ESL-IE-10-05
Contents
1. The collector will rarely be normal to the sun So any angle of incidence that the sun has to the collector will have a similar but opposite effect In that case we will now calculate the actual sunlight by first multiplying the horizontal radiation by the cosine of the solar angle of incidence to the collector 0 and then dividing by the cosine of the solar zenith angle 0z All of this is done automatically in the modeling template Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 I Thxcos cos amp Equation 1 where I insolation on the tilted collector Ih insolation on the horizontal surface solar angle of incidence to the collector z solar zenith angle Analysis of Eeee of Collecfoe Tilt ANGIE on Injensity oF Solar Insolation Now examining a graph of the daily integrated data from NASA figure 6 presented as the Insolation on a collector at 4 different tilt angles we can determine the best starting tilt angle for our application Monthly Average Insolation at Various Tilt Angles 2 Jan Feb Mar Apr May Jun Jul Aug Sep t No Dec Figure 6 We see that the line representing a tilt of 18 or Latitude 15 provides us the best overall starting tilt angle for an application that is more interested in air conditioning during the months April through September than any other angle If on the other hand our appli2. been collecting solar and meteorological data since 1961 It covers approximately 1440 locations within the US and its territories A portion of those locations are actually measured data The remaining locations are modeled based on the measured data We are interested in both the Hourly Statistical Summaries which show the hourly average insolation and the Typical Meteorological Year data This data may be accessed for free from http rredc nrel gov solar old_data nsrdb The Hourly Statistical Summaries will provide a monthly average of solar energy falling ona horizontal plane in Watts per square meter for the hour ending at the indicated time So for January the 1300 hours data will be the average of every day in January of the irradiance falling on the ground between 1200 hours and 1300 hours in Watt hours per square meter This data is given for global horizontal horizontal diffuse and direct or beam radiation This data is also available for each of 15 years You should download all 15 years and then average them The modeling template will do the averaging for you Typical Meteorological Year data TMY 3 files provides multiple measurements on an hourly basis for both the sunlight and the meteorological data It is based on a 30 year period of time and chooses for each month that month from the 30 year period of time that is statistically the most typical You are likely to find that you have a different year chosen for e
3. chiller bouncing on and off Further the chiller must be able to control the flow of heat medium fluid into the chiller to prevent the absorbent from precipitating out of the solution as salt You must not continue the flow of hot water into the chiller if it has no call for cooling This is critical So you should have the chiller control this flow Simply tell the chiller when it is to operate and allow the chiller to control those flows Finally as mentioned above there should be some means of relieving excess heat from the collector array The best way to do this is with some kind of thermostatically controlled heat dump located between the collector array and the buffer storage It is best to locate the heat dump here to protect the collector array as well as the chiller The last thing you want to do is damage the collectors by stagnating them They are the single most expensive component of the system Figure 20 shows how the hot water side of the system interfaces to the chiller Array Field Design A Thermostatically controlled in heat dump protects chiller se and collectors Isolate here Figure 20 For the chilled water side figure 21 presents a representative diagram If this system is to be the primary chilled water plant for the facility an auxiliary boiler will be required Should you place an auxiliary boiler into the system it is imperative that you NOT attempt to operate the solar heat source
4. energy available as an output from your collector in terms of Watts M at the design time for each month during your design season Then average that output Divide the energy required for design conditions by this average output to calculate the total gross collector area required to meet design conditions Then divide the total gross area required by the gross area of the collector C Hr Hox CAg Equation 6 where C the number of collectors required Hr heat required for design conditions in Btuh or Watts and don t forget your losses Ho average heat output at design time in Btu ft or Watts m during design season CAg Collector gross area in ft m The template uses SI units for all of these calculations as the solar and meteorological data is ESL IE 10 05 03 supplied in SI units The template then converts and displays primarily in IP units All of the chiller based calculations are then done in IP units STEP 6 CALCULATE A HEAT BALANCE What comes next is sizing the cooling tower for design conditions It is imperative that 100 of the heat input to the chiller is rejected With an absorption chiller there are 2 sources of heat input There is of course the load but since it is a thermally driven chiller there is the power source as well The heat balance is calculated as Qe Qg Qe Equation 7 Where Qc heat rejected to the cooling tower Qg Heat input to the generator Qe Co
5. from the other side the air conditioning engineer designs for extremes also termed design conditions When designing a chilled water system to meet the 1 design condition which is the only logical way to insure that the load is met satisfactorily you may still lack the data needed to determine how much solar energy is available during those extremes Once again the system ends up grossly underpowered So far only one of the Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 systems evaluated seems to be over powered and only one system appears to be adequately powered that is still operating And it is a derated system One other issue which has contributed to this shortfall of energy is the application of the absorption process to solar thermal energy Even if we adequately determine the load and accurately estimate the energy available over the course of an average of average days we are likely to find the chiller short of power Simply because an absorption chiller s Coefficient of Performance is affected by both the temperature of the heat medium the energy source and the temperature of the cooling water the condenser loop If we size based on the daily average load and the daily average solar energy we will find that the energy available around noon is greater than the energy demand of the chiller This results in the heat medium fluid temperature in the
6. in the tropics The second variable is the intensity of the sunlight The brighter the sunlight the more efficiently it can convert the sunlight into heat This is more related to the heat absorption mechanism and neither type of collector necessarily has an advantage over the other on this one Flat panel collectors have a greater aperture area to gross area ratio than evacuated tube collectors With a flat panel a very large proportion of the area within the frame is absorption material While with the evacuated tubes only the tubes themselves represent the aperture area There is a space between the tubes where the sunlight passes straight through This means there is less sunlight striking the working surface of the array resulting in less sunlight being converted into heat for an equivalent gross collector area When the collectors are lab tested the results are converted into a quadratic efficiency equation which represents the performance of the collectors relative to the two primary variables which affect them One is the intensity of the sunlight and the other is the AT temperature difference between the fluid in the collector and the outdoor ambient This modeling tool utilizes these empirically derived efficiency equations to simulate the performance of the collectors There are two agencies doing independent performance testing on these collectors In the US it is the Solar Radiation Certification Corporation SRCC and
7. south azimuth while most of the evacuated tube collectors will provide the best afternoon sun with a 0 to 15 west of south orientation STEP 5 SIZE COLLECTOR ARRAY It is true that most any glazed flat panel or evacuated tube collector will meet the needs of our application provided we use enough of them It is also true that most any collector we want to use will cause us difficulties if we have too many of them So it is important that we size the collector array in the most appropriate manner and have a strategy for managing the heat The solar modeling template will calculate the optimum size of the collector array number of selected collectors for a number of different defined conditions Once the solar data for your location is entered and selected and the desired collectors are entered and selected you can begin to set up your application Start by choosing the best overall beginning tilt angle Since we are planning to Peak Shave an existing chilled water plant we will start out with a tilt angle of latitude minus 15 We will set the array azimuth to 45 west of south for our flat panel or 15 west of south for our chosen evacuated tube Next we need to set the cooling tower approach The approach is defined in this template as the Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 difference between the wet bulb temperature and the temperature of
8. 600 0 500 400 500 0 400 600 700 800 900 Efficiency 0 300 0 200 0 100 0 000 0 10 20 30 40 50 60 70 80 90 AT Deg C Figure 12 As you can see this graph displays a different slope for each of 9 different intensities of Insolation I For an insolation intensity of 500 Watts m and a AT of 50 C look where the purple line crosses the 50 C vertical grid line Now looking across to the Y axis you can see that this particular flat panel collector will convert about 33 of the 500 Watts into heat in the fluid But at an Insolation level of 300Watts m this particular collector is virtually useless at a AT of 50 C Now refer to figure 13 to compare this collector with an evacuated tube collector The first obvious observation is the apparently low zero loss efficiency This collector only converts about 41 of the sunlight striking the gross area of the collector into heat This difference is due to the much lower aperture area to gross area ratio But notice that at a AT of 50 C the collector still produces a good deal of energy at an Insolation intensity of 300 Watts m It becomes obvious that this type of collector will provide more hours of contribution while at the same time requiring more gross area during the hours of peak Insolation We next want to compare the Incidence Angle Modifier IAM for these two types of collecto
9. ESL IE 10 05 03 Practical Solar Thermal Chilled Water K A Bud Leavell Sales Engineer ABSTRACT With the pressing need for the United States to reduce our dependence upon fossil fuels it has become a national priority to develop technologies that allow practical use of renewable energy sources One such energy source is sunlight It has the potential to impact America s use of non renewable energy beyond its own design capacity by applying it to the optimization of an existing building s system Solar thermal chilling systems are not new However few of them can be described as a practical success The primary reason for these disappointments is a misunderstanding of solar energy dynamics by air conditioning designers combined with a similar misunderstanding by solar engineers of how thermally driven chillers react to the loads and energy sources applied to them With this in mind a modeling tool has been developed which provides the flexibility to apply a strategy which can be termed Optimization by Design THE CURRENT STATE OF SOLAR THERMAL CHILLING SYSTEMS IN THE US As of January 2010 there are about 22 solar thermal chilling systems installed in North America Of these only a handful can be said to be providing a useful function The majority are in place for research or as a showcase while never realizing system potential Yet the interest in the possibilities of this technology is growing rapidly The sim
10. account for the extra losses Remember that the peak building load occurs later in the day and tends to continue into the night So your chiller load does not necessarily drop off immediately when the sun goes down It will drop off gradually just like it built up STEP 8 DEFINE YOUR CONTROL STRATEGY Among the items with which we must be concerned is determining when there is adequate heat to run the chiller when the cooling water is to be on or off how to control the temperature of the heat medium fluid and the cooling water and how to minimize losses when the system is shut down for the night To begin with it is necessary to define a temperature of the heat medium fluid below which you will stop the system Then once the system is stopped you need to thermally isolate the buffer tank from the chiller and most importantly from the collector array Experience has shown that unless the buffer is isolated you will lose 60 or more of the heat in the tank over night even with the pumps off Thermal siphoning will literally suck the heat right out of your tank Then come morning you will find that the mass of fluid in the collectors and distribution piping will be at ambient temperature This mass will be significant and might even be greater than the mass in your buffer tank It is extremely important that you not start pumping this fluid into the buffer tank until it is at a higher temperature than the fluid in the tank S
11. ach of the 12 months of the Typical Meteorological Year Since there are differences from one year to the next some smoothing is done on the transition days between these months The modeling template will give you a choice of using either the 15 year average or the TMY3 data for the solar energy and goes to the TMY3 data for all dry bulb and relative humidity values ESL IE 10 05 03 We will also want to download some daily integrated data from NASA We will use this for the purpose of determining the most advantageous initial tilt angle for the collector array To retrieve this data visit http eosweb larc nasa gov sse Tilt of the collector is defined as that angle from the ground to the collector on the side of the collector away from the equator figure 4 Ste Equator Facing Collector Figure 4 Tilt angle can have a profound effect on the amount of energy delivered to the aperture surface of the collector The data which we will be using is that sunlight which is falling on the horizontal surface of the ground Tilting the collector to the point where it is normal to the sun perpendicular to the sun s rays will increase the amount of sunlight to which the collector aperture area is exposed Figure 5 illustrates the difference In the case that the collector is normal to the sun we take the amount of sunlight falling on a horizontal surface Ih and divide it by the cosine of the solar zenith angle 0z
12. and the boiler simultaneously Use one or the other but not both at the same time This will at the very least result in the boiler bouncing on and off but most likely will result in the boiler providing 100 of the heat to the chiller and eventual damage to the solar array due to stagnation Depending on the type and ESL IE 10 05 03 design of your collector array you will also very likely find a large volume of the heat from the boiler being lost through the collector array by emission Auxiliary Boiler only with array isolated __ City water Runs any time the air conditioner is on Figure 21 STEP 9 EVALUATE ANTICIPATED MODELED SYSTEM PERFORMANCE The template will provide a graph of the anticipated output in Refrigerant Tons of chilled water on an hour by hour basis for the average day of each month This data may now be used as the basis of test for your what if choices Figure 22 shows a representative output from a 10 Ton chiller with flat panel collectors pointed 45 west of south at an 18 Tilt and a 10 F cooling tower approach This makes it easy to examine the impact of multiple what if scenarios Among them are changes to the type of collector orientation cooling tower size glycol solutions etc Chiller Output in Refrigerant Tons Model SC10 Flat Panel B 18 Tilt 45 Azimuth 10 F Tower Approach Jan gt Feb Jf SN es Mar 8 0 lt 7 MES SS A
13. ate does that for us for a select day each month You should decide what day of any given month is the best representation of the month for your application and then enter those dates in the Solar Geometry tab The default dates in this template represent the day which has extraterrestrial radiation closest to the average for the month as determined by Klein 1976 This template does not take into account the apparent changes in solar position due to refraction through the atmosphere Those are most pronounced much later in the day anyway and would have very little effect on our model Figure 11 shows where to expect the sun in the sky at this given location in March The X axis depicts the solar azimuth angle where 0 is due south in the northern hemisphere a negative azimuth is east of south and a positive azimuth is west of south The Y axis represents the solar altitude angle The altitude angle is equal to 90 minus the solar zenith angle The solar zenith angle represents the angle of the sun relative to horizontal Position of the Sun inthe Skyin March v 80 0 j bo aae al Do 130 v 1000 T e H500 m 80 D 0900 40 0 YY 1600 lt __ t_ s007 DES E7 2604 3 ed __ 1200 al D 007 Degrees of Azimuth from Due South Figure 11 No two collectors from different manufacturers are the same and due to a
14. buffer tank increasing This is what we want Store the energy for use later The only trouble is the increased temperature in the tank increases the amount of work the chiller can perform while at the same time reducing the COP of the chiller Later in the afternoon as the solar energy is decreasing and the building load is increasing the engineer discovers too late that he has less energy stored than he thought The result is an inadequately powered chiller when you need it the most So how do we remedy these shortcomings Utilizing a recently developed modeling template in a step by step process the engineer can adequately size the energy source and buffer storage position the collector array for best performance and evaluate the predicted output including the financial benefits of the application STEP 1 EXPLICITY DEFINE THE EXPECTATION We need to start with a clear definition of what we expect the sun to do for us There are a lot of opportunities for increasing the energy efficiency of a building HVAC system Solar provides a lot of possibilities limited only by the engineer s imagination and the economics of the individual application In looking for the place where we can have the most effect the low hanging fruit would be those applications where we can get more savings out of the investment than the capacity of the invested system If we can find a way to get greater than 40 reduction in electric use by correctly app
15. cation was for dehumidification and we ESL IE 10 05 03 needed to accomplish that during the winter months as well as the summer months we might opt for Latitude 15 instead Overall in North America Latitude 15 seems to work best for air conditioning This is the only use we will make of daily integrated values of solar data Everything else will be done with hourly data STEP 3 WHAT TYPE OF COLLECTOR Now we need to select the most appropriate collector for our application The key here is remaining focused on the application This modeling tool recognizes only 2 types of collectors Those are the flat panel and evacuated tube collectors Since this tool is designed around single effect low temperature water fired chillers high heat medium temperatures are not required What is required is a high volume of heat For instance a 30 Refrigerant Ton Chiller at rated conditions will require 512MBtuh input hot water to produce 30 Refrigerant Tons It will in fact produce greater than 30 Refrigerant Tons 1f the heat input is increased With this in mind we need to find the most cost effective way to get heat into the chiller when it is required or can be utilized The primary differences between flat panel and evacuated tube collectors lies in their loss profile ratio of aperture area to gross area and incidence angle modifier The choice of one type of collector over the other is dependent on multiple factors Cost is always
16. d capacity and heat input These of course will vary with the changes in solar and meteorological conditions The template will make those calculations for you Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 STEP 7 SIZE THE BUFFER TANK You will need some mass of heat medium fluid to smooth out the fluctuations in heat medium temperature and to insure adequate energy to start the chiller at morning start up There have been rules of thumb used in the past of 1 5 to 2 0 gallons of buffer storage tank for every ft of collector area These rules are probably not valid any more due to the increases in the efficiency of today s collectors Instead you should take advantage of the template s ability to estimate the temperature of the storage medium under differing conditions It is critically important that you not allow the heat medium temperature to exceed 203 F entering the chiller This will cause the chiller to shut down requiring a hard reset to protect from crystallization of the solution Since the solar and meteorological data are typical or averages of averages rather than peak conditions it is very important that you anticipate receiving far more sun from time to time than you can handle It is probably not a good idea to under power the chiller to prevent over heating That may make sense for domestic hot water but will make for a very unsatisfactory exp
17. dvances in collector technology Rules of Thumb will no longer be adequate Fortunately there is plenty of empirical data describing the performance of the majority of those collectors currently marketed in the United States and Europe There is provision in the template to enter efficiency equations for up to 48 different collectors And there is provision in the template to compare the efficiencies and Incidence Angle Modifiers of these collectors to enable an intelligent selection of the most appropriate collector for your application Figure 12 depicts the efficiency of a flat panel collector The X axis show the AT temperature difference between the fluid in the collector and the outdoor ambient while the Y axis reflects the actual efficiency of the collector If 100 of the sunlight is transferred to the fluid as heat then the efficiency is Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 1 0 The variable nO is defined as the zero loss efficiency or how much sunlight is transferred to the fluid as heat with no losses from temperature differential O AT At zero loss efficiency this collector converts about 77 of the sunlight striking the gross area of the collector into heat Notice how the efficiency decreases as the AT increases Efficiency of Collector Flat Panel B 0 900 0 800 lin Wim 0 700 100 200 300 0
18. e Energy Laboratory Technical Report NREL TP 581 43156 Revised May 2008 Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010
19. ed systems inadequate and incorrect control strategies and a lack of critical safety devices Another obvious shortcoming was the application of too small an amount of common sense and an inaccurate understanding of the technology Solar Thermal is one area of engineering which is rife with misinformation yet there is a wealth of accurate information available on the internet for free It comes from reputable sources including the US Government and third party product evaluation laboratories not to mention some very good texts Today there is no reason for a system to be designed in the United States based on other than empirical data As an example the most common problem observed in these systems is their lack of adequate solar energy The explanation for this turned out to be very simple it is caused by the disconnect between air conditioning disciplines and solar thermal disciplines Traditionally the solar thermal guys have used integrated daily solar energy values which are themselves derived from averages of averages They also calculate the loads they intend to service in the same way And for simple loads like domestic hot water this seems to be adequate On the other hand air conditioning loads are very difficult to average over a day much less over a longer period of time So how do you adequately match the load with the energy source Following these methods you are bound to find yourself under powered Looking at it
20. er system by using hourly solar and meteorological data than attempting to use daily integrated values There are several weaknesses with the daily data The first is knowing how many hours per day the integrated data covers Even if you know how many hours it covers that doesn t guarantee that your collector array will be able to collect heat for the entire period It probably cannot due to the angle of the sunlight to the collector aperture So how can you then estimate the average hourly solar intensity since the total daily amount available is in question Secondly it is impossible to get a single dry bulb temperature and solar intensity that adequately represents the average value during the entire day You will inevitably get one or both values such that you are either overestimating or underestimating the output of the collector The observed tendency is that the output of the collector array is overestimated resulting in many of the existing North American solar chillers being grossly underpowered If on the other hand you use hourly data the anticipated load on the chiller during a given time period may be compared to the expected heat output from the collectors during that same period This will allow for a more precise method of sizing the collector array and buffer storage tank Calculating collector output is one area that is rife with misinformation There is much oversimplification in the marketing materials of some equ
21. erience with air conditioning It is a very good use of this excess heat to supplement domestic hot water or some other application You do need to insure there is a place to send 100 of the excess heat whenever it arrives or else you must have a heat dump The larger the buffer tank the less likely you are to dump heat but do not over size the buffer tank You will pay a price in both the lost contribution in the early part of the day and in reduced capacity of the chiller due to a reduced temperature of the heat medium Not to mention the expense of the larger tank The modeling template will show that the best all around tank size will be somewhere between 8 to 20 minutes of heat medium flow rate Do not attempt to oversize the buffer tank with the idea of working for any significant period of time after the sun goes away In order to support off sun operations you will need to store excess energy in an adequate volume to operate the chiller To do that will require that you collect significantly more heat than the chiller can use during periods of sunshine This will require even more collector area Significantly more collector area and that expense alone is likely to render the project impractical If you have a summer day with about 7 hours of collectible sunshine and you want to run the system for 7 hours after the sun goes down you will essentially require twice the collector area plus a ESL IE 10 05 03 little extra to
22. in Europe Solar Keymark This modeling tool will recognize the performance data from both of these agencies with one caveat In North America all performance data is based on the gross area of the collector where in Europe it is based on the aperture area This modeling tool is set to display results in gross area However if you enter the Solar Keymark data into this system using the aperture area then the system will still calculate the required number of collectors but the display showing gross area should be interpreted as aperture area instead There is significant disagreement among many in the solar thermal industry as to the appropriateness of using gross collector area as the basis for collector efficiency For one thing it makes the efficiency of some collectors appear far lower than they would be if aperture area is chosen But using gross area vs aperture area has absolutely no impact on the number of collectors used nor does it affect in any way the ESL IE 10 05 03 amount of roof space required And after all what we need to know when sizing the collector array is how many collectors are needed how much roof area is required and how much spacing is needed between rows to prevent shading In the southern latitudes flat panel collectors will require less roof space than evacuated tubes In the northern latitudes that can be just the opposite due to the greater AT Flat panel collectors will provide more shading on
23. ing agency AT is the difference in temperature between the fluid entering the collector and the outdoor ambient Make certain that you choose the proper constants The SRCC displays the equations for both SI and IP units The template uses the SI units for these calculations IAML is the longitudinal Incidence Angle Modifier AMT is the transverse Incidence Angle Modifier IAM and AMT are normally represented as Ka and are a quadratic equation and IAML is a first order equation from SRCC But with Solar Keymark you normally get a table of values You can either calculate based on these values or use a spreadsheet to create the quadratic equation from the table and then use those values in the template ESL IE 10 05 03 Ka 1 b1 1 cos 1 b2 1 cos 1 Equation 4 IAML 1 53 1 cos 1 Equation 5 Where b1 b2 and b3 are signed values supplied by the SRCC is the angle of incidence Incidence Angle Modifier Evacuated Tube A 1 50 ee ee ee a 1 40 0 8 IAM _ j oO 0 10 20 30 40 50 60 7 Degrees from Due South AM Transverse IAM Longitudinal Figure 15 STEP 4 CHOOSE OPTIMUM COLLECTOR AZIMUTH So which direction should we point the collector array It is not necessarily true that the collector should be pointed directly south Unless the array is a bi axial tracking array there will be a limited n
24. ipment manufacturers that lead the engineer to make performance calculations based on solar and meteorological conditions which do not exist in the real world This of course is completely unnecessary as data describing real world conditions in the United States is readily available The source for this hourly data in the United States and its territories is the National Solar Radiation Database If you are outside of the US or its territories and your government does not have this data available you will need to model it from the daily average data available from NASA Don t forget that you need solar irradiance plus dry bulb and wet bulb Often when using data from somewhere other than the National Solar Radiation Database you will only get the data in 3 hour Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 averages You will need to interpolate this data in order to get a reasonable approximation of the actual hourly conditions And if you are modeling your own data you must differentiate between Direct or Beam radiation and diffuse Normally this daily data is described as Global Normal or Global Means This is a combination of Beam and Diffuse and you will have to separate them The data from the NSRDB is already broken out as Global Means and Diffuse Beam radiation is then calculated by subtracting diffuse from global The National Renewable Energy Labs has
25. lying solar energy to 25 of the load then we will have achieved an exponential savings This is possible in at least two scenarios where the physics of an ESL IE 10 05 03 existing process is optimized by strategically applying the energy from the sun Since many commercial buildings have existing chilled water systems and since virtually all chillers have a COP Coefficient of Performance which improves as the load on the system decreases then we have an opportunity to use solar energy in a peak shaving configuration to gain just such savings Figure is a graph of the power curve of a popular water cooled screw chiller A close examination shows that if we can keep this screw chiller running between 25 and 75 of full capacity then the electric demand per ton of chilled water is at its most advantageous point Part Load Performance For Chiller Type 0 800 0 700 0 600 0 500 0 400 0 300 0 200 kWatts per Refrigerant Ton 0 100 0 000 T T T T T T T T T 0 10 20 30 40 50 60 70 80 90 100 Percent of Full Load Figure 1 Figure 2 illustrates the financial effect of part load operation on this same chiller Notice that at 100 capacity the COP is 4 88 So for every 1 kW of electric energy applied the chiller does 4 88kW of work This is actually quite good But reduce the load to 75 of capacity and that COP increases by another 21 So the expectation f
26. n eastern laying collector from a panel to the southwest of it This worksheet takes into consideration the altitude and azimuth of the sun and the azimuth of the collector array at 2 00PM standard time for collectors pointed west of south and at 10 00AM for collectors pointed east of south To calculate the spacing required you will need to know the length of the collector and the sun s azimuth and altitude angles on the day which you define as the shortest day of the year for your calculations Normally you would select December 21 as the shortest day of the year But if you do not care about solar energy during the winter you may prefer to keep the spacing minimized in order to maximize the shading on the roof So perhaps April 15 is more appropriate for your application The spacing between the rows of collectors is as follows D b b b Lc sin 90 Tilt b a sin 90 0 sin 0 a Lesin Tilt Equation 8 Equation 8 1 Equation 8 2 Equation 8 3 Where D distance between leading edge of the rows of collectors Le length of collector Tilt the collector tilt angle from horizontal solar altitude angle a height of the collector as a function of tilt angle Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 You may want to perform this step much earlier in the process to insure that you have adequate space for your collecto
27. nting 1978 5 Basics of Refrigerant Chillers Vincent Schraida P E featured article in Engineered Systems December 1 2008 6 DIRECTORY OF SRCC CERTIFIED SOLAR COLLECTOR RATINGS Solar Rating Certification Corporation January 21 2009 ESL IE 10 05 03 7 National Solar Radiation Database 1991 2005 Update User s Manual Technical Report NREL TP 581 41364 April 2007 8 Optimized Design of Hot Water Storage in Solar Thermal Cooling Facilities R Salgado P Rodriquez M Venegas A Lecuona M C Rodriquez ITEA Research Group Universidad Carlos HI de Madrid 9 Power From The Sun by William B Stine and Michael Geyer copyright 2001 by William B Stine and Michael Geyer www powerfromthesun net 10 Solar Energy S A Klein 1977 as quoted by Duffie Beckman 11 Solar Engineering of Thermal Processes John A Duffie William A Beckman John Wiley amp Sons Inc 3 Edition 2006 12 Solar Heating and Cooling Active and Passive Design om Edition Jan F Freider and Frank Kreith 1982 Hemisphere Publishing Corp 13 Solar Heating and Cooling in Japan T Ishibashi Yazaki Parts Co LTD 1978 14 Solar System Design Manual Yazaki Corporation 1980 15 Surface Meteorology and Solar Energy SSE Release 6 0 Methodology Version 2 3 November 6 2008 NASA 16 Users Manual for TMY3 Data Sets S Wilcox and W Marion National Renewabl
28. o with the distribution and collector array isolated from the tank establish a process where you can determine if there is adequate sunlight to begin heating water One way you might choose to do that would be with a solar PV collector at the same tilt angle and azimuth as the solar thermal array Connect this PV to the control system and use it to measure the sunlight Once an adequate amount of solar energy is striking the array as measured by the solar P V device have the control system turn on the fluid pump and begin circulating the fluid through the pipes and array only Many of the solar thermal collector manufacturers already have this capability available as an option Monitor the temperature in the tank and the distribution lines Once the temperature of the fluid circulating through the array and pipes reaches degree higher than the tank open the valves to the buffer tank and begin charging it Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 Once the temperature of the fluid in the tank reaches the predetermined chiller start temperature this should be much higher than the chiller stop temperature turn on the system and start the chiller The reason for the higher start temperature is that the chiller at first start will take a great deal more energy until it gets balanced and will return a AT on the heat medium much greater than the design AT resulting in the
29. oling capacity Since we are talking about a single effect chiller the COP at rated conditions will be 0 7 That means for every 1 000Btu of chilled water we must input 1 429Btu of heat to drive the process Thus we must reject 2 429Btu of heat to the cooling tower Failure to adequately reject the heat input to the chiller will cause the chiller to perform below expectations and possibly affect its life expectancy The flow rates for the cooling tower tend to be much higher than for an electric chiller and are generally fixed You should plan on having a constant flow rate through the condenser loop regardless of the load and allow the chiller to control the flow on and off You will also want to have a strategy in place to maintain a minimum cooling water temperature Absorbers are rated at one temperature but often produce higher capacities at a lower temperature There is typically a temperature below which you will no longer gain capacity but will in fact negatively impact the COP of the chiller Allowing the chiller to control the fan on the cooling tower will provide some measure of control but to reach the ideal 80 F cooling water temperature you will probably want to add a thermostatically controlled mixing valve The capacity of the chiller will also change with the temperature of the heat medium In order to take advantage of this potential extra capacity the heat balance should be calculated based on the greatest anticipate
30. one of them Perhaps available roof space or the existence of large quantities of snow should be considered The application s needs must be paramount If the collector array choice brings more negatives than it does positives then the system implemented could easily turn out useless There are two variables which primarily affect the efficiency of these collectors The 1st and greatest effect is the temperature difference between the outdoor ambient and the average temperature of the heat medium fluid in the collector The greater this AT is the greater the thermal energy losses and the lower the efficiency of the collector Evacuated tubes have a significant advantage here as they essentially have the heat collection medium sealed up in a thermos bottle But this can turn into a serious disadvantage in locations with a large amount of snow Due to their inherently low losses it is very easy for evacuated tubes to be rendered useless by snow cover On the other hand the losses of flat panel collectors can effectively keep the snow cleared away providing solar contribution when evacuated tubes would be buried in the snow Evacuated tubes Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 are most often designed using heat pipe technology This brings the advantage of lower losses through emission but also forces them into a minimum tilt angle which may prove a disadvantage
31. op the electric chiller The VFD Variable Frequency Drive in the chilled water loop must have a lower limit of the flow rate for the absorption chiller and a maximum rate of 100 of the load capacity Another advantage to this type of configuration 1s in the capital cost savings Since the building already has an airside system with chilled water we can ignore that portion of the system when calculating the costs And there is no need to supply a back up heat source for when the sun is bashful or sleeping since the building is already equipped with a chilled water system sized for 100 capacity STEP2 HOW MUCH SUN DO WE HAVE There are a number of factors affecting the intensity and duration of solar radiation available to power our system Among these factors are latitude air pollution especially ozone and particulates atmospheric moisture content cloud cover frequency and duration of precipitation Other factors which must be considered are the ambient dry bulb and wet bulb temperatures The intensity of the solar radiation and the dry bulb temperature are required to estimate ESL IE 10 05 03 the heat production of the collector s The wet bulb temperature is required not only for sizing the cooling tower but also for estimating the temperature of the water leaving the tower as this affects both the chilled water output and the volume of heat input to the chiller It is far better to size a solar array for a chilled wat
32. or this example application will be to keep this existing screw chiller from exceeding 75 of full capacity while the sun is shining This will provide an absolute reduction in power required to air condition the building during the peak period of about 25 Plus we will realize an additional savings for the remaining portion of the load through the increased COP of the chiller It should not be unexpected to realize an electric consumption reduction of over 40 during these peak periods The method we will use is a sidestream configuration where the solar driven absorption chiller is pre cooling a portion of the return chilled water By reducing the temperature of the chilled water returning to the electric chiller it will then unload as the solar system is reducing the work it is expected to do See figure 3 Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 100 Cap 75 Cap COP 4 88 5 91 21 Input KW elec 1054 Output kW cooling 513 2 KW electric Ton 0 720228 0 595 Cost per KW 0 1266 Cost per Ton Hour 0 0912 0 0753 0 0159 Figure 2 Combined Cycle Side Stream Piping O VFD Chilled jur Water a Supply Water Fired Chiller Using Solar or Waste Heat Chilled 52 F ggf Water Chilled Water Retum Figure 3 Anytime the load is smaller than 25 of the electric chillers full capacity and the sun is shining we can completely st
33. ple prospect of having the sun do for free what we have traditionally paid hard cash for is very compelling Add to that the changing political climate driving State and Federal Governments to offer cash and tax incentives and now the demand for engineers to evaluate the potential of this technology in various applications is tremendous If the technology can be made practical especially from a financial point of view the impact can be significant So what is wrong with those few systems installed in North America If we are going to make these systems practical it is imperative that we understand what motivated the installation how it was designed and implemented and why certain design practices were followed especially on those less successful systems What was discovered from an examination of many of these systems was a disconnect between Yazaki Energy Systems Inc Plano Texas the two different disciplines involved in this technology While air conditioning designers were competent and comfortable with hydronic cooling plumbing building loads and chilled water in general they seem to have a misunderstanding of solar energy dynamics When combined with a similar misunderstanding by solar engineers of how thermally driven chillers react to the loads and energy sources applied to them you have the recipe for a very expensive system incapable of performing satisfactorily Among the issues observed have been grossly underpower
34. pr 9 May JUN Jul ome Aug Sep Oct Nov 1700 1800 __ Dec i 1 1200 1300 1400 1500 1600 CentralStandard Time Figure 22 J 1 1 900 1000 1100 Once we have settled on a basic design as expressed above we should evaluate the systems impact on the existing installation to determine 1f it meets our specified design criteria For instance we determined at the beginning of the process that we were going to keep the existing chilled water plant running at or below 75 of full capacity To do this we will need to know what the existing chiller s Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 performance curve looks like as well as the building load hour by hour The template provides the basic framework around which this may be built It also includes a number of graphs which can be used to confirm that your defined goals for system COP carbon footprint reduction electric consumption and cost reduction were realized It must be emphasized that this is a template and not a completed tool for the simple reason that no two of these installations will be identical Therefore it should be expected that the template will be modified by the engineer to meet his anticipated application Figures 23 and 24 demonstrate from the template that our stated goals of maintaining the existing chiller at 75 or less of full load is met and that the sys
35. r array The calculations done for the amount of collector gross area do not take this minimum spacing into account and therefore do not reflect completely the amount of roof space or other area required to accomplish the desired mission So don t forget this step Conclusion The application of common sense engineering principles along with the use of this template will provide the best chance of designing a solar thermal powered chilled water system with a financial payback and an end user satisfied with the installation It is not necessary to design around very high heat medium temperatures or to have complex design strategies It is important to take a realistic approach to how much contribution can be made with the sun and to minimize the initial investment as much as practical However it is best to minimize the recurring costs while at the same time maintaining simplicity as a key design component Avoid the temptation to store heat for use when the sun is unavailable and instead size the array to match the load at the time the load is greatest This will insure a satisfactory installation that provides decades of trouble free operation References 1 Active Solar Heating Systems Design Manual ASHRAE pub 1988 2 ASHRAE Handbook Fundamentals 2001 3 ASHRAE Handbook HVAC Applications 1999 4 ASHRAE STANDARD 93 77 Methods of Testing to Determine the Thermal Performance of Solar Collectors corrected pri
36. rs Once we have the efficiency for our sizing conditions we will compute the IAM and multiply the efficiency by the IAM s As you can see from figure 14 the IAM for the flat panel begins to drop off at a fairly steady rate to the point where the panel ESL IE 10 05 03 losses are compounded above 60 Figure 15 shows the IAMs for the evacuated tube collector The longitudinal IAM is very similar to the IAM for the flat panel But notice how the Transverse IAM seems to provide a gain in efficiency This combination of low thermal losses with the Transverse IAM is what causes the flatter output graph seen in figure 10 Efficiency of Collector Evacuated Tube A 0 900 0 800 lin W m 0 700 100 200 300 0 500 ERO 500 0 400 600 700 800 900 0 600 Efficiency 0 300 0 200 0 100 0 000 0 10 20 30 40 50 60 70 80 90 AT Deg C Figure 13 Which collector strategy you choose should be driven by your application The objective is to get the most solar contribution for your dollar invested Since there are many factors which will contribute to this decision you should use the template as a tool for What If calculations in order to make the best overall selection One final note on collectors it is a myth that heat medium temperatures in excess of 200 F are required to drive an absorption chiller There are single effec
37. t absorption chillers on the market which will effectively produce chilled water with heat medium temperatures as low as 158 F And 175 F seems to be the best compromise between what the collectors can effectively produce and what the chiller can effectively put to work So you should plan on sizing your collector array to meet the load demand with the solar energy available at that time at 175 F heat medium input You will also discover from the template that the heat medium temperature often exceeds the 175 F design temperature as the heat input exceeds the chillers capacity You should also engineer the AT of your collector array to match the AT of the chiller under your design conditions Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 Incidence Angle Modifier Flat Panel B 0 10 20 30 40 50 60 70 80 Degrees from Due South AM Transverse IAM Longitudinal Figure 14 Heat Output Ho for a flat panel collector Ho N0 alAT I a2 AT 1 x IAM Equation 2 Heat Output Ho for an evacuated tube collector Ho N0 alAT I a2 AT 1 x IAMLx IAMT Equation 3 where al and a2 are provided by the testing agency They are usually but not always negative values and must be entered into the template as signed values noO is the zero loss efficiency supplied by the test
38. t another hour Since this is an air conditioning application it is imperative that we provide adequate energy to drive the chiller at the time we need it the most It appears that the best over all fit for sizing an air conditioning collector array is to meet the design requirements at 3 00PM This will provide just enough over sizing during the mid day period to have adequate energy stored in the buffer tank for later in the afternoon Figure 18 shows the solar energy output from a flat panel array positioned due south While figure 19 shows the output from the same array facing 45 west of south If you are able to position the array this far west you can see a 50 increase in energy available at 3 00pm versus pointing it due south The tradeoff is in the reduced solar contribution in the morning Since our application is focused on afternoon peak shaving we get more value from afternoon sun than from morning sun ESL IE 10 05 03 Solar Heat Output in Watts adjusted for Solar Angle of Incidence and 0 Degrees Azimuth 900 a A MEOT TONE o A NT wi N 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Figure 18 Solar Heat Output in Watts adjusted for Solar Angle of Incidence and 45 Degrees Azimuth 900 i tN e e a i A e T YT as SY gt A A AAT TTN 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Figure 19 Most of the flat panel collectors will meet this demand with a 45 west of
39. tem COP is improved by more than the 25 of the load we are taking with the solar system Optimization Effect on Existing Chiller Load Solar Application 2 Typical Screw August of Full Load D So 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Standard Time eee Un optimized Chiller Load Optimized Chiller Load Figure 23 Optimization Effect on Existing Chiller Efficiency Solar Application 2 Typical Screw July kW RTon 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Standard Time oe Unoptimized KW RTON Optimized KW RTON Optimized System kW RTon Figure 24 STEP 10 SHADE AVOIDANCE This step is taken last as the final step in engineering the system only because the template does nothing from a performance evaluation point based on shade prevention Of course 1f the collector is shaded it is not gathering energy from the sun So the last item is to determine the minimum spacing required from an obstruction such as a parapet adjacent building trees etc and of course the minimum spacing required between rows of collectors to keep them from shading one another Figure 25 illustrates the process ESL IE 10 05 03 a he Le Sjn G0 Tilt SiN Figure 25 Shade avoidance is the simple application of the Law of Sines as illustrated above It is important to keep in mind that the azimuth of the sun changes as the day wears on and so the calculations must prevent shading of a
40. the cooling water leaving the tower The ideal cooling water temperature is 80 F but that may not be practical to achieve Use the template to determine what you can live with We will next choose the size of chiller either 10 20 or 30 Refrigerant Tons and the target capacity of the chiller And we need to decide the best method and season for sizing If we choose automatic sizing the template will average the heat output from the selected collector at the selected time of day over the selected months For our example application we will choose automatic sizing at 1500 hours during the months of May through September The template will then take the amount of heat required for the target capacity and divide it by the amount of heat output from the selected collector to calculate the number of collectors The number of collectors required and the needed gross collector area are then displayed We also have the option of limiting the number of collectors In that case the template will calculate the number of collectors required to achieve the stated capacity with a maximum of the number you set for a limit In order to size the collector array to meet the demand we must establish how much energy is required to drive the chiller our losses in the heat distribution mechanism how much solar energy is available on average at the time of our design load and the physical characteristics of our collectors To start calculate the solar
41. the roof but may in turn place a greater structural load on the building Finally the Incidence Angle Modifier can have a profound effect on the amount of energy collected When the collector is normal to the sun virtually 100 of the direct solar radiation is captured by the panel Although some is lost and some re radiated none is reflected back into space However as the sun s angle of incidence advances either side of normal a portion of that solar radiation is reflected back into space by the glazing of the panel A bi axial tracking array would eliminate that effect With any fixed array we must take the angle of incidence into account For a glazed flat panel calculating the angle of incidence and its effect 1s quite simple Not quite so simple with an evacuated tube collector Here there are two separate Incidence Angles we must deal with First is the longitudinal incidence angle This is the way the sunlight strikes the tube lengthwise and is normally calculated from the sun s altitude angle The second is the transverse angle of incidence This is the way the sunlight appears to the tubes as the sun s azimuth changes relative to the collector As the collector is normal to the sun the spacing between the tubes is most pronounced But as the sun moves laterally the spacing between the tubes as seen from the sun decreases This has the effect of increasing the ratio of aperture area to gross area Graphing the heat o
42. umber of sunlight hours during which your collectors can effectively capture energy So it is first necessary to decide whether we need morning sun midday sun or afternoon sun For our example application optimizing an existing chilled water plant by peak shaving it is logical to target afternoon sun Let s start by comparing the available sunlight with the peak temperatures Figure 16 shows the Insolation available on an hour by hour basis while figure 17 shows the typical outdoor ambient temperature on the same hour by hour basis Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 Insolation adjusted for Time of Day Solar Angle of Incidence and Cosine Effect 0 80 I May Jun jul Aug kWatts m2 oO Qo oO Sep o D 0 20 0 00 j 0900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Figure 16 Average Temp May June July Degrees F August foe Oo September 70 800 900 1000 1100 1200 bee 1400 1500 1600 1700 1800 1900 ime Figure 17 As you can see maximum solar energy appears around solar noon at this location just slightly past 12 00PM while the warmest outdoor temperatures occur between 3 00 and 4 00PM And the peak load on the building will probably lag the peak outdoor temperature by ye
43. utput from these two different collectors shows that a flat panel collector s output is bell shaped While the evacuated tube collector s output is much flatter The evacuated tube collector will have the advantage of greater output both earlier and later in the day See figures 9 and 10 respectively for a graph of the collector heat output from a flat panel and an evacuated tube collector Proceedings from the Thirty second Industrial Energy Technology Conference New Orleans LA May 19 22 2010 Take into account solar angle of incidence 08 00 12 00 16 00 Figure 7 Solar Incidence Angle on a flat panel Longitudinal Incidence Angle Transverse Incidence Angle Figure 8 Incidence angles on an Evacuated Tube Solar Heat Output in Watts M2 600 500 400 300 200 100 0 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Figure 9 Solar Heat Output in Watts M2 450 400 350 300 250 200 150 100 50 0 T 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Figure 10 In order to accurately size the collector array we need to be able to calculate the output of the collectors at a given time of day To accomplish this we need to know the outdoor ambient temperature ESL IE 10 05 03 the intensity of the sunlight and the position of the sun in the sky This templ
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