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Economic analysis of phase change energy storage fresh air unit operation (Part 2)

3.2 operation cost analysis

fresh air load in one weather (the concept of fresh air load in this paper is different from the traditional concept) can be expressed as

(10)

where q is the fresh air load, kJ; G is air volume, m3/s; Is the air density, kg/m3; H is the specific enthalpy kj/kg. Subscript d indicates one day, in indicates air intake, out indicates air exhaust, and h, a and L respectively indicate the peak period, peak period and trough period of electricity price

in this paper, q1=0 is taken, that is, qd= QD + QA, that is, the unit's treatment of fresh air during the nighttime electricity price trough period is not considered

when Q QD, Q ex= Q - Q D, Q ex, h= Q - Q h, Q sh, h= 0, Q sh, a= 0

when Q QH and Q Q D, Q ex= 0, Q ex, h= Q - Q h, Q sh, h= 0, Q sh, a= Q D - Q

when QH, qex= 0, Q ex, h= 0, Q sh, h= Q H - Q, Q sh, a= Q A

operation cost of energy storage unit:

m r = [(Q -q Ex) p1 + Q sh, H P H + Q sh, a p a]/(3600 COP) (11)

operation cost of conventional unit:

m0r = (QH p H + q a p a) "/(3600 cop) (12)

operation cost saved on day J: yj= m0r - M r

in one year, the total operation cost saved is

the above types make it a strong in the growth point, q is the cold storage capacity, kJ; P is the electricity price, yuan/(kW h); Cop is the coefficient of refrigeration; Y is the operation cost saved every day, yuan. The subscript ex indicates excessive, and sh indicates insufficient

4 selection of optimal cold storage capacity and operation scheme

phase change energy storage fresh air units store cold at night during the low electricity price period, and cool the fresh air during the day, so as to reduce the operation cost of the unit. For the economic analysis of the phase change cold storage fresh air unit, it is necessary to consider the daily cold storage capacity and operation scheme of the unit, which are interrelated and affect each other, and need to be comprehensively considered

4.1 selection of operation scheme

whether the operation scheme is reasonable is an important factor affecting the economy and operation effect of energy storage fresh air units. The operation of energy storage air conditioning system is generally divided into two types: full load energy storage and partial load energy storage. Although the full load energy storage mode has low operation cost, it has high equipment investment and large area of energy storage device; Part of the cooling capacity required for the whole day of the partial load energy storage mode is provided by the cooling surface. The required refrigerator capacity and the capacity of the energy storage system are small, but the operation cost saved is less than that of the full load mode. According to the electricity price in Figure 2, the following operation scheme is adopted in this paper: ① ensure that the outlet temperature of the treated air is 24 ℃; ② For the full load energy storage mode, the purpose of cold storage at night is to meet the fresh air load throughout the day; For the partial load energy storage mode, the nighttime cold storage is mainly to meet the fresh air load during the peak period of electricity price. If the selected daily cold storage capacity has excessive cooling capacity during the peak period of electricity price, it is considered that the excessive part can be transferred to the peak period of electricity price. In case of insufficient cooling capacity during the peak period and peak period, the refrigerator will provide insufficient cooling capacity. In order to realize this operation scheme, it is necessary to optimize the structure of the phase change energy storage heat exchanger and adopt reasonable operation control means

Figure 2 electricity price structure in Beijing

4.2 selection of cold storage capacity

the essence of selecting the best cold storage capacity is to match the cold storage capacity with the new load, and strive to ensure the air treatment effect and the economy of the system. The cold storage capacity required by the fresh air unit is related to the operation scheme and outdoor meteorological parameters. In a day, the longer the unit operates, the higher the outdoor fresh air temperature, the greater the fresh air volume, and the greater the cold storage capacity required. Because the outdoor air temperature changes greatly during the unit operation in a year, the daily fresh air load is different under a certain operation scheme, and the maximum and minimum fresh air load in a year are highly correlated, which brings great difficulties to the determination of the cold storage capacity. The selection of cold storage capacity is small, the required energy storage equipment capacity and equipment investment are small, the utilization rate of phase change materials is high, and because most of the energy stored at night can be supplemented in the peak of electricity price, the corresponding performance price is relatively high. However, because the energy stored at night is small, there is still relatively large amount of cooling capacity to be provided by the refrigerator during the day, and the operation cost is relatively high; The selection of cold storage capacity is large, the required energy storage equipment capacity and equipment investment are large, and because the energy stored at night is likely to be replenished after the peak of the electricity price, it will be transferred to the peak period of the electricity price, and even part of the cold storage capacity will not be used. Therefore, the corresponding performance price ratio and the theme index of CSI new materials are from rare earth functional materials, rare metal materials, high-quality special steel, high-performance fiber The listed companies related to new materials such as nano materials have been selected to take 50 shares as constituent stocks, and the utilization rate of phase change materials is low. However, due to the large energy storage at night, the energy provided by the refrigerator during the day is small, and the operation cost is relatively low

in order to clearly express the relationship between cold storage capacity and operation scheme and system performance and economy, the following concepts are proposed in this paper:

utilization rate of phase change materials (13)

satisfaction rate of air treatment effect (14)

operation cost saving rate (15)

where Q sh = q SH, H + Q sh, a, q = q d at full load cold storage; Q sh = q sh, h, q = q h

where Z1 is the ratio of the cold storage used in the daytime to the total cold storage at night; Z2 refers to the ratio of the cold storage capacity utilized during the day to the fresh air load of the day (the total fresh air load in the daytime peak and peacetime under the full load mode, and the load in the daytime peak under the partial load mode). It also indirectly indicates the cooling capacity still required to be provided by the refrigerator during the day; Z is the proportion of the saved operation cost to the operation cost of the conventional unit, which indicates the saving of the operation cost of the energy storage unit. Originally, Q ex = q ex, H should be used for partial load cold storage, but in practical application, it is inevitable that some excessive cold storage capacity will be transferred to the peak period, so Q ex is still taken here

it can be seen that Z1, Z2 and Z are important indicators for economic analysis of phase change energy storage fresh air units. These indexes, combined with the saved operation cost and critical initial investment of the system, are helpful to determine the optimal cold storage capacity and reasonable operation scheme of the system. In the full load cold storage, because the refrigerator does not work during the day, Z2 can be obtained according to the air treatment requirements to calculate the corresponding optimal cold storage capacity; In the case of partial load cold storage, since the refrigerator can still provide insufficient cooling capacity in the daytime, the optimal cold storage capacity can be selected by comprehensively considering the Z-index and the saved operation cost. 5 calculation example

taking the fresh air unit with an air volume of 3000m3/h as an example, the meteorological data in Beijing are taken as the outdoor working conditions, and the outdoor meteorological parameters are obtained from the literature [2]. According to the operation scheme described above, the critical initial investment and indicators Z1, Z2 and Z of the unit under different cold storage capacity are studied. The unit operation time is calculated from May 15 to September 10

it can be seen from Figure 3 and Figure 4: ① with the increase of cold storage capacity, the annual saved operation cost and the allowable critical additional initial investment gradually increase, but the rate of increase is slower; ② When the cold storage capacity is increased from 200mj to 300MJ, the annual operation cost saved is only increased from 485 yuan to 535 yuan. The latter is because the cold storage capacity is small, and the cold storage capacity at night is mainly transferred to the peak of electricity price, so the material utilization rate is high and the economy is strong. However, with the increase of cold storage capacity, more and more cold storage capacity is transferred to the peak period, or even can not be used, and the material utilization rate is getting lower and lower, so the economy is weakened. However, too small cold storage capacity will lead to poor air treatment effect

Figure 3 annual operation cost saved by different cold storage capacity Figure 4 critical additional initial investment n of different cold storage capacity is the payback period

Figure 5 ~ 8 curves describe the Z indicators of each month, namely Z1, Z2 and Z, under the full and partial cold storage modes when different cold storage capacity is selected. It can be seen that from May to September, the law of Z1 is small at both ends and large in the middle, while the law of Z2 and Z is large at both ends and small in the middle (under partial load mode, Z2 has reached 100% due to small fresh air load), which is caused by the law that the fresh air load increases and then decreases with the change of outdoor working conditions

Figure 5 Z index (full load) when the cold storage capacity is 100mJ Figure 6 Z index (full load) when the cold storage capacity is 300MJ Figure 7 Z index (partial load) when the cold storage capacity is 100mJ figure 8 Z index (partial load) when the cold storage capacity is 200mj Figure 9 and figure 10 describe the relationship between the cold storage capacity and the annual average Z index under the full load cold storage mode and the partial load cold storage mode. The optimal cold storage capacity can be determined from the Z2 index in the figure according to the dissatisfaction rate of air treatment required by the user. If the dissatisfaction rate is taken as 4% (i.e. the annual operation period has a 5-day non assurance rate), then z2=96%. Under the full load cold storage mode, the corresponding optimal cold storage capacity is 250mj, but at this time, the utilization rate of phase change materials is only 61%, and the operation cost saving rate is 63%. At this time, the annual operation cost saved is 510 yuan; Under the partial load cooling storage mode, the optimal cooling storage capacity corresponding to z2=96% is 30MJ, and the utilization rate of phase change materials is 97%. However, it can be seen that at this time, the growth rate of Z is large, so the cooling storage capacity can be appropriately increased, and part of the cooling storage capacity can be transferred to the peak period. It can be seen that when the cooling storage capacity is 200mj, the air treatment satisfaction rate reaches 100%, the utilization rate of phase change materials is 71%, and the operation cost saving rate is 60%. Compared with the full load cooling storage mode, The material is saved by 20%, but the operation cost saving rate is basically not reduced. At this time, the capacity of the refrigerator required is also reduced compared with the full load cold storage mode. Therefore, from the perspective of economic analysis of operation cost, the partial cold storage mode is economical and reasonable

Figure 9 annual average Z index of different cold storage capacity (full load cold storage) Figure 10 annual average Z index of different cold storage capacity (partial load cold storage)

when the daily optimal cold storage capacity is 200mj, it can be seen from the curve in Figure 3 that the annual operation cost is saved by 485 yuan; According to the curve in Figure 4, when the payback period is 5A and 10a, the allowable critical additional initial investment is 2100 yuan and 4023 yuan respectively, that is, the cost of additional equipment after using phase change material fresh air unit can be 2100 yuan and 4023 yuan more than the sum of saved power capacity increase fee, refrigeration unit cost and surface cooler cost. The manufacturer can judge whether the investment should be made based on this value and the estimated cost data

taking the 3000m3/h unit in this paper as an example, according to the design conditions mentioned above, the heat transfer rate of the phase change energy storage heat exchanger is about 11kw. If the refrigerator cop=3 is taken, the refrigerating unit capacity and electric power installed capacity that can be reduced are 11 kw and 3.7 kW respectively. If the cost of the refrigeration unit is estimated at 850 yuan/kW, and the power capacity increase fee is calculated at 5700 yuan/kva according to the price in Beijing, while the saved surface cooler is estimated to be 6000 yuan, and the land occupation cost of the unit is set at 7500 yuan, then the cost of the phase change energy storage heat exchanger is allowed to be 28940 yuan from the perspective of initial investment. At this time, the initial investment is the same as that of the conventional system; If the payback period is 5a, the critical additional initial investment allowed from the perspective of operating cost is 2100 yuan, that is, the phase change energy storage fresh air unit can save 2100 yuan of present value than the conventional unit within 5A; Therefore, from the perspective of total investment, the total cost of phase change energy storage heat exchanger is allowed to be 31040 yuan. Through preliminary calculation, the pipeline and processing cost of the phase change energy storage heat exchanger is estimated to be 8500 yuan, and about 1500kg of phase change materials are required. It can be judged that the production should be invested only when the price of phase change materials is less than 15 yuan/kg

This paper introduces a method to determine the optimal daily cooling storage capacity and operation scheme of phase change material energy storage fresh air unit by comprehensively considering the utilization rate of phase change material, the satisfaction rate of air treatment and the saving rate of operation cost, and puts forward the concept of critical additional initial investment. At 3000 m3/h
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