Techno-Economic Comparison of Snow Melting Based on District Heating with Its Removal to Dumps
Abstract Generated abstract
This paper evaluates the technical and economic feasibility of melting urban snow using heat from district heating networks instead of transporting it to disposal sites. It develops a cost comparison method based on capital investment, operating costs, snow volume, hauling distance, installation capacity, heat and electricity costs, and payback period, then applies the calculation to conditions in Tomsk. The results indicate that larger annual snow-melter capacities shorten the payback period, and that for a 3 km average hauling distance a unit handling about 300 tons per year or more can fall below the standard eight-year payback period. The study also notes potential system benefits from lowering return-water temperatures and calls for industrial operating data to refine sizing and labor assumptions.
Full Text
TECHNO-ECONOMIC COMPARISON OF SNOW MELTING BASED ON DISTRICT HEATING WITH ITS REMOVAL TO DISPOSAL SITES
N. A. POPOV
(Presented by the Department of Industrial Thermal Power Engineering and by the scientific seminar of the Thermal Power Engineering Faculty)
When city territories are cleared of snow, substantial expenditures of funds and motor transport are required for hauling it to disposal sites. Reducing these expenditures through the use of more rational methods of combating snow will bring undoubted benefits to the municipal economy.
One such method may be melting snow in place instead of hauling it to dumps. Figure 1 shows the schematic diagram of a snow-melting installation. The snow is melted in the body of snow melter 1 by direct contact of heated meltwater circulating with the snow.
Fig. 1. Schematic diagram of a surface-contact snow-melting installation
Heating of the circulating water is carried out in heat exchanger I by means of the heat of the district-heating water during those hours of the day when the subscribers have no heat load for hot-water supply and ventilation, and in heat exchanger II by means of the heat of the return water from the local systems and heat exchanger I.
Water circulation through the heat exchangers is effected by pump 2, and its spraying by nozzles 3. Snow melting will be economically expedient when the payback period of the additional capital investment in the snow-melting installation \(T\) is equal to or less than the standard payback period \(T_{\mathrm{n}}\).
The payback period of the additional capital investments is determined as:
\[ T=\frac{K_{\mathrm{st}}}{S_{\mathrm{v}}-S_{\mathrm{st}}},\ \text{years} \tag{1} \]
where \(K_{\mathrm{st}}\) is the initial capital investment in the snow-melting installation, rubles;
\(S_{\mathrm{v}}\) is the annual operating cost of transporting snow to dumps, rubles/year;
\(S_{\mathrm{st}}\) is the same for snow melting, rubles/year.
In organizing snow melting, it is assumed that delivery of the scraped snow to the snow melters will be carried out by means of a bulldozer equipped with a special bucket attached to the bulldozer blade.
Such a bulldozer can not only bulldoze the scraped snow but also scrape it at the same time.
Under these conditions, we take the costs of scraping snow and bulldozing it during snow melting as equal to the costs of scraping snow and loading it onto motor vehicles during removal.
Then the annual operating costs for removal and melting of snow are written as:
\[ S_{\mathrm{v}}=s_{\mathrm{v}}G_{\mathrm{s}}, \tag{2} \]
\[ S_{\mathrm{st}}=S_{\mathrm{a}}+(s_{0}+s_{\mathrm{e}}+s_{\mathrm{k}}+s_{\mathrm{t}})G_{\mathrm{s}}. \tag{3} \]
In expressions (2) and (3), the following notation is used:
\(s_{\mathrm{v}}\) is the unit cost of paying for motor transport when removing snow, rubles/ton;
\(G_{\mathrm{s}}\) is the amount of snow subject to removal, tons/year;
\(S_{\mathrm{a}}\) is the annual deductions from the initial investment in the snow-melting installation for depreciation and repairs, rubles/year;
\(s_{0}\) is the unit cost of labor for servicing the snow-melting installation, per one ton of melted snow, rubles/ton;
\(s_{\mathrm{e}}\) is the unit cost of pumping the melt circulation water, per one ton of melted snow, rubles/ton;
\(s_{\mathrm{k}}\) is the unit cost of sewering the melt water, per one ton of melted snow, rubles/ton;
\(s_{\mathrm{t}}\) is the unit cost of the heat energy expended on melting snow, per one ton of melted snow, rubles/ton.
Determination of operating costs
The unit cost of expenses for motor transport is
\[ s_{\mathrm{v}}=a+bl_{\mathrm{v}}, \qquad \text{rubles/ton} \tag{4} \]
where \(a\) and \(b\) are certain coefficients, adopted in accordance with tariff handbooks for the transportation of freight by motor transport;
\(l_{\mathrm{v}}\) is the distance to the dumps, km.
The annual deductions from the initial cost of capital investments in the snow-melting installation are
\[ S_{\mathrm{a}}=f_{\mathrm{st}}K_{\mathrm{st}}, \tag{5} \]
where \(f_{\mathrm{st}}\) is the share of annual deductions as a whole for the installation, taken as 0.06;
\(K_{\mathrm{st}}\) is the initial capital investment at the design capacity of the installation, rubles.
Capital investments in the installation consist of
\[ K_{\mathrm{st}}=K_1+K_2+K_3+K_4, \tag{6} \]
where \(K_1\) is the cost of the reinforced-concrete housing, rubles;
\(K_2\) is the cost of the pump-and-heater installation, rubles;
\(K_3\) is the cost of laying the external circulation pipeline to the snow-melting unit housing, rubles;
\(K_4\) is the cost of laying the sewer pipeline, rubles.
In determining capital costs, the following assumptions were adopted:
1) The internal volume of the housing is \(0.85\ \mathrm{m^3 \cdot h/t}\).
2) The cost of reinforced concrete in place is \(40\ \mathrm{rubles/m^3}\).
3) The heating surface of the heat exchangers was determined at an average temperature difference between the heating and heated media of \(\Delta t_{\mathrm{av}} = 22^\circ\mathrm{C}\), a heat-transfer coefficient \(k = 8.35\ \mathrm{MJ/m^2 h^\circ C}\), and amounts to \(2\ \mathrm{m^2}\) per one ton of hourly capacity of the snow-melting unit.
4) The cost of \(1\ \mathrm{m^2}\) of heat-exchanger heating surface is \(50\ \mathrm{rubles/m^2}\) [2].
5) The cost of two-pipe, channelless laying of circulation water pipelines is
\[ K_3=a_1+b_1 2\cdot d\cdot l, \tag{7} \]
where \(d\) is the diameter of the heat pipeline, m;
\(l\) is the length of the external route, m.
The coefficients are
\[ a_1=0\ \mathrm{rubles/m};\quad b_1=140\ \mathrm{rubles/m^2}\ [2]. \]
6) The cost of laying the sewer pipe is
\[ K_4=2.0\ \mathrm{rubles/m}. \]
7) The length of the route of the external circulation water pipelines and sewerage is \(10\ \mathrm{m}\) each.
8) The pump power was determined for a head of 5 bar and a capacity equal to 50% of the heating-water flow rate.
9) The heating-water flow rate is
\[ G_1 \approx 5G_{\mathrm{st}}. \]
Expressing the cost of the installation as a linear function of its capacity, we obtain
\[ K_{\mathrm{st}}=a_2+b_2G_{\mathrm{st}}, \tag{8} \]
where \(a_2, b_2\) are coefficients whose values are taken as
\[ a_2=400\ \mathrm{rubles};\quad b_2=160\ \mathrm{rubles\cdot h/t}. \]
\(G_{\mathrm{st}}\) is the design hourly capacity of the snow-melting installation, t/h.
It is found from the minimum of the sum of the dependent reduced costs for the installation and is expressed as
\[ G_{\mathrm{st}}=\sqrt{\frac{G_c P T_i}{175b_2\left(f_{\mathrm{st}}+p_n\right)}},\quad \frac{m}{r}, \tag{9} \]
where \(G_c\) is the amount of snow to be melted at this installation, t/h;
\(P\) is the wage of the worker servicing the installation, rubles/month;
$T_i$ — labor time spent by a worker servicing the installation during one hour of its operation, h/h;
$P_{\mathrm{n}}$ — standard efficiency coefficient;
175 — number of hours of operation of the installation during a month. The specific cost of servicing the installation is
\[ S_0=\frac{P\cdot T_i}{175G_{\mathrm{st}}},\quad \text{rub/t}. \tag{10} \]
The specific cost of pumping the circulating water is
\[ S_{\mathrm{e}}=\frac{9.8G'_{\mathrm{c}}H\cdot z_{\mathrm{e}}}{10^6G_{\mathrm{st}}\eta_{\mathrm{n}}},\quad \text{rub/t} \tag{11} \]
where $G'_{\mathrm{c}}$ is the design flow rate of circulating water for the given capacity of the snow melter, t/h,
\[ G'_{\mathrm{c}}\simeq 0.5G'_{\mathrm{r}}; \]
$G'_{\mathrm{r}}$ — design flow rate of heating water for the given capacity of the snow melter, equal to
\[ G'_{\mathrm{r}}\simeq 5G'_{\mathrm{st}}; \]
$H$ — head of the circulating pump, m;
$z_{\mathrm{e}}$ — cost of electric energy, rub/MJ;
$\eta_{\mathrm{n}}$ — pump efficiency. The specific cost of heat consumed for melting snow is
\[ S_t=\left[\frac{\left(r-C_c t_{\mathrm{n}}^{\mathrm{st}}\right)}{10^3\cdot \eta_{\mathrm{st}}}(1-\varphi_t)\right],\quad \text{rub/t}, \tag{12} \]
where $z_t$ is the cost of thermal energy, rub/MJ;
$\zeta$ — latent heat of fusion of snow, J/kg;
$c_c$ — heat capacity of snow, J/kg °C;
$t_{\mathrm{n}}^{\mathrm{st}}$ — design outdoor air temperature for snow melting, °C. It is taken as equal to the average, by duration of occurrence, outdoor air temperature in the range from $t_{\mathrm{n}}=0^\circ\mathrm{C}$ to $t_{\mathrm{n}}^{\mathrm{TPP}}\ ^\circ\mathrm{C}$;
$t_{\mathrm{n}}^{\mathrm{TPP}}$ — design temperature of the TPP, °C;
$\eta_{\mathrm{st}}$ — efficiency of the snow melter, accounting for heat losses to the environment and with the melt water discharged into the sewer system. On the basis of preliminary experiments, its value may be taken as 0.9–0.93.
$\varphi_t$ — a quantity accounting for fuel savings in the combined method of producing electric and thermal energy on the basis of snow melting,
\[ \varphi_t=29.3b_{\mathrm{e}}, \]
29.3 — calorific value of standard fuel, MJ/kg;
$b_{\mathrm{e}}$ — specific fuel saving in combined generation of electric and thermal energy on the basis of snow melting, kg/MJ.
We shall show the comparison of alternatives using a particular example for the city of Tomsk. For the conditions of the city of Tomsk we may take: $P_{\mathrm{otb}}=1.2$ bar; $Z_{\mathrm{k}}=0.03$ rub/t; $a=0.43$ rub/t; $b=0.11$ rub/t·km; $a_2=400$ rub; $b_2=160$ rub·h/t; $G_{\mathrm{c}}^1=0.5G_{\mathrm{r}}^1$; $G_{\mathrm{r}}^1=5.0G_{\mathrm{st}}^1$; $P=80$ rub/month; $T_i=1.0$ h/h; $H=50$ m; $z_{\mathrm{e}}=0.0028$ rub/MJ; $\eta_{\mathrm{st}}=0.9$; $z_t=0.0006$ rub/MJ; $t_{\mathrm{n}}^{\mathrm{TPP}}=-14^\circ\mathrm{C}$; $t_{\mathrm{n}}^{\mathrm{st}}=-7^\circ\mathrm{C}$; $\varphi_t=0.8$; $\eta_{\mathrm{n}}=0.3$; $f_{\mathrm{st}}=0.06$; $P_{\mathrm{n}}=0.125$.
On the basis of the calculations performed, graphs have been constructed, shown in Figs. 2 and 3.
Figure 2 shows the dependence of the payback period for additional costs of a snow-melting installation on the annual capacity of an individual snow melter and on the distance for hauling snow to dumps.
Fig. 2. Payback period of capital investments in a snow-melting installation at different annual capacities, depending on the distance for hauling snow to dumps
Fig. 3. Unit costs of melting and hauling snow to dumps
From the graph it is evident that the greater the annual capacity of the snow melter, the shorter the payback period of the capital costs. For the conditions of the city of Tomsk, with an average distance to the dumps of 3 km, the payback period for a snow-melting installation will be below the standard payback period \(T_n = 8\) years, at a capacity of an individual snow melter of \(300\ \text{t/year}\) and higher.
Figure 3 gives a comparison of the unit costs of hauling and melting snow. When the capacity of the snow-melting unit is more than 160 t/day, the cost of snow melting begins to decrease sharply in comparison with hauling it away.
Conclusion
1) As the studies carried out show, snow melting based on district heating is technically feasible and economically expedient.
2) To resolve a number of issues related to the organization of snow melting, it is necessary to obtain certain data from experience with an industrial installation. In particular, only through time-and-motion studies can one determine the optimal area of the territory to be cleared and, consequently, the amount of snow that determines the design capacity of a single snow-melting unit, the time expenditures of workers in servicing it, etc.
3) As operating experience with heat-supply systems in most urban heating networks shows, the temperature of the return water returned from consumers to the combined heat-and-power plant exceeds the temperature specified by the schedule by 10–20°C or more. Reducing it in snow-melting installations will be a positive factor in improving the quality indicators of the operation of heat-supply systems.
4) In order to stimulate the introduction of snow melting based on district heating, it is necessary, at the expense of the additional savings obtained in the combined generation of electric power, to establish reduced tariffs for the heat supplied for these purposes.
LITERATURE
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A. V. Nikitin. Snow-Melting Units. Publishing House of the Ministry of Municipal Services of the RSFSR, Moscow, 1952.
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E. Ya. Sokolov. District Heating and Heat Networks. Gosenergoizdat, Moscow; Moscow–Leningrad, 1963.