Prospects for the Development of Thermal Power Plants with Increasing Hydropower Supply to Power Systems
Abstract Generated abstract
This paper examines the changing role of thermal power plants in Soviet power systems as hydroelectric generation expands. Using a heat balance for systems that include hydroelectric stations, condensing thermal generation, cogeneration heat extraction, and independent boiler houses, it argues that hydroelectric energy first displaces less efficient condensing output, while increasing district heating from turbine extractions reduces fuel consumption per unit of useful energy delivered. The analysis supports converting and operating thermal plants increasingly as cogeneration facilities, expanding heat networks to residential, industrial, agricultural, and possible municipal uses, and using simple boiler houses mainly for peak heat loads. It concludes that the long-term function of thermal power stations under growing hydropower supply depends on broad development of district heating and advance identification of new heat consumers.
Full Text
Prospects for the Development of Thermal Power Plants with Increasing Supply of Hydroelectric-Plant Energy to Power Systems
I. N. Butakov
In connection with the extensive development of hydroelectric construction in the USSR, the question arises of the role of thermal power plants in power systems.
Let us assume that the power system also includes independent boiler houses capable of supplying heat consumers with \(Q_{\mathrm{kot}}\) kcal over the time interval under consideration (year, month), with their weighted-average efficiency \(\eta'_{ku}\cdot\eta'_{pot}\); in addition to \(Q_{\mathrm{kot}}\) kcal, heat consumers receive from turbine extractions another \(Q_{\mathrm{otp}}\) kcal at an efficiency \(\eta_t\), ensuring the generation of electric energy for heat consumption of \(E_t\) kWh. Then we shall have the heat balance of the power system in the form
\[ \frac{860 E_t + Q_{\mathrm{otp}}}{\eta_t} + \frac{860(E - E_t - E_{\mathrm{gidr}})}{\eta_k} + \frac{Q_{\mathrm{kot}}}{\eta'_{ku}\cdot \eta'_{pot}} = \frac{860(E - E_{\mathrm{gidr}}) + Q_{\mathrm{otp}} + Q_{\mathrm{kot}}}{\eta}. \tag{1} \]
Here \(E\) is the total generation of electric energy by the power system in kWh over the same period of time; \(E_{\mathrm{gidr}}\) is the generation, in kWh, by hydroelectric plants supplying electric energy to the power system under consideration; \(\eta_k\) is the weighted-average efficiency of the condensing parts of the system; and \(\eta\) is the economic efficiency of the thermal part of the entire power system.
Hence we obtain
\[ 1/\eta = \frac{1}{860(E - E_{\mathrm{gidr}}) + Q_{\mathrm{otp}} + Q_{\mathrm{kot}}} \cdot \left[ \frac{Q_{\mathrm{kot}}}{\eta'_{ku}\cdot \eta'_{pot}} + \frac{Q_{\mathrm{otp}}}{\eta_t} + \frac{860(E - E_{\mathrm{gidr}})}{\eta_k} - 860 E_t \left( \frac{1}{\eta_k} - \frac{1}{\eta_t} \right) \right]. \tag{2} \]
For a given value of the total heat consumption, \(Q_{\mathrm{otp}} + Q_{\mathrm{kot}} = \mathrm{const}\), any increase in \(Q_{\mathrm{kot}}\) reduces \(Q_{\mathrm{otp}}\), and consequently also the generation of electric energy \(E_t\) for heat consumption; as is seen from formula (2), this increases the number of kilocalories spent to obtain 1 kcal supplied to consumers by the power system, irrespective of the purpose of this kilocalorie (for electric or heat consumers). On the other hand, the kWh of hydroelectric plants will displace first of all the kWh \((E - E_{\mathrm{gidr}} - E_t)\) obtained in condensing operation, since they entail a greater specific consumption of kcal/kWh.
It is therefore clear that, with the general growth over time of the supply of kilocalories by the power system \([860(E - E_{\mathrm{gidr}}) + Q_{\mathrm{otp}} + Q_{\mathrm{kot}}]\), our efforts must be directed toward the development of district heating, toward increasing \(Q_{\mathrm{otp}}\).
Substituting into expression (2), instead of \(E_m\), its value \(E_m=\dfrac{Q_{\mathrm{otp}}}{q}\), where the system-average value \(q=(i-\eta_{oi}H-t_{\mathrm{kon}})\times\dfrac{860}{\eta_e H}\), kcal/kWh, is most simply determined for the entire power system from the given \(Q_{\mathrm{otp}}\) and \(E_m\), and taking outside the brackets, in the numerator and denominator of the same expression (2), the quantity \(860(E-E_{\mathrm{hidr}})\), we then obtain, after dividing the numerator polynomial by the denominator polynomial of (2),
\[ \begin{aligned} 1/\eta={}&\frac{1}{\eta_m} -\frac{860}{q}\left(\frac{1}{\eta_k}-\frac{1}{\eta_m}\right)+\\ &+\left\{ \frac{Q_{\mathrm{kot}}}{\eta'_{ku}\eta'_{\mathrm{pot}}\,860(E-E_{\mathrm{hidr}})} +\frac{1}{\eta_k} -\left[ \frac{1}{\eta_m} -\frac{860}{q}\left(\frac{1}{\eta_k}-\frac{1}{\eta_m}\right) \right]\times \right.\\ &\left. \times\left[ \frac{Q_{\mathrm{kot}}}{860(E-E_{\mathrm{hidr}})}+1 \right]\right\}: \left[ 1+\frac{Q_{\mathrm{kot}}}{860(E-E_{\mathrm{hidr}})} +\frac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})} \right]. \end{aligned} \tag{3} \]
Thus, the dependence between \(1/\eta\) and \(\dfrac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})}\) is hyperbolic.
The asymptotes of the hyperbola are, on the one hand, for \(\dfrac{1}{\eta}=\infty\),
\[ \frac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})} = -\left[ 1+\frac{Q_{\mathrm{kot}}}{860(E-E_{\mathrm{hidr}})} \right], \]
and, on the other hand, for \(-\dfrac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})}=\infty\),
\[ 1/\eta=\frac{1}{\eta_m} -\frac{860}{q}\left(\frac{1}{\eta_k}-\frac{1}{\eta_m}\right). \]
We find the point for constructing the hyperbola by setting
\[ \frac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})}=0. \]
Let, for a particular example, we have: \(\eta_k=0.20\); \(\eta_m=0.8\); \(q=3000\) kcal/kWh; \(\dfrac{Q_{\mathrm{kot}}}{860(E-E_{\mathrm{hidr}})}=0.1\); \(\eta'_{ku}\cdot\eta'_{\mathrm{pot}}=0.8\). Then one asymptote is \(1/\eta=0.18\); the other asymptote is
\[ \frac{Q_{\mathrm{otp}}}{860(E-E_{\mathrm{hidr}})}=-1.1, \]
and for the point on the ordinate axis we obtain \(1/\eta=4.66\). The hyperbola is plotted in Fig. 1.
Thus, the development of district heating (an increase in \(Q_{\mathrm{otp}}\)) with a simultaneous increase in the supply of current from hydroelectric stations, when \((E-E_{\mathrm{hidr}})\) decreases, ensures a reduction in \(1/\eta\), or in the number of kcal of fuel per 1 kcal delivered by the power system to consumers, regardless of the nature of this calorie. Hence the profound correctness of the directive of the Nineteenth Party Congress: “To ensure the construction of CHP plants and heat networks for the broad district heating of cities and industrial enterprises.” Hence also the necessity for power engineers to seek out and develop in every possible way thermal-
consumption \((Q_{\text{cons}})\), bearing in mind that “economic managers must seek, find, and use hidden reserves, ... make maximum use of existing production capacities, ... reduce the cost of production.”
Fig. 1.
In addition to broadly covering the residential sector by means of district heating—including the supply of hot water to consumers—it is necessary to strive to ensure the heat supply of industrial enterprises from thermal power stations, with the closing of local boiler houses, which are often centers of wasteful fuel consumption; it is necessary to achieve the establishment, in suburban zones, of market gardens under glass1 as major heat consumers. The need for this is especially indicated in the resolution of the Plenum of the Central Committee of the CPSU of September 3, 1953, on the report of Comrade N. S. Khrushchev. Agro-district heating proves, under the climatic conditions of most of our country, to be a year-round consumer of heat, since even in summer soil heating is required in order to make it possible to grow heat-loving plants (melons, watermelons, tomatoes, etc.).
The possibility is not excluded that it may be expedient to use heat for melting snow on the streets of densely populated parts of cities, as well as at large railway junctions,2 where enormous funds and effort are now spent on removing this snow, which hampers traffic. Unfortunately, no one has yet dealt seriously with these questions, so for the time being one can speak of this only tentatively.
At present the possibilities for broad coverage of district-heating facilities are increasing considerably, since the expediency of long-distance heat supply is being established (over 20–50 km and even more).
At the same time, the expediency is also becoming apparent of including independent boiler houses in the power system for shaving peaks of the heat load, with the aim, on the one hand, of increasing the utilization factor of turbine extractions, which makes it possible to expand the range of heat consumers, and, on the other hand, of reducing the cost of fixed assets, since shaving heat peaks can be carried out by boiler houses of the simplest design, with low steam parameters, cheaper than the main equip-
…equipment of CHP plants, bearing in mind that the duration of peak periods is not great, usually amounting to 500–1000 hours per year. It is clear that the most suitable of the existing industrial central boiler houses subject to closure in connection with the centralization of heat supply may be used as peak boiler plants.
The continued existence of condensing stations, especially of low and increased pressures, when power is supplied from hydroelectric stations, will be threatened first of all because of their poor utilization, or even because of their complete exclusion from the operation of power systems. Measures must therefore be taken in good time to use them as cogeneration plants, with justification of the expediency of converting machines to a poorer vacuum while simultaneously extracting steam from intermediate stages for heating network water, and converting unregulated extractions into regulated ones, while, of course, preserving regeneration.
The broad development of district heating raises anew the question of the drive for auxiliary equipment at thermal power stations: for a number of mechanisms the electric drive must be replaced by a steam drive, using the heat of exhaust steam for district-heating purposes. The electric drive will then become a standby drive. The new role of the steam drive poses the task of revising its design so as to make it capable of prolonged and more economical operation. For example, at high-pressure CHP plants the exhaust steam from boiler blowdown expanders should be directed to the intermediate stages of the turbines of feed pumps, and not to deaerators, as is usually done, thereby reducing the efficiency of the regenerative process of the main turbines. It is clear that, under such an arrangement, the steam drive of the feed pumps must be designed differently than at present, receiving several stages of the driving turbine. It is understandable that putting steam drives into operation will significantly ease the acuteness of the issue of electricity consumption for station auxiliaries.
Thus, the future of thermal power stations, with the development of hydroelectric construction in our country, lies in the broad growth of district heating, toward the expansion of which workers on the energy front must take measures in advance, identifying on the spot ever more new consumers of heat.