Outdoor Temperature – an overview

Outdoor Temperature – an overview

Fenestration for reducing building cooling needs

C.G. Granqvist, in Eco-Efficient Materials for Mitigating Building Cooling Needs , 2015

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16.3.1 The importance of low thermal emittance and solar control

The outdoor temperature is usually not the same as the one desired indoors, and hence, it is necessary to heat or cool—by one technique or another—in order to create a comfortable climate inside a building. Good thermal insulation is demanded so that excessive amounts of energy are not wasted for this heating and cooling. Therefore, we now contemplate the thermal properties of glazings.

Heat transfer occurs via three mechanisms—radiation, conduction, and convection—and can be measured and computed. Figure 16.2 illustrates calculated results for glazings embodying one, two, and three vertical glass panes separated by air gaps (Granqvist, 1991); the glass surfaces are numbered consecutively with the outermost surface denoted 1. One of the surfaces has an emittance in the range between zero and 0.85, where the upper value applies to normal float glass, so that the calculations show the effect of a lowered surface emittance. The upper two curves in Figure 16.2 pertain to a single glass pane and show that the heat transfer is diminished somewhat from about 6 Wm− 2 K− 1 when the emittance drops. In the case of double or triple panes, the relative influence of emittance changes is much larger, however, and the heat transfer then goes from 2.8 to 1.4 Wm− 2 K− 1 and from 1.8 to 1.2 Wm− 2 K− 1, respectively, when the emittance approaches zero. It is evident that low emittance should be on one of the surfaces that bound an air gap. One observes that the heat transfer in a double-pane construction can be almost halved when one of the surfaces displays low emittance. (For completeness, we note that modern glazings seldom have air in the spaces between the glass panes; instead, the glazing comprises an “insulated glass unit” (IGU) typically with argon between the panes, which yields an additional decrease of the heat transfer though not as large as the drop governed by a low emittance.)

Outdoor Temperature - an overview

Figure 16.2. Computed thermal transfer for different glazings with one surface having low thermal emittance. The glass surfaces are numbered consecutively with the outermost surface labeled 1, and the panes are separated by air gaps of 12.7 mm (typical for IGUs). The calculations assume a low outdoor temperature and some wind and are not described in detail.From Granqvist (1991).

The obvious question is now how it is possible to create the desired low emittance for a glass surface. To answer this question, it is necessary to consider the properties of window glass, which obviously is transparent for visible light and usually also is fairly transparent for solar radiation, whereas it is opaque for thermal radiation. A low value of E means that A is low, and Equation (16.1) dictates that the corresponding R must be high. This property can be obtained by coating the glass with a thin layer (usually referred to as a “thin film”) with the following ideal property:

(16.3a)

Tλ=1for0.4<λ<3μm

(16.3b)

Rλ=1for3<λ<50μm

A practically useful glazing should be transparent for visible light and have low heat transfer, but what should the transparency be for solar energy? We now recall that Figure 16.1 indicated that about 50% of the solar energy lies in the infrared, so the question above is whether this infrared radiation should be transmitted or not. If the building where the glazing is to be used needs heating (more exactly, if the room where the glazing is placed requires heating), the solar energy ingress should be large so that that the properties described by Equations (16.3a) and (16.3b) are desired. However, many buildings do not need solar heating but have a cooling requirement instead, which is so for almost all modern commercial buildings, irrespectively of their geographic location, and also for residential buildings in warm climates. In this latter case, the wanted properties can be expressed as

(16.4a)

Tλ=1for0.4<λ<0.7μm

(16.4b)

Rλ=1for0.7<λ<50μm

Coatings giving optical properties approximated by Equations (16.3a), (16.3b) and (16.4a), (16.4b) are called “low-E coatings” and “solar control coatings,” respectively. They are electrically conducting and often alternatively referred to as “transparent conductors”; they can be used as electrodes, for example, in electrochromics-based glazings, as discussed in Section 16.5, and in solar cells, light-emitting diodes, etc.

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Types of Compressors and Heat Pumps

Ioan Sarbu, Calin Sebarchievici, in Ground-Source Heat Pumps , 2016

4.2.6 Hybrid Air-to-Water Systems

A hybrid air-to-water system integrates an air-to-water HP unit with another non-renewable heat source, such as a condensation gas boiler, to create a highly energy efficient heating and DHW system. This system can produce water flow temperatures from 25 up to 80 °C, making it suitable for any type of heat emitter, including radiant floor heating and radiators.

The intelligent hybrid HP measures the outdoor temperature, automatically adjusting the flow temperature to the emitters and calculating the efficiency of the HP. The system continuously evaluates whether the efficiency of the HP is higher than that of the condensing gas boiler. Based upon this evaluation, the energy source is selected, ensuring that the most efficient heat source is being used at all times. There are three operating conditions for this system [5]:

HP only: for approximately 60% of the year, when the outdoor temperature is mild, the HP will supply energy for space heating. The primary energy-based efficiency in this mode is approximately 1.5.

hybrid operation: for approximately 20% of the year, when outdoor temperatures are between −2 and 3 °C, the HP and the condensing gas boiler work together to provide energy for space heating. The system efficiency is approximately 1.0 in this mode.

boiler only: when outdoor temperatures are below −2 °C, the condensation gas boiler provides the energy for space heating.

Throughout the year, the overall weighted primary energy efficiency is between 1.2 and 1.5, which is 30–60% higher compared with the best gas condensation boiler.

The hybrid HP system consists of three main components [5]:

1.

The outdoor unit transmits the renewable energy extracted from the air to the indoor unit (hydro-box). The compact and whisper-quiet outdoor unit contains the inverter-driven compressor, which has a modulation ratio from approximately 20–100%. In partial load conditions, the outdoor heat exchanger is oversized, which increases the efficiency by up to 30%.

2.

The hydro-box is mounted on the wall behind the condensing boiler. It contains the water-side elements of the system, such as the expansion tank and pump, as well as the controls for the system and the heat exchanger, which converts the renewable energy extracted from the air into hot water.

3.

The condensing gas boiler is installed in front of the hydro-box. The combined dimensions of the boiler and hydro-box are approximately the same as a conventional wall-hung boiler.

The hybrid HP has been field tested in various climates and house types (i.e. size, age and energy rating) with a range of different heat emitters. The seasonal performance factor (SPF) measured during the winter of 2011–2012 varied between 1.25 and 1.6.

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Vulnerability of Human Health to Climate

S. Kovats, in Climate Vulnerability , 2013

1.08.5 Risk Assessment for Current and Future Burdens of Heat-Related Mortality

Future populations are likely to be exposed to increasing outdoor temperatures. The implications to human health from such changes have been evaluated primarily in the context of climate scenarios from greenhouse gas forcings (IPCC 2012). Other aspects of climate change (e.g., heat islands) are less well studied. Many factors have a substantial influence on the burden of mortality and morbidity attributable to the ‘direct’ effects of temperature, which can be considered determinants of population vulnerability and adaptive processes. In epidemiological terms, such factors are effect modifiers of the temperature–mortality relationship.

Health impact assessments of climate change (due to greenhouse gas emissions) have been undertaken at city, national, regional, and global levels. Most future climate impacts assessments use a scenario-driven approach to take into account the uncertainty about future climate and the future world in which those climates will be experienced. Scenarios are plausible and often simplified description of how the future may develop, based on a coherent and internally consistent set of assumptions about driving forces and key relationships. Climate scenarios are generally derived from the output of global or regional climate models. Sometimes the model output is downscaled to local areas (such as cities), but it is not clear if this reduces the uncertainty in the future projection.

As nearly all aspects of our natural, physical, and social environment will change in the future, it is important that these changes are reflected in the scenarios used in climate impacts studies. In practice, however, only population growth and economic growth (in GDP per capita) have been quantified and routinely included in climate impact studies. Several studies have now been published that estimate future heat-related mortality under a range of climate scenarios (to estimate a set of plausible future changes in local temperature exposures) (Huang et al. 2011). Some (but not all) use population scenarios to estimate the future total population and/or the proportion of people belonging to older age groups, who are much more susceptible to heat-related mortality. A common limitation of these studies is that they must make assumptions about how future populations will respond to exposure to high temperatures, including the rate of acclimatization. The studies so far have focused on cities in high income populations and show that heat-related mortality is likely to increase, even when some acclimatization is assumed (Huang et al. 2011; Kinney et al. 2008).

Populations in Europe and North America are aging. In addition, the number of persons with chronic diseases is anticipated to increase. High-income populations are expected to experience a high burden of disease from both cardiovascular and other chronic diseases such as diabetes in the next few decades. These trends are likely to increase the susceptibility to heat (Table 1) (O’Neill and Ebi 2009). In contrast, populations may become less sensitive to temperature extremes due to improvements in the underlying health of the population and improvements in the infrastructure, particularly the uptake of air-conditioning or passive cooling systems to reduce indoor temperatures.

Table 1. Risk of heat-related mortality associated with chronic disease category

Chronic disease category Risk of heat-related death
All respiratory ?
COPD +
Asthma +
Neurological damage, Parkinson’s disease, etc. +++
Schizophrenia, psychotic illnesses ++
Dementia, Alzheimer’s disease +++
Diabetes, renal disease +++
Previous stroke ++
Obesity + (elderly)

++ (adults)
Ischemic heart disease ++

+, small risk

++, moderate risk

+++, high risk

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Experimental Methods to Compare Building Component Alternatives

Francesca Stazi PHD, in Advanced Building Envelope Components , 2019

4.4.2 External Envelope Study

The west-facing wall of the mock-up was used to install the ventilated skin prototypes. Three ventilated skins were simultaneously compared (Fig. 4.29).

Outdoor Temperature - an overview

Fig. 4.29. West view of the mock-up before (A) and after (B) the vented facades realization.

The first configuration (L) is the lightweight typology without mass. It has an aerated gap enclosed by a lightweight panel and it was used as reference for comparisons (Fig. 4.30). Screwed vertical wooden battens (6 cm width) and an external white plastered oriented strand board (OSB) panel were used.

Outdoor Temperature - an overview

Fig. 4.30. Realization phases of lining L with wooden battens and OSB panel (before plastering).

The second configuration (IM) has an internal mass.

In this solution, the clay blocks are laid adjacent to the insulation material; the gap is then confined through a white plastered OSB panel, as in the first solution (Fig. 4.31). For the realization of this wall, stainless steel supporting brackets were anchored (also providing a thermal cutting plate) at the cross-laminated structure in the bottom of the walls (Fig. 4.31A); then, the massive layer was assembled adjacent to the existing insulation layer, using also cavity ties distributed for the entire surfaces (Fig. 4.31B and C). Finally, an external plastered OSB was used by leaving an interspace of 6 cm with respect to the massive layer.

Outdoor Temperature - an overview

Fig. 4.31. Realization of wall IM: (A) stainless steel brackets; (B) realization of massive layer with perforated bricks; (C) massive layer complete, with fixing ties.

The third configuration (EM) has an external white plastered mass.

Stainless steel supporting brackets (as those used in IM wall) were anchored at the cross-laminated structure in the bottom of the walls (Fig. 4.32A); then, the massive layer was assembled leaving an interspace (6 cm width) with the existing insulation layer. Cavity ties (longer than those of IM wall) were adopted for the entire surface to fix the wall; the bricks were then plastered.

Outdoor Temperature - an overview

Fig. 4.32. Realization of wall EM: (A) stainless steel brackets and (B) completed wall.

Obviously, the stainless steel supporting brackets of EM solution were longer than those of IM (Fig. 4.33).

Outdoor Temperature - an overview

Fig. 4.33. Details of the different stainless steel supports used for IM (A) and EM (B and C).

To reach the same optical properties for all the walls the same external white shaving was used. Honey-combed nets were placed to protect all the cavities. Lateral insulation boards were used to seal and insulate each wall.

The yearly extensive monitoring activity was carried out to examine the thermal behavior of the proposed ventilated skin technologies. The probes were installed either in the internal surfaces of the cavities or in the external surface of each cladding.

The following sensors were used (Fig. 4.34):

Outdoor Temperature - an overview

Fig. 4.34. Monitoring plan of the three ventilated prototypes, section, and plant.

Outdoor temperature, relative humidity, global solar radiation, speed, and direction of the wind. The data were collected using a weather station;

Surface temperatures of the outermost and innermost layers measured at the bottom (60 cm), at the middle (115 cm), and at the top (168 cm) of each façade. The data were collected adopting thermoresistance sensors (accuracy ± 0.05°C);

Incoming and outgoing heat fluxes measured at mid-height (115 cm) in the innermost side of the air chamber. Data were collected through heat flux plates (precision ± 3%);

Air velocity and air temperature inside the ventilation chamber recorded at mid-height (115 cm). The data were collected using hot-sphere anemometers (tolerance of ±0.03 m/s).

The data acquisition involved gathering analog signals from measurement sources and digitizing them, adopting NI-DAQ hardware for storage. LabVIEW software was used and implemented for real-time data presentation. The sample rate of the data acquisition system was set every 5 minutes.

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Energy auditing and measurements in practice

Patrik Thollander, … Jakob Rosenqvist, in Introduction to Industrial Energy Efficiency , 2020

6.7.11 Temperature

Temperature data are particularly useful for calculating the energy use of the unit processes Space Heating and Cooling and Ventilation. Temperature measurements can be used to calculate the efficiency of heat exchangers for AHUs. To calculate the heat losses, the exhaust air temperature is crucial, but seldom registered by the monitoring system and even more seldom logged. To use standard values for efficiency of heat exchangers is a useful shortcut.

Temperature logging can be used to identify operating time for equipment. A temperature probe can be attached directly to the surface of a machine or in an air flow or other medium connected to the studied equipment (see Figs. 6.26 and 6.28).

Typical temperature measurements in an industrial energy analysis are:

Outdoor temperature. To calculate heating and cooling energy demand and heat losses,

Room temperature. To calculate heating and cooling energy demand and heat losses,

Air temperatures in air handling units. Along with air flows to calculate heat losses, heat supply, and heat exchanger efficiency,

Supply and return temperatures in heating or cooling systems. To make a diagnosis of the regulation in the system or together with flow to calculate the energy use,

Surface temperatures. On pipes to get an idea of the temperature of the liquid inside the pipe. On machines to see when they are used and when they are turned off,

Water temperatures, and

Flue gas temperatures.

You will have use for a set of different temperature probes for air, surface temperatures, and immersion probes for liquids. A thermometer with standard thermocouple plug is good to have. This allows you to have many interchangeable probes for the same thermometer and you are not bound to any brand.

Several loggers will be needed for your temperature probes. One or two channels loggers, robust, preferably waterproof is a good choice. Use loggers with wired or wireless probes, to be able to place the equipment anywhere you want.

You will want to use several temperature loggers in many different locations simultaneously. You will not be able to install long cables for measurement data collection. You don’t have much benefit from many channels—rather many loggers with one or two channels per logger.

You can have several different types of probes for your loggers, but temperature probes that are waterproof, small enough to be taped as surface sensor and at the same time quick enough to handle even measurements in air are a good compromise that can handle most situations.

The temperature range depends of course on what to measure. In most cases, you will be able to work in energy auditing contexts with accuracy as low as 0.3°C, allowing you to use temperature sensors with a large measuring range, for example, −30°C to 200°C.

If you have a thermometer for thermocouples, a temperature logger with standard contact for thermocouple sensors is valuable. The combination gives you many choices and will be useful when the probes to your standard loggers for some reason cannot handle the task. There may be special requirements, for example, on size, corrosion resistance, or temperature range.

You will probably want to log with minute intervals rather than second intervals, so the probes don’t have to react faster than that.

Fig. 6.16 displays loggers and probes for measuring temperatures.

Outdoor Temperature - an overview

Figure 6.16. Temperature loggers and probes. To the left, a robust and waterproof logger with a stainless probe. In the middle, a temperature and relative humidity logger with a probe slim enough to fit in a small hole in a ventilation duct. It also has a display that allows you to use it for instantaneous measurements. To the right, a four-channel logger for thermo-element probes and thermistor probes. One good feature is that it can log temperature difference directly. Another good feature is that you can use it with any of the many thermo-element probes on the market.

In Fig. 6.16 you see a thin standard probe and a probe with a hook-and-loop fastener strap intended for pipes, but also suitable for fluorescent tubes.

+61404532026 Infrared cameras and thermometers

An infrared camera is a good way of showing temperature differences and thereby, for example, illustrating leaks of hot or cold air or variations in heat transport through building constructions, poorly insulated pipes, and so on. If the difference between indoor and outdoor temperatures is large enough, one can also get a picture of where there are wooden joints or masonry joints inside the wall behind the facade. Superheated fuses and conductors in a switchgear can also be detected with an infrared camera and can give you a reason to suggest measures that reduce the power need. An infrared camera, with high resolution, or infrared thermometer, with narrow-angle lens (e.g., 12:1), is also a good way to measure temperatures on surfaces out of reach, as a basis for calculations or to see if a machine is working at all. The advantage of the camera, in comparison with the thermometer, is the visibility of the result both for yourself as an energy auditor as well as for those you want to inform with the results from the audit.

The emissivity of the studied surface plays a major role in obtaining a good result from an infrared measurement. If the surface is within reach, which allows you to also measure the temperature with a thermometer with a contact sensor, you get a very good reference value. Pasting on a tape with known, high emissivity on the surface you want to measure is also a good way to make sure you get useful values. The tape tip is also a good way to obtain reliable temperatures from surfaces with very low emissivity. Since these surfaces have high reflectivity, they will reliably reflect the ambient temperatures, rather than showing the surface temperature. The tape needs to cover at least one pixel in the thermal camera sensor for the reference to become useful.

To use an infrared camera or thermometer to measure the temperature of surfaces out of reach, you need to know something about the surface emissivity and set it in the camera/thermometer. There are emissivity tables for many materials, but it may be worth knowing that the emissivity may vary with the temperature of the material and with your viewing angle.

+61404532026 Typical measurement locations

You will measure temperatures:

Inside ventilation ducts,

In rooms,

Outdoors,

On pipes and ducts, and

On operating equipment and machines.

+61404532026 In the toolbox

For temperature measurements you need:

Thermometer for instantaneous measurements. With different probes for different purposes.

Loggers with associated probes.

Infrared camera or infrared thermometer for noncontact measurement.

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Fig. 6.17 displays an IR thermometer and portable IR camera.

Outdoor Temperature - an overview

Figure 6.17. To the left, an IR thermometer that is lightweight and easy to use. To the right a very competent but still portable IR camera.

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Transfer Function Approach

Nicolae Lobontiu, in System Dynamics for Engineering Students , 2010

(a)

Find the transfer function corresponding to the indoor and outdoor temperatures θ1 and θ2 assuming that θ2 > θ1.

(b)

Determine θ1 and plot it as a function of time when θ2 = 40°C. Known also are the following data for the wall: height h = 4 m, width w = 10 m, length l = 0.2 m, thermal conductivity k = 0.04 W/m-C; and for the air in the room space, mass density ρ = 1.1 kg/m3, and constant-pressure specific heat cp = 1100 J/kg-C and θ1(0) = 20°C.

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A new dynamic heat pump simulation model with variable speed compressors under frosting conditions

N. Park, … B. Chung, in 8th International Conference on Compressors and their Systems , 2013

2.6 Heat pump model validation

Developed heat pump model is used to compute heating cycle at standard outdoor temperature, or 7°C (DB) / 6°C (WB), and low temperature, or -10°C, conditions. As shown in Figure 9, simulated cycle data closely mimics actual cycle even during initial phase. Sharp changes of inverter speed at around 7 minute and 25 minute correspond to turning on and off of the constant speed compressor.

Outdoor Temperature - an overview

Figure 9. Cycle simulation data at standard heating condition (upper) and the validation of simulation against experimental data (lower)

Qualitative comparison with experimental data shows that the present model underestimates heating capacity by 11~13%, while COP predictions are within 3% error. Reminding the level of prediction accuracy for all cycle components as shown in the previous subsections, this level of error is not surprising. Note also that experimental data used for the comparison represent the best performance from manually tuned cycle, while the simulation results are from automatic runs with P-D controller mentioned earlier. Thus, no further tuning is pursued here to closely match the experimental data.

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Environmental Benefits of Heat Pumps

Torbjörn Svensson, in Heat Pumps , 1990

Air source heat pumps

Ambient air heat pumps must handle large quantities of air to provide the heat. When the out-door temperature approaches zero degrees Centigrade, regular defrosting is required, and the condensed water has to be discharged. From an energy balance point of view it is obvious that the air cooling will have no impact on the air temperature, since the heat is being returned to the air from the heated buildings. Locally, however, cooled air might reach the ground and possibly be recirculated into the heat pump

The major negative impact of air heat pumps is considered to be the emission of noise, as discussed above, and the risk of CFC-leakage. It is anticipated that the large evaporators and complicated defrosting equipment necessary result in a somewhat higher risk of CFC-leakage than for most other heat pump types, unless a separate air heat exchanger is employed.

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Sustainable Built Environment & Sustainable Manufacturing

D.H.C. Chow, in Encyclopedia of Sustainable Technologies , 2017

General criteria for adaptive comfort to work

As well as satisfying the aforementioned criteria, for adaptive comfort to work, the rate of how quickly the outdoor temperature changes is also important. In general, if the temperature changes are small (< 2°C) in the course of the day, then dissatisfaction amongst occupants is unlikely. For larger temperature changes, this may be acceptable if there is sufficient control of the building for the occupants (e.g., they can move to different area, change clothing level, open windows, etc.). For temperature changes over longer time periods (e.g., in summer heat wave), this may be acceptable if this occurs sufficiently gradually to allow building occupants to adapt to the change.

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District Heating with Combined Heat and Electric Power Generation

Richard H. Tourin, in Advances in Energy Systems and Technology, Volume 1 , 1978

C Some Characteristics of District Heating Hot Water Systems

Figure 6 illustrates the temperatures and water flow used in district heating hot water systems, as a function of outdoor temperature. Figure 7 is a typical load curve. The combined heat and power plant with back-pressure turbines is sized to carry up to 50% of the maximum heat load. To cover the winter peaks, heat is supplied directly from boiler plants, often fired with municipal waste. This is the most economical arrangement. The dashed line in Fig. 7 indicates a possible additional load of absorption air conditioning.

Outdoor Temperature - an overview

Fig. 6. Water temperature and water flow in relation to outdoor temperature.

Outdoor Temperature - an overview

Fig. 7. Annual load diagram. Heat consumption in a small town in the middle of Sweden. The equivalent full load operating time is about 2200 h/yr. Power factor depending on the number of consumers connected to the heat producer is 0.8.

When using back-pressure turbines, all steam is condensed in the hot water condenser for district heating, and there is no need for cooling water, because the generated electricity is produced according to the hot water demand. If there is a demand for electricity without a corresponding heat demand, either the heat is stored or the necessary steam for power production can be condensed in a recooler chilled with cold water. This subject has been discussed by Muir (1973). A brief treatment, adapted from Muir’s paper, is given below.

Electricity is the more valuable product in a combined heat and power system for district heating. The ratio of electricity to heat is called the α-ratio, and varies from 0.45 up to 0.70. It depends on the steam inlet data, the number of heating condensers, and the temperature of the district heating water. Once a turbine is built, the first two parameters are fixed, and it is only the third factor which varies. It is desirable to maximize the α-ratio to achieve greatest economy.

The α- ratio can be improved by heating the circulating water in a number of stages. Two stages are normally used. These stages, or hot water condensers, are connected in series on the water side and this gives about 4–5% greater electrical output than would heating in one stage. Apart from the change brought about by seasonal weather variations, the a ratio is fixed, which means that full electrical output is not available if the heat demand is less than the maximum heat output capacity of the turbine. Some means must therefore be provided so that the output of electrical energy can be varied independently of the heat output. There are two ways in which this can be done.

The first way of increasing the electrical output is by artificially increasing the heat load. This is done by means of a recooler, which is a large heat exchanger cooled either by air or by water and connected into the hot water system, as shown in Fig. 8. When the load is low, pumps A and B are running fairly slowly or have their guide vanes set for low flow. But the output of pump C can be increased, thus circulating more or less of the water through the recooler. More water passes through the turbine condenser, which means that more steam can be passed through the turbine and thus more electricity generated.

Outdoor Temperature - an overview

Fig. 8. District heating turbine with recooler and accumulators.

Hot water accumulators are used to smooth daily variations in heat load. These are large insulated vessels connected into the hot water circuit as shown in Fig. 8. Suppose that the maximum electrical output is required at a time when the heat load is not at a maximum. Full steam flow is passed through the turbine and to the hot water condensers. The flow through the condensers is increased by means of pump C, and some of the water is taken off and passed into the top of the accumulator. The flows are adjusted so that the right amount of water is left to be pumped out into the distribution system. The colder water in the accumulator is displaced and pushed out of the bottom and back to the hot water condensers. Thus the heat load on the turbine has been effectively increased: the rate of flow of water has been increased and colder water is being supplied for reheating. More electricity can be obtained from the turbine, and the extra heat which has been transferred to the hot water is stored.

The accumulators are big enough for it to take some hours before they are full of hot water. At some other time during the day, if the electrical demand drops while the heat load rises, or if perhaps the heat load rises to more than the turbine can supply, the accumulators are discharged by changing over the valves so that the hot water which was heated earlier is supplied to the distribution system.

The turbine can also be constructed with a stage for extraction of steam to heat exchangers for district heating and a cold condensing “tail.” In the last case there will be need of cooling water, but the amount required is considerably smaller than in a pure condensing turbine, and the heat rejected as waste is also much smaller. The production of hot water district heat from turbine extractions gives a considerable heat saving, as only one fifth as much heat needs to be fed into the turbine in the form of admission steam as is extracted. This appears in the turbine schematics in Fig. 9. The output of 1000 MW (e) is shown only to illustrate the calculation. Units of this type now in operation are below 300 MW (e) in size.

Outdoor Temperature - an overview

Fig. 9. (a) Pure condensing turbine, (b) Combined condensing and extraction turbine arranged to obtain two different exhaust pressures.

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