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Energy conservation and improvement of reliability in centralized heating systems
0360-5442/87 $3.00 +O.OO Pergamon Journals Ltd
Energy Vol. 12,No. lO/ll, pp. 1013-1016,1987 Printed in Great Britain
ENERGY CONSERVATION AND IMPROVEMENT OF RELIABILITY IN CENTRALIZED HEATING SYSTEMS V. VARVARSKY,’
YA. KOWLIANSKY’
and S. CHISTOVICH~
‘VNIPI Energoprom, Moscow 105266, U.S.S.R. ‘VNIGGS, Leningrad, U.S.S.R. Abstract-Centralized heating systems are being used increasingly to supply heat to the cities and towns of the U.S.S.R. These systems are usually based either on boilers of 50MW or greater thermal capacity or on cogeneration (combined heat and power) plants. Cogeneration not only saves energy but also reduces air pollution and thermal pollution and raises labor productivity. In some new systems, not only baseload but also loadfollowing power plants are being used for cogeneration, which allows improvements in the system-wide load factor for electricity. This paper outlines some of the characteristics of modem systems, including scale, layout, pipeline materials, and systems for control.
1. CENTRALIZED
HEATING SYSTEMS IN THE U.S.S.R.
Centralized heating systems are being used increasingly to supply heat to the cities and towns of the U.S.S.R. These systems have considerable advantages from the points of view of economics, sociology, and ecology. The most desirable centralized heating systems are combined heat and power systems associated with electric power plants; compared to separately generating electricity at condensing plants using either fossil or nuclear fuel and separately generating heat in district or local heating systems, combined heat and power systems use less fuel, improve regional air quality, and provide higher quality heat supply. Representative characteristics of heat supply sources in the U.S.S.R. are presented in Table 1. It is seen that, in 1980, 55% of all heat demand in the residential and commerical sector was met by centralized sources: 39% by thermoelectric plants and 16% by district heating systems. In fact, restricting the estimate to cities and towns, about 70% of residential and commerical heat demand was met by centralized sources. Energy savings in 1980 due to the use of power plants for combined heat and power amounted to 33 million tonnes of coal equivalent (tee), or 1.0 x lo’* J. The average specific fuel consumption for electricity at plants used for combined heat and power was 75% of the specific fuel consumption at condensing plants, 265 vs 356 grams of coal equivalent (gee) (7.8 vs 10.4 MJ) per kWh of electricity produced. (According to our energy accounting convention, the entire energy savings associated with combined heat and power is assigned to electricity production in these calculations.) Table 1 shows that there are dramatic differences in labor productivity between centralized and local heat supply systems. There are also considerable differences in energy requirements; one estimate attributes energy savings of 21 million tee (6.4 x 1017J) to the use of centralized rather than local heat supply systems. Moreover, centralized heat supply has further merits beyond savings of manpower and fuel, including reduced air pollution and thermal pollution; quantitatively, nationwide in 1980 the emission of harmful effluents to the atmosphere was at least 10% less than it would have been in the absence of centralized heat supply.
2. ENERGY SOURCES AND MODES OF OPERATION
In the European portion of the U.S.S.R. the energy sources for future centralized heat will be nuclear electric power plants, nuclear thermal plants (producing heat only), and large boilers burning coal or natural gas supplied from eastern portion of the U.S.S.R. In the eastern portion itself (Siberia and Kazakhstan), the source for centralized heat will be 1013
1.
Heating
Total
Waste-heat
Recovery
Plant
Plant
System
Local Heat Supply Small Boiler Individual Heating
District
(TEP)
Energy
Centralized Heat Supply Thermoelectric Plant
Table
-12350(100%)
8700(100X)
450(<1%)
3400(28%) 2250(17X)
1900(16X)
4800(39%)
lOI and % of total
Energy Use 1980
Productivity
--
190(11%)
2400(28%) 2810(31%)
590(7X)
2900(34X)
lOI and X of total
Annual 1970
Use and Labor
5 2 __
_-
_I
20
40
Labor Productivity (CJ/person-yea?:)
Systems
7 5
1.3
1.0
Normalized Number of Workers (T.E.P.=l.O)
and Local Heat Supply
1.14 1.33
0.98
1.00
Normalized Fuel-to-heat Ratio (T.E.P.=l.OO)
in Centralized
Energy conservation in centralized heating systems
1015
fossil fuels burned in power plants, as this region possesses large reserves of cheap coal which will be used to produce electricity for new industries. It is economical to modify both nuclear and fossil fuel condensing power plants so that they operate in the cogeneration mode. By 1981, 325 condensing turbines had been modified, saving 1.8 million tee (5.3 x 1Ol6 J) of energy. Moreover, it has become increasingly economical to operate load-following power plants as cogeneration plants. This mode of operation is particularly important in the European part of the U.S.S.R., to supplement the nuclear power plants providing base load. The use of renewable sources of energy for centralized heat is also being investigated. The principal geothermal sources are located in the Caucasus, Siberia, the Far East, Kamchatka, Kazakhstan, and Central Asia. The nation’s geothermal heat supply has been estimated to be able to supply 30-40 million tee (0.9-1.2 x 1018J) annually. More than 40 geothermal fields are currently being used, all in the Caucasus or on the Kamchatka Peninsula; in 1980, they produced 40 million m3 of hot water and 300,000 tonnes of steam, saving 0.5 million tee (1.5 x 1016J). By 1985 geothermal sources were providing 60 million m3 of hot water, and more than 800,000 people in Tbilisi, Makhachkala, and other cities and towns were served by geothermal heat. As for centralized heating by solar energy, the most favorable areas, where the sun shines 2200-3000 h per year and the annual solar energy on a horizontal surface is 12OC-1700 kWh/m2, are in the Central Asian republics, the southern Ukraine, Kazakhstan, Moldavia, and southern portions of the Russian Republic. Energy for centralized heat can also be provided by waste-heat recovery plants attached to industrial heat supply systems. In 1980 energy savings of 58 million tee (1.7 x lO”J) were achieved by such plants in the ferrous metal, chemical, and petrochemical industries. Long-term energy planning in the U.S.S.R. envisions large-scale heat recovery from sewage treatment plants and from the ventilation air exhausted from buildings. Heat recovery can be accomplished either by heat exchangers or heat pumps.
3. SYSTEM DESIGN
Because new thermoelectric plants are being built tens of kilometers from population centers, centralized heat supply systems tend to have a comparable dimension. The effective radius of some networks is more than 30 km, and the diameter of a trunk pipeline is sometimes 1.4m. For example, the trunk supply line connecting the Mid-Urals thermoelectric plant to the city of Sverdlovsk is 25 km long; the line has been operating continuously since 1962. The heat supply line planned to connect the Odessa nuclear power plant with the city of Odessa is also more than 25 km long; as for the heat supply networks for Moscow, Leningrad, Novosibirsk, Kharkov, each has an overall length of hundreds of kilometers. The total length of the heat supply lines in the U.S.S.R. exceeds 180,000km. Of the total investment for a centralized heat supply system, typically 70-80% is for the network in a dedicated heating system and 35-40% is for the network in a combined heat and power system. Heat supply lines laid above ground are more reliable, less susceptible to corrosion, more easily inspected, and more easily repaired, thus, whenever possible, pipes are laid above ground, on low trestles. Underground siting of heat supply lines is necessary in cities. In Leningrad, pipelines with reinforced concrete foam insulation have been in operation since 1948. Nowadays, to save labor, heat insulation is applied to the pipes at the factory. Anticorrosion coatings are applied to the outer surface, as well as electrolytic protection (cathode polarization). There is practically no internal corrosion, because the supply water is deaerated at the heat source and sodium silicate is added. Increasingly, open-circuit heat supply systems are being built; relative to closed systems these are more energy efficient and they can recover lower quality heat. Roughly half of the heat supply systems in our cities and towns today are open systems, for example in Leningrad, Tashkent, Alma Ata, Kalinin, Khabarovsk, Ivanovo, Krasnoyarsk, and Norilsk.
1016
V. VARVARSKYet al. 4. THE ROLE OF OPTIMIZATION
AND CONTROL
A systems approach is increasingly used in the planning for centralized heat supply, whether at the scale of a town or industrial center, at the scale of a region or interconnected electric grid, or at the scale of the nation as a whole. Mathematical modeling techniques such as nonlinear programming, dynamic programming, and combinatorics allow the consideration of time-dependent growth in heat loads and heat sources. Typical problems include determining the optimum number of plants of each type and size, optimum pipeline diameter, optimum time of deployment, and optimum supply temperature. Computational analysis of operating heat supply systems, combined with long-term experience, has revealed considerable shortcomings in distribution systems, especially in critical circumstances where the system is functioning below design conditions. New principles have been developed to guide the modernization of such systems. One preferred arrangement has a two-pipe circular trunk network, subdivided by means of shutters (gate valves); 25-60 MW of thermal power are managed by each distribution center connected to the circular trunk. Downstream of the distribution center, heat distribution typically runs through three-pipe networks; two pipes supply hot water and one is the common return. Automation of heat control is mandatory to assure economical and reliable operation. Modern control systems permit prompt redistribution of heat load among heat sources, to take advantage of differential costs of supply in base-load and peak-load plants. Modern control systems allow the system to respond to weather conditions, to production schedules of industrial sources and consumers, and to consumption patterns dependent on the time of day. Modern control systems, moreover, greatly improve responses to emergencies, detecting and localizing the problem and minimizing the system-wide damage. The control of heat networks requires the sensing and regulation of two parameters, temperature and pressure, whereas one parameter suffices for water supply networks (pressure), gas supply networks (pressure), and electricity networks (voltage). Control systems for heat networks being designed today have two interdependent subsystems, one to control pressure and one to control temperature.
Energy Vol. 12,No. lO/ll, pp. 1013-1016,1987 Printed in Great Britain
ENERGY CONSERVATION AND IMPROVEMENT OF RELIABILITY IN CENTRALIZED HEATING SYSTEMS V. VARVARSKY,’
YA. KOWLIANSKY’
and S. CHISTOVICH~
‘VNIPI Energoprom, Moscow 105266, U.S.S.R. ‘VNIGGS, Leningrad, U.S.S.R. Abstract-Centralized heating systems are being used increasingly to supply heat to the cities and towns of the U.S.S.R. These systems are usually based either on boilers of 50MW or greater thermal capacity or on cogeneration (combined heat and power) plants. Cogeneration not only saves energy but also reduces air pollution and thermal pollution and raises labor productivity. In some new systems, not only baseload but also loadfollowing power plants are being used for cogeneration, which allows improvements in the system-wide load factor for electricity. This paper outlines some of the characteristics of modem systems, including scale, layout, pipeline materials, and systems for control.
1. CENTRALIZED
HEATING SYSTEMS IN THE U.S.S.R.
Centralized heating systems are being used increasingly to supply heat to the cities and towns of the U.S.S.R. These systems have considerable advantages from the points of view of economics, sociology, and ecology. The most desirable centralized heating systems are combined heat and power systems associated with electric power plants; compared to separately generating electricity at condensing plants using either fossil or nuclear fuel and separately generating heat in district or local heating systems, combined heat and power systems use less fuel, improve regional air quality, and provide higher quality heat supply. Representative characteristics of heat supply sources in the U.S.S.R. are presented in Table 1. It is seen that, in 1980, 55% of all heat demand in the residential and commerical sector was met by centralized sources: 39% by thermoelectric plants and 16% by district heating systems. In fact, restricting the estimate to cities and towns, about 70% of residential and commerical heat demand was met by centralized sources. Energy savings in 1980 due to the use of power plants for combined heat and power amounted to 33 million tonnes of coal equivalent (tee), or 1.0 x lo’* J. The average specific fuel consumption for electricity at plants used for combined heat and power was 75% of the specific fuel consumption at condensing plants, 265 vs 356 grams of coal equivalent (gee) (7.8 vs 10.4 MJ) per kWh of electricity produced. (According to our energy accounting convention, the entire energy savings associated with combined heat and power is assigned to electricity production in these calculations.) Table 1 shows that there are dramatic differences in labor productivity between centralized and local heat supply systems. There are also considerable differences in energy requirements; one estimate attributes energy savings of 21 million tee (6.4 x 1017J) to the use of centralized rather than local heat supply systems. Moreover, centralized heat supply has further merits beyond savings of manpower and fuel, including reduced air pollution and thermal pollution; quantitatively, nationwide in 1980 the emission of harmful effluents to the atmosphere was at least 10% less than it would have been in the absence of centralized heat supply.
2. ENERGY SOURCES AND MODES OF OPERATION
In the European portion of the U.S.S.R. the energy sources for future centralized heat will be nuclear electric power plants, nuclear thermal plants (producing heat only), and large boilers burning coal or natural gas supplied from eastern portion of the U.S.S.R. In the eastern portion itself (Siberia and Kazakhstan), the source for centralized heat will be 1013
1.
Heating
Total
Waste-heat
Recovery
Plant
Plant
System
Local Heat Supply Small Boiler Individual Heating
District
(TEP)
Energy
Centralized Heat Supply Thermoelectric Plant
Table
-12350(100%)
8700(100X)
450(<1%)
3400(28%) 2250(17X)
1900(16X)
4800(39%)
lOI and % of total
Energy Use 1980
Productivity
--
190(11%)
2400(28%) 2810(31%)
590(7X)
2900(34X)
lOI and X of total
Annual 1970
Use and Labor
5 2 __
_-
_I
20
40
Labor Productivity (CJ/person-yea?:)
Systems
7 5
1.3
1.0
Normalized Number of Workers (T.E.P.=l.O)
and Local Heat Supply
1.14 1.33
0.98
1.00
Normalized Fuel-to-heat Ratio (T.E.P.=l.OO)
in Centralized
Energy conservation in centralized heating systems
1015
fossil fuels burned in power plants, as this region possesses large reserves of cheap coal which will be used to produce electricity for new industries. It is economical to modify both nuclear and fossil fuel condensing power plants so that they operate in the cogeneration mode. By 1981, 325 condensing turbines had been modified, saving 1.8 million tee (5.3 x 1Ol6 J) of energy. Moreover, it has become increasingly economical to operate load-following power plants as cogeneration plants. This mode of operation is particularly important in the European part of the U.S.S.R., to supplement the nuclear power plants providing base load. The use of renewable sources of energy for centralized heat is also being investigated. The principal geothermal sources are located in the Caucasus, Siberia, the Far East, Kamchatka, Kazakhstan, and Central Asia. The nation’s geothermal heat supply has been estimated to be able to supply 30-40 million tee (0.9-1.2 x 1018J) annually. More than 40 geothermal fields are currently being used, all in the Caucasus or on the Kamchatka Peninsula; in 1980, they produced 40 million m3 of hot water and 300,000 tonnes of steam, saving 0.5 million tee (1.5 x 1016J). By 1985 geothermal sources were providing 60 million m3 of hot water, and more than 800,000 people in Tbilisi, Makhachkala, and other cities and towns were served by geothermal heat. As for centralized heating by solar energy, the most favorable areas, where the sun shines 2200-3000 h per year and the annual solar energy on a horizontal surface is 12OC-1700 kWh/m2, are in the Central Asian republics, the southern Ukraine, Kazakhstan, Moldavia, and southern portions of the Russian Republic. Energy for centralized heat can also be provided by waste-heat recovery plants attached to industrial heat supply systems. In 1980 energy savings of 58 million tee (1.7 x lO”J) were achieved by such plants in the ferrous metal, chemical, and petrochemical industries. Long-term energy planning in the U.S.S.R. envisions large-scale heat recovery from sewage treatment plants and from the ventilation air exhausted from buildings. Heat recovery can be accomplished either by heat exchangers or heat pumps.
3. SYSTEM DESIGN
Because new thermoelectric plants are being built tens of kilometers from population centers, centralized heat supply systems tend to have a comparable dimension. The effective radius of some networks is more than 30 km, and the diameter of a trunk pipeline is sometimes 1.4m. For example, the trunk supply line connecting the Mid-Urals thermoelectric plant to the city of Sverdlovsk is 25 km long; the line has been operating continuously since 1962. The heat supply line planned to connect the Odessa nuclear power plant with the city of Odessa is also more than 25 km long; as for the heat supply networks for Moscow, Leningrad, Novosibirsk, Kharkov, each has an overall length of hundreds of kilometers. The total length of the heat supply lines in the U.S.S.R. exceeds 180,000km. Of the total investment for a centralized heat supply system, typically 70-80% is for the network in a dedicated heating system and 35-40% is for the network in a combined heat and power system. Heat supply lines laid above ground are more reliable, less susceptible to corrosion, more easily inspected, and more easily repaired, thus, whenever possible, pipes are laid above ground, on low trestles. Underground siting of heat supply lines is necessary in cities. In Leningrad, pipelines with reinforced concrete foam insulation have been in operation since 1948. Nowadays, to save labor, heat insulation is applied to the pipes at the factory. Anticorrosion coatings are applied to the outer surface, as well as electrolytic protection (cathode polarization). There is practically no internal corrosion, because the supply water is deaerated at the heat source and sodium silicate is added. Increasingly, open-circuit heat supply systems are being built; relative to closed systems these are more energy efficient and they can recover lower quality heat. Roughly half of the heat supply systems in our cities and towns today are open systems, for example in Leningrad, Tashkent, Alma Ata, Kalinin, Khabarovsk, Ivanovo, Krasnoyarsk, and Norilsk.
1016
V. VARVARSKYet al. 4. THE ROLE OF OPTIMIZATION
AND CONTROL
A systems approach is increasingly used in the planning for centralized heat supply, whether at the scale of a town or industrial center, at the scale of a region or interconnected electric grid, or at the scale of the nation as a whole. Mathematical modeling techniques such as nonlinear programming, dynamic programming, and combinatorics allow the consideration of time-dependent growth in heat loads and heat sources. Typical problems include determining the optimum number of plants of each type and size, optimum pipeline diameter, optimum time of deployment, and optimum supply temperature. Computational analysis of operating heat supply systems, combined with long-term experience, has revealed considerable shortcomings in distribution systems, especially in critical circumstances where the system is functioning below design conditions. New principles have been developed to guide the modernization of such systems. One preferred arrangement has a two-pipe circular trunk network, subdivided by means of shutters (gate valves); 25-60 MW of thermal power are managed by each distribution center connected to the circular trunk. Downstream of the distribution center, heat distribution typically runs through three-pipe networks; two pipes supply hot water and one is the common return. Automation of heat control is mandatory to assure economical and reliable operation. Modern control systems permit prompt redistribution of heat load among heat sources, to take advantage of differential costs of supply in base-load and peak-load plants. Modern control systems allow the system to respond to weather conditions, to production schedules of industrial sources and consumers, and to consumption patterns dependent on the time of day. Modern control systems, moreover, greatly improve responses to emergencies, detecting and localizing the problem and minimizing the system-wide damage. The control of heat networks requires the sensing and regulation of two parameters, temperature and pressure, whereas one parameter suffices for water supply networks (pressure), gas supply networks (pressure), and electricity networks (voltage). Control systems for heat networks being designed today have two interdependent subsystems, one to control pressure and one to control temperature.