RETScreen - Solar Water Heating Project Analysis - Speaker's notes
SLIDE 1: Solar Water Heating Project Analysis
This is the Solar Water Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the use of the sun's energy to heat water using solar collectors, such as those mounted on this building in Ontario, Canada.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of solar water heating (SWH) systems; second, to illustrate key considerations for solar water heating project analysis; and third, to introduce the RETScreen Solar Water Heating Project Model.
SLIDE 3: What do SWH systems provide?
Simply put, solar water heating systems use the energy in sunlight to heat water. They can be used wherever moderately hot water is required. "Off the shelf" solar systems can provide hot water to the bathroom and kitchen of a house; larger systems can be used for bigger buildings, such as multi-unit apartments, restaurants, hotels, motels, hospitals, and sports facilities, and even for industrial and commercial processes, such as car washes, laundries, and fish farms. Solar systems can also be very cost-effective for heating swimming pools.
In addition to these energy benefits, solar water heating systems may provide increased hot water storage and lengthen the season during which an outdoor swimming pool can be used. Because it will not always be sunny when hot water is needed, many solar water heating systems include a dedicated hot water storage tank. This increases the total hot water storage capacity of the system, such that periods of heightened hot water use will not cause the system to "run out" of hot water. When seasonal air temperatures are too chilly to permit swimming in an unheated outdoor swimming pool, there may be solar energy available to heat the pool to comfortable temperatures, thus extending the period during which it can be used.
The top photo on this slide shows the conference centre for the Bethel Business and Community Development Centre, in Bethel, Lesotho. The solar hot water system consists of the large tanks on the roof and the black strip down the centre of roof. In addition, the building makes use of daylighting, active solar heating, and photovoltaic systems.
The bottom photo shows a housing development in Kungsbacka, Sweden. The dark sections of roof along the top of the photo are solar collectors.
SLIDE 4: Components of SWH Systems
Solar water heating systems can be configured in a number of different ways, and the
components they contain vary from system to system. Let's look at an example that contains most of the components one would ever expect to see in SWH system.
There are three major components in this system. At the top left of the slide, we see the solar collectors, which absorb the sun's light and heat water or a water/glycol mix that passes through the collectors. Below the collectors is a heat exchanger, which transfers the heat from the hot fluid coming from the collectors to the water destined for the enduser. This solar-heated water is kept in a preheat tank, to the right of the heat exchanger, that permits the system to store heat from sunny periods for use during non-sunny periods of up to several days.
The heat exchanger tank and preheat tank are shaded red at the top of the tank and blue at the bottom. This indicates that the tanks are stratified: they have been designed to operate with the hottest, lightest water at the top of the tank and the coldest, heaviest water at the bottom of the tank. Thus, as hot water is drawn from the standard auxiliary storage tank, cold water supply enters at the bottom of the preheat tank and hot water is transferred from the preheat tank to the standard auxiliary storage tank. In solar systems that are not designed to meet the entire hot water demand year-round, a conventional gas or electric water heater in the standard tank will raise the temperature of the water to the desired level.
Water from the preheat tank flows through the heat exchanger by the thermosyphon effect - no active pumping is required. The heaviest water, at the bottom of the preheat tank, enters the heat exchanger, is warmed, and rises in the heat exchanger, drawing more cold water into the heat exchanger behind it. The hottest, lightest water rises to the highest point in the system, which is the connection of the heat exchanger to the preheat tank. Note that the hot fluid from the collectors flows in the opposite direction. This "crossflow" heat exchanger configuration maximizes the temperature of the water entering the preheat tank. It also minimizes the temperature of the fluid entering the solar collectors, which in turn maximizes their efficiency.
In this system, the fluid in the solar collectors is in a closed loop: it remains completely separate from the hot water destined for the end-user. This permits the addition of antifreeze, such as glycol, to the collector loop. In cold climates, another way to protect the system against freezing is the use of a drain-back tank: the heat transfer fluid is pumped up from a tank in the heat exchanger to the collectors, and then drains back down to the heat exchanger by gravity. During freezing periods, the system is not operated and all water drains from the collectors. In the system shown on this slide, the pump is powered by a photovoltaic module, which generates electric current when sunlight strikes it. Thus, the pump operates only when there is sunshine available to heat the collectors. Not all solar systems employ photovoltaic modules and closed loop configurations.
SLIDE 5: Unglazed Solar Collectors
There are several different types of solar collectors. First, we will examine unglazed solar collectors. As their name implies, these collectors have no glazing. Rather, a thin plastic panel connects two large supply and return headers; that is, pipes which transport water to and from the collector. The plastic panel, which is flexible in some collectors, contains numerous parallel channels that pass water from the supply to the return header. Sunlight is absorbed by the black plastic panel, which warms up and in turn heats the water passing through the channels.
Because glazing does not cover them, unglazed solar collectors absorb most of the energy from the sun. But for the same reason, they also lose heat quickly. There is nothing to prevent losses by convection to the air around the collector, nor losses by radiation. No insulation covers the rear of the collector, so losses occur there, too. Losses become more severe the larger the difference between the water temperature and the outside air temperature; these losses are exacerbated in windy weather, due to enhanced convection. For this reason, unglazed solar collectors are used when only moderately warm water is required, such as with swimming pools and fish hatcheries, and in seasonal applications.
The unglazed collector has a number of compelling advantages. It is the lowest cost type of solar collector and it is rugged, durable, and lightweight. These advantages have made it the most widely installed type of solar collector in North America.
Being a plastic collector, the unglazed collector may be unable to withstand high water pressures, such as those that would occur if the collector were connected to the residential water supply. Before using the collector at high water pressures, consult the manufacturer's specifications. For swimming pools, a filtration pump normally circulates water through the unglazed collectors, resulting in much lower pressures.
The plastic of the collector is treated to resist degradation by the ultraviolet rays in sunlight. Collector warranties generally cover a period of 10 to 15 years.
Individual unglazed collector panels are often fairly large, measuring around 1.5 m by 2.5 m. These panels can be interconnected to make a larger collector field.
SLIDE 6: Glazed Flat Plate Solar Collectors
Glazed flat plate solar collectors address some of the shortcomings of unglazed solar collectors. Insulation on the rear of the collector and one or more glazed coverings reduce heat losses, permitting the collector to operate successfully at higher water temperatures and in cooler climates. Typically the headers and the riser tubes that connect the headers are copper, and able to withstand high water pressures. The riser tubes are attached to a metal absorber plate which will often have a selective surface; that is, it will absorb nearly all incident sunlight but reradiate little heat over its range of operating temperatures.
These advantages come at a cost. Per unit area, glazed flat plate collectors typically cost two to three times as much as unglazed collectors. They are also considerably heavier and, because of the glazing, more fragile. This complicates installation.
SLIDE 7: Evacuated Tube Collectors
Evacuated tube collectors minimize heat losses from the collector. This is achieved by enclosing the collector in a glass vacuum tube, thus eliminating convection losses from the collector to the environment. If the collector has a selective surface and is not in contact with the glass tube, the heat losses by radiation and conduction will also be minimal. As long as there is solar energy available, the collector can generate very hot water - at temperatures up to 80ºC - even when outside air temperatures are well below freezing.
In order to keep the glass vacuum tube to a manageable size, the absorber is a narrow strip. In most modern evacuated tube collectors, the absorber is bonded to a heat pipe that contains a heat transfer fluid which, when heated, evaporates. Then, as a gas, it migrates to the end of the heat pipe, which protrudes out of the glass vacuum tube. This end of the heat pipe is in thermal contact with a pipe carrying water to be heated. The gas transfers its heat to the water, condenses, and drains back down into the section of the heat pipe that is enclosed by the vacuum tube. This cycle occurs continuously.
Because one thin absorber strip does not collect much solar energy, multiple evacuated tubes are generally arrayed in a parallel fashion and assembled into an individual collector. Like glazed flat plate collectors, these evacuated tube collectors are fragile, heavy, and somewhat complicated to install.
Evacuated tube collectors are the most expensive of the solar collector types, but they are the collectors that work best when outside temperatures are low and very hot water is required. They operate year-round in cold climates, and the spaces between the parallel evacuated tubes encourage the shedding of snow and ice. They are not necessarily the most efficient collectors; however, when outside air temperatures are high and only low water temperatures are required, there is little heat loss to be eliminated. Under these conditions, unglazed collectors and glazed flat plate collectors can be more efficient.
SLIDE 8: Solar Water Heating in Various Climates
The amount of hot water produced by a solar system over the course of a year will depend not just on the sunshine available at the site, but also on the outdoor air temperature and the desired water temperature, due to their influence on the heat losses from the collector. Furthermore, some hot water may be generated when there is already hot water in excess of the daily demand, and is therefore wasted; the amount of wasted hot water is determined by the variation in the solar resource over time as well as the hot water storage capacity of the system. Climate therefore plays a complex role in the performance of a solar hot water system.
The capabilities of a representative solar hot water system in various climates are shown on this slide. The chosen system contains 6 m² of glazed collector and a tank capable of storing one day's hot water demand, or 300 L. This is typical of a European or North American domestic solar hot water system. It is assumed that water at a temperature of 60ºC is desired.
Such a system has a solar fraction of 20 to 85%; that is, it can deliver between 20 and 85% of the total annual demand for hot water. The remainder of the demand would have to be met by conventional energy sources.
Although lower latitude sites tend to have higher solar fractions, this is not always the case - for example, consider Tokyo, Japan, and Warsaw, Poland, which have cloudy climates. The most equatorial sites shown here, Jakarta, Indonesia, and Puerto Limon, Costa Rica, have a tropical climate that does not give as high a solar fraction as drier, less cloudy sites further north and south. Also, colder sites are not necessarily worse: Yellowknife, Canada, is far colder than Warsaw, Northern Norway, or Puntas Arenas, Chile, but it is quite sunny, and generates a higher solar fraction.
SLIDE 9: Examples of SWH System Costs and Benefits
The financial costs and benefits associated with a solar water heating system will depend on the type of system, the climate it is installed in, and the cost of competing conventional energy sources. This slide shows three example systems. The first is a glazed flat plate collector system with storage installed in La Paz, Bolivia. This system operates yearround and annually generates approximately 2.2 GJ of useful heat per square meter of collector. The installed system cost is around $400/m² of collector. The second example, also operating year-round and with storage, is an evacuated tube system in Copenhagen, Denmark. Per square meter of collector, the system generates 1.8 GJ of heat annually and costs $1,000. The third example, located in Montreal, Canada, heats a swimming pool during summertime. Per square meter of collector, the system generates 1.5 GJ of heat annually and costs $150.
The annual value of the hot water provided by these systems is shown in the figure, as a function of the avoided cost of energy. The three sloped lines show that the annual savings associated with the solar system increase as the cost of conventional energy increases. The green vertical lines demark the range of energy costs associated with natural gas purchased at 15 ¢/m³ to 50 ¢/m³. The orange vertical lines demark the range of energy costs associated with electricity purchased at 5 ¢/kWh to 15 ¢/kWh.
Note that despite its very low cost and summer-only operation, the annual benefits associated with the unglazed system are only slightly less than those provided by the much more expensive glazed and evacuated tube systems. When the application permits low temperature heating and seasonal operation, the unglazed collector is very attractive.
The figure underlines the importance of conventional energy costs in determining the cost-effectiveness of a solar energy system. Paybacks can be quick when there are no low cost alternatives, such as cheap gas, but a more careful analysis will be required when such alternatives do exist.
SLIDE 10: Solar Water Heating Project Considerations
A number of situations tend to lead to successful solar water heating projects. First, a large demand for hot water reduces the importance of the fixed costs associated with a solar system. Second, as indicated on the previous slide, high conventional energy costs make solar more attractive. Third, where the conventional energy supply is unreliable, using a second source of energy - the sun - and having extra hot water storage can ensure that the end-user will have access to hot water on demand. Fourth, a building owner or operator with a strong environmental interest will value the solar system beyond simply its ability to save on energy costs.
The level of storage that will be required depends on a number of factors, one of which is the correlation between when sunshine is available and when hot water is needed. If hot water is required at night, or early in the morning, more storage will be required.
As pointed out in the previous slide, lower cost solar systems that operate only during those months when the air temperature is relatively warm and there is a strong solar resource, can be more cost-effective than more expensive systems operating year-round.
The operator of a solar water heating project must be committed to regular maintenance and timely repairs. While the burden of maintenance is not particularly high, being similar to that required of other plumbing, the system cannot be ignored. An operator lacking this
commitment may soon have a broken or leaky system.
SLIDE 11: Domestic Hot Water Systems - Examples: Australia, Botswana, and Sweden
When located on the electrical and natural gas grids, houses with domestic solar water heating systems need support from a committed homeowner. Even with the system providing 20 to 80% of the home's hot water, payback periods will be long when the cost of competing conventional energy sources is low.
The photo on the bottom left shows two on-grid homes in Malmö, Sweden, with solar hot water systems on their roofs. In this northern climate, collectors furnish up to 40% of the home's hot water requirements.
Off-grid or in locations where the supply of conventional energy for heating is unreliable, domestic hot water systems are much more financially attractive. The photo on the bottom right shows a thermosyphon solar water heater and a photovoltaic system on the roof of a house for staff working at a medical clinic in rural Botswana. The two solar systems provide hot water and electricity and make it easier to attract health professionals to this off-grid location.
The photo on the top right shows an Australian thermosyphon system, similar to the one used on the Botswana house, which can be used on or off-grid. The term "thermosyphon" indicates that the system makes use of the buoyancy of hot water to circulate water through the collectors, without any active pumping.
SLIDE 12: Swimming Pool Systems - Examples: USA and Canada
Many swimming pools are heated with low cost unglazed solar collectors. These can be used in cold climates for outdoor pools operated only during the summer, and in warm climates to extend the season during which the water temperature is comfortable. These types of systems can be very cost-effective, with simple payback periods of one to five years. The photo on the right of this slide shows an unglazed solar system for a pool in the USA.
Glazed collectors can also be used for swimming pool heating. They can provide heat to indoor pools operating year-round in colder climates. The photo at the bottom of the slide shows a large glazed collector on an indoor swimming pool in Ontario, Canada.
In swimming pool systems, the pool acts as its own water storage unit and the pool's filtration pump can often be used to circulate the water through the solar collector.
SLIDE 13: Commercial/Industrial Hot Water Systems - Examples: Greece and Canada
A number of commercial and industrial activities require large quantities of warm or hot water. Solar water heating has been successfully used for hotels, motels, apartment buildings, office buildings, health centers, hospitals, car washes, laundromats, restaurants, sports facilities, schools, public showers, aquaculture, and other small industry.
In the photo on the left, solar hot water systems are seen on nearly every rooftop of this hotel complex in the city of Agio Nikolaos, in Crete, Greece. In the photo on the right, unglazed collectors provide low temperature hot water to an aquaculture operation in British Columbia, Canada. Because the desired water temperature is sometimes below the ambient air temperature, these collectors occasionally operate with apparent efficiencies in excess of 100%!
SLIDE 14: RETScreen Solar Water Heating Project Model
The RETScreen Solar Water Heating Project Model is a simple but useful tool for the preliminary investigation of the technical and financial feasibility of solar water heating projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. Swimming pool applications, with or without pool cover, and service hot water systems, with or without storage, can be analysed; the system can utilise unglazed, glazed flat plate, or evacuated tube collectors.
To conduct this analysis, the user provides the site's average air temperature, average
daily solar radiation on a horizontal surface, average wind speed, and average relative humidity for each month. The software includes a large database of solar radiation, temperature, and wind data from around the world. These monthly data are more easily obtained and entered into the software than the 8,760 values needed for an hour-by-hour simulation.
While RETScreen can be applied to a vast range of solar hot water applications and systems, it has several limitations. There is no provision for changing the volume of hot water used in service hot water applications from day-to-day or season-to-season - it is instead assumed that this is constant. Stand-alone service hot water systems, which rely on solar energy for 100% of the hot water requirement without any conventional energy input, are not modeled. RETScreen assumes that service hot water systems without storage make use of all the water heated by solar energy, so its calculations concerning such systems with high solar fractions should be treated with caution. Similarly, its application to swimming pools with no back-up heater and solar fractions less than 70% is questionable. Finally, RETScreen does not model sun tracking collectors, concentrator systems, and integral collector storage systems.
SLIDE 15: RETScreen SWH Energy Calculation
The RETScreen Solar Water Heating Energy calculation determines the performance of the SWH system over the period of a year, using monthly solar radiation, temperature, relative humidity, and wind speed data. The calculation is based on monthly average values, and does not involve a detailed, hour-by-hour simulation of the operation of the solar system. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
The first step in the RETScreen calculation is the determination of various environmental variables. These include the monthly average daily irradiance in the plane of the collector, used to calculate the collector efficiency and the solar energy collected; the "sky" temperature, or the apparent temperature of the sky for long-wave radiation exchange between a collector and the sky, used to calculate the energy collected by unglazed collectors and the heat loss from a swimming pool due to radiation; the temperature of the cold water supply, used to determine the heating load on the system; and the service hot water load.
Next, RETScreen calculates the solar energy that can be collected. The method of calculation depends on the type of system and the application. For service hot water systems with storage, RETScreen employs the f-chart method. This long-established method predicts the fraction of the hot water demand satisfied by solar energy based on the results of many numerical experiments and simulations correlated in terms of several dimensionless parameters. For service hot water systems without storage, RETScreen employs the utilisability method. This method is based on the observation that to permit heat gain at the desired collector temperature, the solar irradiance must exceed a critical threshold, below which heat losses to the environment are greater than the solar energy absorbed. The utilisability is the fraction of the monthly solar energy incident on the collector that exceeds this critical threshold, and therefore generates useful heat gain.
The utilisability calculation is also employed in the solar energy calculation for swimming pools. RETScreen evaluates the pool's hot water demand, based on heat losses due to evaporation, convection, conduction, radiation, and the addition of make-up water, as well as passive solar gains, gains from the solar system, and gains from an auxiliary heater. The gains from the solar system are determined by the utilisability calculation; a heat balance is used to determine the auxiliary heat needed, if any, to maintain the pool at the desired water temperature.
Finally, RETScreen performs various other calculations, such as those required to determine the suggested solar collector area, the pumping energy required, and the system efficiency, specific yield, and solar fraction.
SLIDE 16: Example Validation of the RETScreen SWH Project Model
In order to validate the RETScreen software, it has been compared to other simulation tools and monitored data. For service hot water systems, RETScreen was compared to the WATSUN hourly solar simulation program. The test case was a domestic hot water system, located in Toronto, Canada, with a 5 m² glazed collector and 400 L of storage. As shown in the table on the right of this slide, RETScreen and WATSUN agreed to within a few percent in their calculation of incident radiation on the collector, the domestic hot water load, the energy delivered by the solar system, and the pump run time. RETScreen was also compared to monitored data from ten domestic hot water systems in Guelph, Canada. RETScreen appears to overestimate the performance of these systems, by around 29% on average. RETScreen performed better for those systems with a high hot water load, but in any case, this accuracy is acceptable at the prefeasibility level.
For swimming pools, RETScreen was compared to the ENERPOOL, an hourly simulation tool dedicated to solar heating of swimming pools. The test case was a 48 m² pool in Montreal, Canada, kept at 27ºC, uncovered for 8 hours a day, and operating from May through September. RETScreen and ENERPOOL agreed to within 2% in their estimates of the energy required to heat the pool; their estimates of the solar energy provided by the collector are not strictly comparable because RETScreen does not include energy that heats the pool above the desired temperature while ENERPOOL does.
RETScreen was also compared to monitored data for a 1,200 m² swimming pool in Mohringen, Germany, open from May through September. RETScreen's prediction of the energy required to heat the pool was within 3% of the measured value and its estimate of the solar energy production was 14% off the measured value. Once again, this level of accuracy is acceptable for pre-feasibility studies.
SLIDE 17: Conclusions
Solar water heating systems can be used to provide warm or hot water in any climate. Unglazed collectors are best suited to applications, such as pool heating, that do not require high water temperatures, and to operation at moderate air temperatures, such as during summertime. Glazed flat plate collectors, while more expensive, can provide moderately hot water even in cold climates. Evacuated tube collectors are well suited to year-round operation in cold climates, and can generate very hot water, but are more expensive yet.
Solar water heating projects are most attractive when a high demand for hot water dilutes the fixed costs associated with installation, where conventional energy costs are high, and where the owner and operator are strongly committed to and interested in the system.
The RETScreen Solar Water Heating Project Model calculates the hot water load for service hot water applications and swimming pools, as well as the performance of solar systems, with or without storage, for these applications. Providing an annual analysis based on monthly resource data, and with an accuracy comparable to hourly simulation tools, RETScreen considerably reduces the cost and difficulty of conducting a prefeasibility study of solar water heating projects.
SLIDE 18: Questions?
This is the end of the Solar Water Heating Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
This is the Solar Water Heating Project Analysis Training Module of the RETScreen Clean Energy Project Analysis Course. Here, we discuss the use of the sun's energy to heat water using solar collectors, such as those mounted on this building in Ontario, Canada.
SLIDE 2: Objectives
This module has three objectives. These are first, to review the basics of solar water heating (SWH) systems; second, to illustrate key considerations for solar water heating project analysis; and third, to introduce the RETScreen Solar Water Heating Project Model.
SLIDE 3: What do SWH systems provide?
Simply put, solar water heating systems use the energy in sunlight to heat water. They can be used wherever moderately hot water is required. "Off the shelf" solar systems can provide hot water to the bathroom and kitchen of a house; larger systems can be used for bigger buildings, such as multi-unit apartments, restaurants, hotels, motels, hospitals, and sports facilities, and even for industrial and commercial processes, such as car washes, laundries, and fish farms. Solar systems can also be very cost-effective for heating swimming pools.
In addition to these energy benefits, solar water heating systems may provide increased hot water storage and lengthen the season during which an outdoor swimming pool can be used. Because it will not always be sunny when hot water is needed, many solar water heating systems include a dedicated hot water storage tank. This increases the total hot water storage capacity of the system, such that periods of heightened hot water use will not cause the system to "run out" of hot water. When seasonal air temperatures are too chilly to permit swimming in an unheated outdoor swimming pool, there may be solar energy available to heat the pool to comfortable temperatures, thus extending the period during which it can be used.
The top photo on this slide shows the conference centre for the Bethel Business and Community Development Centre, in Bethel, Lesotho. The solar hot water system consists of the large tanks on the roof and the black strip down the centre of roof. In addition, the building makes use of daylighting, active solar heating, and photovoltaic systems.
The bottom photo shows a housing development in Kungsbacka, Sweden. The dark sections of roof along the top of the photo are solar collectors.
SLIDE 4: Components of SWH Systems
Solar water heating systems can be configured in a number of different ways, and the
components they contain vary from system to system. Let's look at an example that contains most of the components one would ever expect to see in SWH system.
There are three major components in this system. At the top left of the slide, we see the solar collectors, which absorb the sun's light and heat water or a water/glycol mix that passes through the collectors. Below the collectors is a heat exchanger, which transfers the heat from the hot fluid coming from the collectors to the water destined for the enduser. This solar-heated water is kept in a preheat tank, to the right of the heat exchanger, that permits the system to store heat from sunny periods for use during non-sunny periods of up to several days.
The heat exchanger tank and preheat tank are shaded red at the top of the tank and blue at the bottom. This indicates that the tanks are stratified: they have been designed to operate with the hottest, lightest water at the top of the tank and the coldest, heaviest water at the bottom of the tank. Thus, as hot water is drawn from the standard auxiliary storage tank, cold water supply enters at the bottom of the preheat tank and hot water is transferred from the preheat tank to the standard auxiliary storage tank. In solar systems that are not designed to meet the entire hot water demand year-round, a conventional gas or electric water heater in the standard tank will raise the temperature of the water to the desired level.
Water from the preheat tank flows through the heat exchanger by the thermosyphon effect - no active pumping is required. The heaviest water, at the bottom of the preheat tank, enters the heat exchanger, is warmed, and rises in the heat exchanger, drawing more cold water into the heat exchanger behind it. The hottest, lightest water rises to the highest point in the system, which is the connection of the heat exchanger to the preheat tank. Note that the hot fluid from the collectors flows in the opposite direction. This "crossflow" heat exchanger configuration maximizes the temperature of the water entering the preheat tank. It also minimizes the temperature of the fluid entering the solar collectors, which in turn maximizes their efficiency.
In this system, the fluid in the solar collectors is in a closed loop: it remains completely separate from the hot water destined for the end-user. This permits the addition of antifreeze, such as glycol, to the collector loop. In cold climates, another way to protect the system against freezing is the use of a drain-back tank: the heat transfer fluid is pumped up from a tank in the heat exchanger to the collectors, and then drains back down to the heat exchanger by gravity. During freezing periods, the system is not operated and all water drains from the collectors. In the system shown on this slide, the pump is powered by a photovoltaic module, which generates electric current when sunlight strikes it. Thus, the pump operates only when there is sunshine available to heat the collectors. Not all solar systems employ photovoltaic modules and closed loop configurations.
SLIDE 5: Unglazed Solar Collectors
There are several different types of solar collectors. First, we will examine unglazed solar collectors. As their name implies, these collectors have no glazing. Rather, a thin plastic panel connects two large supply and return headers; that is, pipes which transport water to and from the collector. The plastic panel, which is flexible in some collectors, contains numerous parallel channels that pass water from the supply to the return header. Sunlight is absorbed by the black plastic panel, which warms up and in turn heats the water passing through the channels.
Because glazing does not cover them, unglazed solar collectors absorb most of the energy from the sun. But for the same reason, they also lose heat quickly. There is nothing to prevent losses by convection to the air around the collector, nor losses by radiation. No insulation covers the rear of the collector, so losses occur there, too. Losses become more severe the larger the difference between the water temperature and the outside air temperature; these losses are exacerbated in windy weather, due to enhanced convection. For this reason, unglazed solar collectors are used when only moderately warm water is required, such as with swimming pools and fish hatcheries, and in seasonal applications.
The unglazed collector has a number of compelling advantages. It is the lowest cost type of solar collector and it is rugged, durable, and lightweight. These advantages have made it the most widely installed type of solar collector in North America.
Being a plastic collector, the unglazed collector may be unable to withstand high water pressures, such as those that would occur if the collector were connected to the residential water supply. Before using the collector at high water pressures, consult the manufacturer's specifications. For swimming pools, a filtration pump normally circulates water through the unglazed collectors, resulting in much lower pressures.
The plastic of the collector is treated to resist degradation by the ultraviolet rays in sunlight. Collector warranties generally cover a period of 10 to 15 years.
Individual unglazed collector panels are often fairly large, measuring around 1.5 m by 2.5 m. These panels can be interconnected to make a larger collector field.
SLIDE 6: Glazed Flat Plate Solar Collectors
Glazed flat plate solar collectors address some of the shortcomings of unglazed solar collectors. Insulation on the rear of the collector and one or more glazed coverings reduce heat losses, permitting the collector to operate successfully at higher water temperatures and in cooler climates. Typically the headers and the riser tubes that connect the headers are copper, and able to withstand high water pressures. The riser tubes are attached to a metal absorber plate which will often have a selective surface; that is, it will absorb nearly all incident sunlight but reradiate little heat over its range of operating temperatures.
These advantages come at a cost. Per unit area, glazed flat plate collectors typically cost two to three times as much as unglazed collectors. They are also considerably heavier and, because of the glazing, more fragile. This complicates installation.
SLIDE 7: Evacuated Tube Collectors
Evacuated tube collectors minimize heat losses from the collector. This is achieved by enclosing the collector in a glass vacuum tube, thus eliminating convection losses from the collector to the environment. If the collector has a selective surface and is not in contact with the glass tube, the heat losses by radiation and conduction will also be minimal. As long as there is solar energy available, the collector can generate very hot water - at temperatures up to 80ºC - even when outside air temperatures are well below freezing.
In order to keep the glass vacuum tube to a manageable size, the absorber is a narrow strip. In most modern evacuated tube collectors, the absorber is bonded to a heat pipe that contains a heat transfer fluid which, when heated, evaporates. Then, as a gas, it migrates to the end of the heat pipe, which protrudes out of the glass vacuum tube. This end of the heat pipe is in thermal contact with a pipe carrying water to be heated. The gas transfers its heat to the water, condenses, and drains back down into the section of the heat pipe that is enclosed by the vacuum tube. This cycle occurs continuously.
Because one thin absorber strip does not collect much solar energy, multiple evacuated tubes are generally arrayed in a parallel fashion and assembled into an individual collector. Like glazed flat plate collectors, these evacuated tube collectors are fragile, heavy, and somewhat complicated to install.
Evacuated tube collectors are the most expensive of the solar collector types, but they are the collectors that work best when outside temperatures are low and very hot water is required. They operate year-round in cold climates, and the spaces between the parallel evacuated tubes encourage the shedding of snow and ice. They are not necessarily the most efficient collectors; however, when outside air temperatures are high and only low water temperatures are required, there is little heat loss to be eliminated. Under these conditions, unglazed collectors and glazed flat plate collectors can be more efficient.
SLIDE 8: Solar Water Heating in Various Climates
The amount of hot water produced by a solar system over the course of a year will depend not just on the sunshine available at the site, but also on the outdoor air temperature and the desired water temperature, due to their influence on the heat losses from the collector. Furthermore, some hot water may be generated when there is already hot water in excess of the daily demand, and is therefore wasted; the amount of wasted hot water is determined by the variation in the solar resource over time as well as the hot water storage capacity of the system. Climate therefore plays a complex role in the performance of a solar hot water system.
The capabilities of a representative solar hot water system in various climates are shown on this slide. The chosen system contains 6 m² of glazed collector and a tank capable of storing one day's hot water demand, or 300 L. This is typical of a European or North American domestic solar hot water system. It is assumed that water at a temperature of 60ºC is desired.
Such a system has a solar fraction of 20 to 85%; that is, it can deliver between 20 and 85% of the total annual demand for hot water. The remainder of the demand would have to be met by conventional energy sources.
Although lower latitude sites tend to have higher solar fractions, this is not always the case - for example, consider Tokyo, Japan, and Warsaw, Poland, which have cloudy climates. The most equatorial sites shown here, Jakarta, Indonesia, and Puerto Limon, Costa Rica, have a tropical climate that does not give as high a solar fraction as drier, less cloudy sites further north and south. Also, colder sites are not necessarily worse: Yellowknife, Canada, is far colder than Warsaw, Northern Norway, or Puntas Arenas, Chile, but it is quite sunny, and generates a higher solar fraction.
SLIDE 9: Examples of SWH System Costs and Benefits
The financial costs and benefits associated with a solar water heating system will depend on the type of system, the climate it is installed in, and the cost of competing conventional energy sources. This slide shows three example systems. The first is a glazed flat plate collector system with storage installed in La Paz, Bolivia. This system operates yearround and annually generates approximately 2.2 GJ of useful heat per square meter of collector. The installed system cost is around $400/m² of collector. The second example, also operating year-round and with storage, is an evacuated tube system in Copenhagen, Denmark. Per square meter of collector, the system generates 1.8 GJ of heat annually and costs $1,000. The third example, located in Montreal, Canada, heats a swimming pool during summertime. Per square meter of collector, the system generates 1.5 GJ of heat annually and costs $150.
The annual value of the hot water provided by these systems is shown in the figure, as a function of the avoided cost of energy. The three sloped lines show that the annual savings associated with the solar system increase as the cost of conventional energy increases. The green vertical lines demark the range of energy costs associated with natural gas purchased at 15 ¢/m³ to 50 ¢/m³. The orange vertical lines demark the range of energy costs associated with electricity purchased at 5 ¢/kWh to 15 ¢/kWh.
Note that despite its very low cost and summer-only operation, the annual benefits associated with the unglazed system are only slightly less than those provided by the much more expensive glazed and evacuated tube systems. When the application permits low temperature heating and seasonal operation, the unglazed collector is very attractive.
The figure underlines the importance of conventional energy costs in determining the cost-effectiveness of a solar energy system. Paybacks can be quick when there are no low cost alternatives, such as cheap gas, but a more careful analysis will be required when such alternatives do exist.
SLIDE 10: Solar Water Heating Project Considerations
A number of situations tend to lead to successful solar water heating projects. First, a large demand for hot water reduces the importance of the fixed costs associated with a solar system. Second, as indicated on the previous slide, high conventional energy costs make solar more attractive. Third, where the conventional energy supply is unreliable, using a second source of energy - the sun - and having extra hot water storage can ensure that the end-user will have access to hot water on demand. Fourth, a building owner or operator with a strong environmental interest will value the solar system beyond simply its ability to save on energy costs.
The level of storage that will be required depends on a number of factors, one of which is the correlation between when sunshine is available and when hot water is needed. If hot water is required at night, or early in the morning, more storage will be required.
As pointed out in the previous slide, lower cost solar systems that operate only during those months when the air temperature is relatively warm and there is a strong solar resource, can be more cost-effective than more expensive systems operating year-round.
The operator of a solar water heating project must be committed to regular maintenance and timely repairs. While the burden of maintenance is not particularly high, being similar to that required of other plumbing, the system cannot be ignored. An operator lacking this
commitment may soon have a broken or leaky system.
SLIDE 11: Domestic Hot Water Systems - Examples: Australia, Botswana, and Sweden
When located on the electrical and natural gas grids, houses with domestic solar water heating systems need support from a committed homeowner. Even with the system providing 20 to 80% of the home's hot water, payback periods will be long when the cost of competing conventional energy sources is low.
The photo on the bottom left shows two on-grid homes in Malmö, Sweden, with solar hot water systems on their roofs. In this northern climate, collectors furnish up to 40% of the home's hot water requirements.
Off-grid or in locations where the supply of conventional energy for heating is unreliable, domestic hot water systems are much more financially attractive. The photo on the bottom right shows a thermosyphon solar water heater and a photovoltaic system on the roof of a house for staff working at a medical clinic in rural Botswana. The two solar systems provide hot water and electricity and make it easier to attract health professionals to this off-grid location.
The photo on the top right shows an Australian thermosyphon system, similar to the one used on the Botswana house, which can be used on or off-grid. The term "thermosyphon" indicates that the system makes use of the buoyancy of hot water to circulate water through the collectors, without any active pumping.
SLIDE 12: Swimming Pool Systems - Examples: USA and Canada
Many swimming pools are heated with low cost unglazed solar collectors. These can be used in cold climates for outdoor pools operated only during the summer, and in warm climates to extend the season during which the water temperature is comfortable. These types of systems can be very cost-effective, with simple payback periods of one to five years. The photo on the right of this slide shows an unglazed solar system for a pool in the USA.
Glazed collectors can also be used for swimming pool heating. They can provide heat to indoor pools operating year-round in colder climates. The photo at the bottom of the slide shows a large glazed collector on an indoor swimming pool in Ontario, Canada.
In swimming pool systems, the pool acts as its own water storage unit and the pool's filtration pump can often be used to circulate the water through the solar collector.
SLIDE 13: Commercial/Industrial Hot Water Systems - Examples: Greece and Canada
A number of commercial and industrial activities require large quantities of warm or hot water. Solar water heating has been successfully used for hotels, motels, apartment buildings, office buildings, health centers, hospitals, car washes, laundromats, restaurants, sports facilities, schools, public showers, aquaculture, and other small industry.
In the photo on the left, solar hot water systems are seen on nearly every rooftop of this hotel complex in the city of Agio Nikolaos, in Crete, Greece. In the photo on the right, unglazed collectors provide low temperature hot water to an aquaculture operation in British Columbia, Canada. Because the desired water temperature is sometimes below the ambient air temperature, these collectors occasionally operate with apparent efficiencies in excess of 100%!
SLIDE 14: RETScreen Solar Water Heating Project Model
The RETScreen Solar Water Heating Project Model is a simple but useful tool for the preliminary investigation of the technical and financial feasibility of solar water heating projects. For an installation anywhere in the world, it can provide an analysis of the energy production, life-cycle costs, and greenhouse gas emissions reductions. Swimming pool applications, with or without pool cover, and service hot water systems, with or without storage, can be analysed; the system can utilise unglazed, glazed flat plate, or evacuated tube collectors.
To conduct this analysis, the user provides the site's average air temperature, average
daily solar radiation on a horizontal surface, average wind speed, and average relative humidity for each month. The software includes a large database of solar radiation, temperature, and wind data from around the world. These monthly data are more easily obtained and entered into the software than the 8,760 values needed for an hour-by-hour simulation.
While RETScreen can be applied to a vast range of solar hot water applications and systems, it has several limitations. There is no provision for changing the volume of hot water used in service hot water applications from day-to-day or season-to-season - it is instead assumed that this is constant. Stand-alone service hot water systems, which rely on solar energy for 100% of the hot water requirement without any conventional energy input, are not modeled. RETScreen assumes that service hot water systems without storage make use of all the water heated by solar energy, so its calculations concerning such systems with high solar fractions should be treated with caution. Similarly, its application to swimming pools with no back-up heater and solar fractions less than 70% is questionable. Finally, RETScreen does not model sun tracking collectors, concentrator systems, and integral collector storage systems.
SLIDE 15: RETScreen SWH Energy Calculation
The RETScreen Solar Water Heating Energy calculation determines the performance of the SWH system over the period of a year, using monthly solar radiation, temperature, relative humidity, and wind speed data. The calculation is based on monthly average values, and does not involve a detailed, hour-by-hour simulation of the operation of the solar system. Here, we provide an overview of this calculation; for more information, see the RETScreen Engineering and Cases Textbook, available online and free-of-charge.
The first step in the RETScreen calculation is the determination of various environmental variables. These include the monthly average daily irradiance in the plane of the collector, used to calculate the collector efficiency and the solar energy collected; the "sky" temperature, or the apparent temperature of the sky for long-wave radiation exchange between a collector and the sky, used to calculate the energy collected by unglazed collectors and the heat loss from a swimming pool due to radiation; the temperature of the cold water supply, used to determine the heating load on the system; and the service hot water load.
Next, RETScreen calculates the solar energy that can be collected. The method of calculation depends on the type of system and the application. For service hot water systems with storage, RETScreen employs the f-chart method. This long-established method predicts the fraction of the hot water demand satisfied by solar energy based on the results of many numerical experiments and simulations correlated in terms of several dimensionless parameters. For service hot water systems without storage, RETScreen employs the utilisability method. This method is based on the observation that to permit heat gain at the desired collector temperature, the solar irradiance must exceed a critical threshold, below which heat losses to the environment are greater than the solar energy absorbed. The utilisability is the fraction of the monthly solar energy incident on the collector that exceeds this critical threshold, and therefore generates useful heat gain.
The utilisability calculation is also employed in the solar energy calculation for swimming pools. RETScreen evaluates the pool's hot water demand, based on heat losses due to evaporation, convection, conduction, radiation, and the addition of make-up water, as well as passive solar gains, gains from the solar system, and gains from an auxiliary heater. The gains from the solar system are determined by the utilisability calculation; a heat balance is used to determine the auxiliary heat needed, if any, to maintain the pool at the desired water temperature.
Finally, RETScreen performs various other calculations, such as those required to determine the suggested solar collector area, the pumping energy required, and the system efficiency, specific yield, and solar fraction.
SLIDE 16: Example Validation of the RETScreen SWH Project Model
In order to validate the RETScreen software, it has been compared to other simulation tools and monitored data. For service hot water systems, RETScreen was compared to the WATSUN hourly solar simulation program. The test case was a domestic hot water system, located in Toronto, Canada, with a 5 m² glazed collector and 400 L of storage. As shown in the table on the right of this slide, RETScreen and WATSUN agreed to within a few percent in their calculation of incident radiation on the collector, the domestic hot water load, the energy delivered by the solar system, and the pump run time. RETScreen was also compared to monitored data from ten domestic hot water systems in Guelph, Canada. RETScreen appears to overestimate the performance of these systems, by around 29% on average. RETScreen performed better for those systems with a high hot water load, but in any case, this accuracy is acceptable at the prefeasibility level.
For swimming pools, RETScreen was compared to the ENERPOOL, an hourly simulation tool dedicated to solar heating of swimming pools. The test case was a 48 m² pool in Montreal, Canada, kept at 27ºC, uncovered for 8 hours a day, and operating from May through September. RETScreen and ENERPOOL agreed to within 2% in their estimates of the energy required to heat the pool; their estimates of the solar energy provided by the collector are not strictly comparable because RETScreen does not include energy that heats the pool above the desired temperature while ENERPOOL does.
RETScreen was also compared to monitored data for a 1,200 m² swimming pool in Mohringen, Germany, open from May through September. RETScreen's prediction of the energy required to heat the pool was within 3% of the measured value and its estimate of the solar energy production was 14% off the measured value. Once again, this level of accuracy is acceptable for pre-feasibility studies.
SLIDE 17: Conclusions
Solar water heating systems can be used to provide warm or hot water in any climate. Unglazed collectors are best suited to applications, such as pool heating, that do not require high water temperatures, and to operation at moderate air temperatures, such as during summertime. Glazed flat plate collectors, while more expensive, can provide moderately hot water even in cold climates. Evacuated tube collectors are well suited to year-round operation in cold climates, and can generate very hot water, but are more expensive yet.
Solar water heating projects are most attractive when a high demand for hot water dilutes the fixed costs associated with installation, where conventional energy costs are high, and where the owner and operator are strongly committed to and interested in the system.
The RETScreen Solar Water Heating Project Model calculates the hot water load for service hot water applications and swimming pools, as well as the performance of solar systems, with or without storage, for these applications. Providing an annual analysis based on monthly resource data, and with an accuracy comparable to hourly simulation tools, RETScreen considerably reduces the cost and difficulty of conducting a prefeasibility study of solar water heating projects.
SLIDE 18: Questions?
This is the end of the Solar Water Heating Project Analysis Training Module in the RETScreen International Clean Energy Project Analysis Course.
