Renewable energy in agro-industrial complex: Requirements to the wind power systems for heat production, storage and supply of green houses

The autonomous power supply systems of greenhouse farm must correspond to the following requirements (fig. 4.36):

 

·        round-the-clock supply of greenhouse farm by the necessary energy resources with the part of renewable energy for middle latitudes on the level of 70%, which reduce external energy supply, continue the season of plants growing, raise yields, and expand the assortment of plants;

·        thermal power supply to heat the water, soil and air (specific thermal power is ~ 2 MW per one hectare of closed ground);

·          electrical energy supply to power the pumps, compressors, transporters and especially, system for optimal illumination of plants within 6000...20000 lx (0.1 kW/m2 of useful area) during the year in places with colder climates. Spectral range of optical radiations favorable for photosynthesis is 0,4…0.7 µm. Specific electrical power is ~ 2 MW per one hectare of closed ground; 

·        possibility of parallel operation with other thermal energy sources: water boiler, thermal generators and cogeneration systems;

·        accounting of external climate indicators: air temperature, light, humidity, wind speed and direction, the presence and intensity of rain, the cloudiness  and Sun position;

·        accounting of the internal green house parameters: temperature and humidity of soil and plants in different parts of the building, and concentration of carbon dioxide.

 

Rational choice for autonomous energy supply of greenhouse farm can be a wind system shown in fig. 4.37. Its basic scheme includes the following components: wind turbine; thermal accumulator; electrical heater, and heat exchanger. Wind power system can work in two modes: production of electricity in real time scale, and production and accumulation of thermal energy. In the presence of the wind the turbine produces electricity that is converted by the electric heater into thermal energy and heats the coolant in the accumulator. At the same time, electricity is used for electrical illumination of plants, and power technological processes and instrumentation.

In the absence of wind, the system delivers heat energy accumulated in thermal storage container to the greenhouse providing all-weather growing of vegetables and other plants in a closed soil. 

Wind power system (WPS) is a wind turbine with a horizontal axis of rotation, which includes a rotor with blades, transmission, and generator, established and fixed on the tower.

In most cases the thermal accumulator is insulated container with a double-layer construction, made in the form of a Dewar flask (vessel) providing good thermal insulation. It can also consist of several separated reservoirs. Each of these tanks is designed for the storage of different temperature coolant. A well-maintained heat exchange between the coolants in different reservoirs is executed with the aid of pipelines. Heating of coolant in reservoirs is done cyclically, according to the growth of solar activity. Heat stored in tanks in the future is used for heating and hot water supply of green houses. The thermal accumulator can be performed in the overland (ground) and underground tank location variants. The choice of the design is conditioned by the specific tasks that are solved during creation of a thermal system.

Working fluid is characterized by high temperature of boiling, high heat capacity and/or large enthalpy of phase transition. In a simplified case the water can be used as a working coolant. At more precise approach the solutions of salts are employed as the coolant. They have higher boiling point and larger specific heat capacity. Specific heat capacity is the ratio of heat that is absorbed by mass unity of the body (substance) at endlessly small change in temperature, to the temperature change value [J/(kg × K)]. The amount of heat absorbed by a body depends on the heating method and its temperature. Heat capacity may be measured at constant volume and at constant pressure and temperature. At constant pressure a part of heat is used to perform a work for extension of the body. Other part of heat is employed to increase its internal energy, whereas at constant volume all the heat is spent only to increase internal energy.

Wind energy system of heat supply functions as follows. System orientation determines the direction of the wind and performs the correction of angular position for turbine’s axis in order to maximize airflow through the wind turbine. The electrodynamic system converts the kinetic energy of the wind turbine into electrical energy. Electrical energy is then transported to the electric heater that transforms it into the Joule’s heat and thus heats the coolant in the water thermal storage unit. Accumulation of heat is going to the moment until the coolant temperature in the tanks reach the critical level. From this moment, wind power system produces electricity for green house needs, or to electrical network.

Construction principles of the basic scheme can be applied to create wind power systems for different powers. In combination with other energy technologies, especially based on the combustion of fossil fuel, wind system can effectively provide heat to residential buildings, communal infrastructure of settlements, as well as to power technological processes of agro-industrial objects. Such systems, in particular, can be used for heating and conditioning of cattle-breeding farms, granaries and factories of primary processing of agricultural products. Depending on the purpose and location the wind systems for heat supply have various constructive solutions and are characterized by different power parameters ad characteristics.

 

 

By Vasil Sidorov on October 15, 2012

Technopark QUELTA, Queltanews from

Nizhyn Laboratories of Scanning Devices

sidorovvasil@gmail.com

 


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