbed to create a cooling effect. The difference is that the absorption chiller then uses a second refrigerant, the absorbent, which absorbs the first refrigerant, as seen in Figure 3.6.
Figure 3.6. Absorption cycle (Hundy et al., 2008)
After heat is emitted from the absorber, still at a low pressure, the solution is in liquid form, containing both the refrigerant and the absorbent. The solution is then pumped to the generator where external heat is added through a heat exchange by utilizing hot water. During this process the refrigerant evaporates and is separated from the absorbent under high pressure. The absorbent returns to the absorber and the refrigerant continues as a vapor to the condenser
