I. Coal-fired hot water boiler
The fuel used in coal-fired hot water boilers is coal, and the heat transfer medium is heat medium water. Coal has low calorific value and low price. Boilers are used for heating, and hot water pipes can be arbitrarily arranged in the greenhouse, which is closely related to the crop requirements of the greenhouse. The denser the pipeline layout, the more uniform the temperature field distribution in the greenhouse and the smaller the temperature fluctuation. On the contrary, the distribution of temperature field is uneven and the temperature fluctuates greatly. In addition, due to the large specific heat capacity of water, the disturbance to the greenhouse is small, which makes the temperature fluctuation in the greenhouse relatively small.
The thermal efficiency of coal-fired boilers generally increases with the increase of boiler capacity. Therefore, different greenhouse scale and different boiler capacity also affect the efficiency of heating system. By analyzing the heat consumption of a modern glass greenhouse in Shanghai, it is found that two hot water boilers with thermal power of 2.8MW are used in its 3-hectare greenhouse, and the actual thermal efficiency is 68%. The heat required for greenhouse operation in six months in winter is 2.02x 10 10KJ, and the annual coal consumption is 1668 tons. The coal consumption per unit area of system operation is 55.6 kg/m2, and the cost is 15.57 yuan /m2. If the greenhouse area is only 0.5 hectares, according to the simulation calculation of greenhouse heat load, a boiler of 0.35-0.7 MW can be selected, and the thermal efficiency of the boiler will be reduced by 4%. The coal consumption per unit area of the system is 59. 1 kg/m2 and the cost is 16.57 yuan /m2. Therefore, due to the reduction of boiler thermal efficiency, the coal consumption per square meter increased by 3.5 kg, and the cost increased by 0.98 yuan. The smaller the greenhouse area, the smaller the boiler capacity. When the boiler capacity is lower than 0.35MW, the coal consumption per unit area of greenhouse will reach 66.4 kg/m2, and the cost will be 18.59 yuan /m2. Therefore, more attention should be paid to the change of thermal efficiency of heating equipment when considering the heating scheme of smaller greenhouse.
Second, the hot blast stove
Hot stoves in greenhouses are divided into coal-fired stoves, oil-fired stoves and gas-fired stoves according to fuel types. Compared with the hot water boiler, the hot air stove has no heat capacity loss of heat medium when it is started, so the heat capacity of the hot air stove can be used for indoor heating in greenhouse. In addition, because the hot air stove does not need to heat the heat medium first during the start-up, the greenhouse heating lag is short and the greenhouse temperature rises quickly. When the indoor and outdoor temperature difference is small, or the outdoor solar radiation intensity is sufficient during the day, the hot blast stove can be turned on and off at any time to adjust the indoor temperature environment, which increases the heating flexibility of the greenhouse. Due to direct heating, the uniformity of temperature distribution in greenhouse is slightly worse, and the temperature fluctuates greatly.
The air supply temperature of hot blast stove is usually 40℃- 120℃. If the air supply temperature is too high, the hot jet will naturally rise, which will make the lower part of the greenhouse not heated well. The air supply temperature is too low, and the enthalpy of the outlet of the hot blast stove is small, so it is necessary to arrange more hot blast stoves or increase the air supply volume of the hot blast stove. Therefore, the choice of outlet air temperature of hot blast stove has great influence on the temperature distribution in greenhouse. Like hot water boiler, coal-fired hot blast stove has poor regulation performance, but the thermal inertia of greenhouse heating is small. Simple start-stop control can ensure that the inside of the greenhouse will not overheat.
Third, the electric greenhouse
Compared with other forms of energy, electric energy needs secondary energy conversion from heat to electricity and then to heat, with the highest energy level and higher price. In thermoelectric conversion, the efficiency of thermal power generation is only about 30%. Therefore, although the conversion efficiency of electric heating is high, the energy utilization efficiency of electric heating is very low in terms of overall energy utilization. It is uneconomical and unreasonable to use electric heating in greenhouses where vegetables or fruits are grown. But for seedling greenhouse, soil temperature is related to the germination rate of seeds and the normal growth of seedlings, which is the main object of greenhouse environmental control. Practice has proved that it is feasible to heat the soil locally by electric heating.
Four, electric heat pump greenhouse
The so-called heat pump is the general name of the equipment that transports heat from the low temperature area to the high temperature area. The principle of thermal cycle of heat pump and refrigerator is basically the same, but their uses and functions are just the opposite. Electric heat pump greenhouse is a device that uses electricity as power to transport heat from a relatively low temperature environment to a relatively high temperature greenhouse. During the daytime, the ambient temperature is high, and part of solar radiation can be converted into heat to heat the greenhouse, so that the coal consumption is reduced compared with other times. Therefore, most of the heating required by the greenhouse is provided at night. It is assumed that the greenhouse heating accounts for 40% of the total heating during the day and 60% at night. Considering the peak-valley value of power system cost, electricity bill can be saved. If the electricity charge at night is half of the daytime, and the industrial electricity charge at the bee valley price is 1/2 during the daytime, then the electricity consumption of the electric heat pump greenhouse is reduced by half compared with the simple electric heating, cheaper than the oil-fired gas boiler, and only 64.7% more than the coal. If the electricity charge at night is 1/3 in the daytime, then the electric heat pump is 42.50000000014 in the daytime, then the charge of the electric heat pump is 30.9% higher than that of coal burning. Combined with the pollution-free characteristics of electric energy, we can see the application potential of electrically driven heat pump in greenhouse heating.