Abstract
Multi-span greenhouses consume huge amounts of energy for heating in northern China, resulting in reduced profitability and unsustainability. In order to find a solution to this problem, a greenhouse heating system was designed that utilizes energy transfer between greenhouses based on a dual-source heat pump . The system collects excess air heat inside Chinese solar greenhouses (CSGs) to heat multi-span greenhouses. By enabling greenhouse energy transfer in time and space, better utilization efficiency of excess air heat in CSGs can be achieved, leading to an overall reduction in heating costs. This study defines the heating approach and describes the overall system design. The dual-source heat pump acts as the core component with two separate evaporators placed in the CSG and ambient air. Then, calculations for system sizing are presented, including the heating load model of multi-span greenhouses, the excess air heat model of CSGs, the selection of the required equipment (dual-source heat pump, heat storage tank and combined surface air cooler). air conditioning unit) and space matching. Finally, a case study illustrates the implementation processes of the heating system. The available CSG excess air temperature was in the range of 100.8-112.6 W m -2 for system sizing and the minimum area of CSGs was recommended to be twice the area of the multi-span greenhouse. The pilot test showed that the working condition and heating effect of the system were stable. The coefficient of performance (COP) of the heat pump reached 4.3-4.8 when using CSG excess air heat as the heat source, and was 23-26% higher when using ambient air during the same periods. During the entire heat collection process, dual-source heat pumps that switch sources according to their settings achieved an overall COP of 3.4-4.2, a 6-11% increase compared to air-source heat pumps. This study presents a new heating approach and energy-saving system for multi-span greenhouses.
Introduction
The greenhouse horticulture industry across China has adopted the expansion of large-scale greenhouses as a market development strategy. One such representative is the multi-span greenhouse, which exhibits high land use efficiency, strong climate control ability and a high level of mechanized operation. These features make multi-span greenhouses suitable for scale and commercial production. But multi-span greenhouses consume huge amounts of energy for heating, especially during the cold season in northern China. The requirement for these high energy inputs is inefficient, leading to reduced profitability and unsustainability. The energy required to heat a greenhouse accounts for 30-70% of the total greenhouse production cost, depending on different latitudes [1]. According to our research and greenhouse management practices, the annual heating cost of a multi-span greenhouse in Beijing is US$6.3-12.5 m-2, accounting for more than 40% of the total operating cost. In milder climates where multi-span greenhouses are well developed, for example in the Netherlands, greenhouse heating is also an important energy consumer. The annual consumption of natural gas, the primary heat source for greenhouse heating, varies between 25 and 40 m3 m -2 with an average cost of 7.8 USD m -2 [2], [3].
An overview of different energy saving techniques to reduce greenhouse heating costs is provided by Ahamed et al. [4]. These measures that reduce heating demand or direct energy input mainly include design optimization of the greenhouse and its structural components [5], [6], improving greenhouse climate management [7], [8], [9], [10]. and the use of renewable energy or energy efficient technologies to reduce fossil fuel or heating power consumption [11]. At the initial development stage of the multi-span greenhouse industry in China, one of the most effective solutions to the greenhouse heating problem is to establish an energy-saving, cost-effective and stable heating approach and develop the necessary supporting equipment.
Heat pump technology, which is accepted as high energy conversion efficiency, low operating cost, environmentally friendly, is successfully used in greenhouse heating. [12], [13], [14], [15], [16], [ 17], [18], [19]. Moreover, it will play an increasingly important role as it follows stricter requirements to reduce CO2 emissions [2]. For example, the heating cost of a heat pump with a COP of 3.5 is slightly higher than that of a coal-fired boiler, but still well below that of a natural gas boiler and electric heating [20]. Meanwhile, the heat pump has the lowest primary energy consumption and CO2 emissions [20]. Also compared to solar thermal systems [21], [22], heat pumps have more stable heating performances, which is very important for crop safety production. In recent years, air source heat pumps [23], the most widely used types of heat pumps, have been increasingly favored for heating more and more multi-span greenhouses in China because they are relatively cheap and stable and easy to install and maintain. . However, reduced heating capacity in cold weather and low COP [24] remain challenges for the efficient use of air source heat pumps. Since heat source quality largely determines COPs [13], [24], [25], energy-efficient greenhouse heating with air source heat pumps makes a strong case for fully utilizing high-grade resources in cold regions. From a practical point of view, this is not a big deal.
In addition to growing crops, greenhouses also harvest energy. For example, the annual cumulative excess energy inside an ideal indoor greenhouse reaches 164 kW h m -2 [26]. Recovering excess heat energy from a greenhouse during the day and providing it for heating the greenhouse itself at night has proven to be a solution for improving the night thermal environment [27], increasing crop yields [28] and saving energy [29]. However, during the coldest months in cold regions or high latitudes, the excess daytime heat inside commercial greenhouses (below 0.5 MJ m -2 day -1 on most days) contributes little to the greenhouse heating requirement [29], while the energy surplus occurs mostly during the supplementary lighting phase or during the warm seasons [30]. Such excessive heat generated by artificial lights is unsustainable and can be reduced by switching from high pressure sodium (HPS) to light emitting diode (LED) lighting [31]. In addition, seasonal thermal energy storage is not a preferred option due to the high investment cost [32]. In principle, it is not advisable to extract excess energy from multi-span greenhouses for heating in cold areas, especially within the daily heat storage-release model.
The energy-saving and low-cost Chinese solar greenhouse (CSG) is the primary choice for overwintering cultivation in northern China. The total area of CSGs exceeded 570,000 hm2 by the end of 2018, accounting for about 30.5% of the total greenhouse area in China [33] . Due to its unique north wall, CSG has good thermal insulation and solar radiation blocking performance. As a result, the indoor air temperature can easily reach and exceed 35°C at noon even in winter, creating an abundant surplus of air heat. In practice, roof ventilation cools the CSG and protects crops from high temperature stress. But at the same time, a lot of energy is wasted in the process. Researchers have developed many active and passive heating systems to utilize excess air heat energy or both excess solar radiation and air heat inside the CSGs. Active solar heat storage-release systems, which transfer and store energy through water circulation, are the most studied [34], [35], [36], [37], [38]. They focused on improving the heat collection efficiency of indoor collectors by considering the heat dissipation capacity. Other common applications using CSG excess energy include ventilated walls [39], [40]roof skeleton netting, water flowing [41] and phase change material (PCM) [42], [43] and heat conduction components [42] containing walls. 44]. Although these systems are low-cost, energy-saving and benefit the greenhouse climate and crop growth, they have a common technical problem; the effective heat collection time and total heating capacity are limited by the heat transfer and the increase in the temperature of the storage medium. varying degrees. This is because their heat capture depends on or is affected by convection processes with the indoor air. This problem leads to a low efficiency of utilization of CSG excess energy.
Compared to the ambient air source, the excess air heat inside the CSG is higher-grade heat energy and is especially valuable during the cold winter months. This excess energy can be collected and used as a low-temperature heat source for a heat pump to improve COPs. Furthermore, the forced draft of the heat pump will efficiently create a positive temperature difference between the heat storage medium and the indoor air. In order to solve the limitations of the above CSG energy utilization systems in terms of heating capacity and energy use efficiency, unlike complex heat pump systems that indirectly extract greenhouse energy [29], [45], Sun et al. [46] developed a heat pump system with a single source of excess air heat for CSG heating. The upgraded system enabled the heat pump to improve the heating performance, resulting in an overall COP of the heating system reaching 2.7. The results show that the direct collection of excess air heat from the CSG by the heat pump during the daytime to heat the CSG itself at night is effective for greenhouse production.
However, previous studies to improve the utilization efficiency of excess energy in CSGs focused on how to absorb more heat from the energy source under predefined constraints and took into account the heat release performance of the heating system, but did not consider regulation at all. energy sinks that can efficiently digest the heat generated. Imbalances between heat supply and demand can occur due to the thermal energy storage of CSG structures and its location in the ground, especially in well-insulated CSGs. On sunny days when an excess of heat energy is available, the need for CSG heating for nighttime supplementation is reduced or even eliminated. In this case, the stored energy cannot be consumed sufficiently that night [37], which negatively affects the heat collection during the day. The low heat requirement of CSGs can also be confirmed by the fact that CSG can produce vegetables and fruits in Northern China (32-43° K) mostly without additional heating [47]. As a result, based on existing systems, the heating approach that captures excess indoor energy during the day and releases this heat at night to heat the CSG has low efficiency in energy use, given the energy supply chain. Compared to CSGs, multi-span greenhouses have a higher heating load [48]; the daily heating cost per unit area of the multi-span greenhouse has been tested to be roughly 3.6 times that of the CSG over the entire heating period [17]. Moreover, once we enter the heating season, heating needs are hardly affected by external weather conditions. Therefore, collecting CSG excess heat on sunny days to provide heating to multi-span greenhouses can improve overall CSG energy use efficiency and reduce heating costs of multi-span greenhouses using alternative energy sources. Therefore, in this study we present a new heating approach that utilizes energy transfer between greenhouses. However, there is no supporting system in place to implement this heating approach.
The project aims to develop a greenhouse heating system that uses energy transfer between greenhouses based on a dual-source heat pump (ETGHP) to solve problems and exploit opportunities. The system collects excess air heat inside the CSGs for heating multi-span greenhouses. With the efficient use of the air source heat pump, it is expected to ensure the transfer of greenhouse energy in time and space, increasing the utilization and efficiency of CSG excess air heat. Achieving this will reduce the energy consumption required for heating multi-span greenhouses and promote the sustainable development of large-scale greenhouses represented by multi-span greenhouses.
The development of greenhouse energy utilization systems in previous studies has devoted more effort to overall system description and performance evaluation, and few have included detailed design of the basic equipment, system sizing and implementation. At the same time, design calculations for heat pump systems mainly focus on the internal configuration [49], [50]. There is no reliable heating load model applied to the selection of air source heat pumps for greenhouse cultivation. Firstly, there is a research gap in the framework in which the heating approach using energy transfer between greenhouses can be pursued, especially the design implemented by the ETGHP system. Second, the basic component of the system, the dual-source heat pump, is different from the most commonly used solar/air [51], [52] and ground/air [53], [54] techniques. It needs a special design for stable and energy-efficient heating purposes. Third, studying system implementation benefits knowledge discovery related to engineering practice and the heating approach. Therefore, this paper focuses on the design and implementation of the ETGHP system. Its main contributions and innovations are as follows:
- (1)Proposal of a heating approach utilizing inter-greenhouse energy transfer and development of an ETGHP system to implement this approach.
- (2)Designing a dual-source heat pump with two separate evaporators placed in CSG and ambient air respectively, realizing three working conditions, achieving energy saving and stable heating.
- (3)Establishment of the systematic sizing method for engineering design of ETGHP system including heating load model of multi-span greenhouses, excess air heat model of CSGs and equipment selection is improved from the theoretical point of view.
- (4) To examine the implementation processes and pilot test the system; as a result, we answer the following research questions
- -How much excess air heat is available in CSGs?
- How large can the area of the heat source CSGs match the area of the multi-span greenhouse to be heated?
- -To what extent can heat pump COPs be improved by using CSG air source?
The rest of this paper is as follows: Section 2 describes the heating approach, followed by the overall system design. Section 3 presents the heating load model, excess air heat model and calculations for equipment selection. In chapter 4 we illustrate the system implementation process with a case study. A pilot test was also carried out to analyze the operating status and heating effect of the system as well as the performance of the heat pumps. The discussions continue throughout this chapter and conclude with an explanation of perspective and further work. Results Section 5. This paper can provide theoretical support and can be used as a case reference for the design and implementation of the ETGHP system.
Approach proposal
The heating approach using inter-greenhouse energy transfer is defined as the collection of excess heat energy from one or more greenhouses to heat one or more other greenhouses. The introduction of this heating approach has several objectives; by transferring greenhouse energy in time and space, the energy use efficiency of excess greenhouse heat energy can be increased and the overall energy consumption for heating can be reduced. The two types of greenhouses involved could be:
Calculation
The calculation procedures for ETGHP system sizing are shown in Figure 3. These calculations are performed at the system and component level. The configuration of the components themselves (e.g. sizing the evaporators and condensers of the heat pump) is therefore not within the scope of the research, as this is usually the responsibility of the manufacturer. In addition, the dimensioning of water pumps, circulating water pipes and ventilation ducts can refer to the technical materials in the Heating Ventilation Air Conditioning section.
System implementation
According to the system design framework shown in Section 2 and Section 3, the ETGHP system was constructed in Shouguang City (36° 54′ N, 118° 51′ E), Shandong Province, China. It took air heat from more than six CSGs to heat a multi-span greenhouse, a CSG for seedling production and an equipment room (Figure 7). The system consisted of dual-source heat pump units, heat storage tank, fans and surface air coolers, water pumps, ventilation ducts, circulating water pipes and a control system.
Results
This study proposes a heating approach that utilizes energy transfer between greenhouses. The ETGHP system was designed and built to implement this approach. The system enables the transfer of greenhouse energy in time and space by collecting excess air heat inside the CSGs to heat the multi-span greenhouse. A pilot study showed that the operating states and heating effects of the system are stable.
References (79)
- MS Ahamed et al.Energy saving techniques to reduce the heating cost of conventional greenhousesBiosystEng(2019)
- MJ Gupta et al.Impact of greenhouse design parameters on energy savings for greenhouse environmental controlEnergy(2002)
- A. Vadiee et al.Energy management strategies for commercial greenhouses Application Energy (2014)
- N. Sigrimis et al.Energy savings in greenhouses using temperature integration: a simulation studyComputingElectron Agriculture (2000)
- PJM Van Beveren et al.Minimum heating and cooling in a modern rose greenhouseApplicationEnergy (2015)
- PJM van Beveren et al.Optimal control of greenhouse climate using minimum energy and grower-set limitsApplicationEnergy(2015)
- PJM van Beveren et al.Optimal use of boilers, combined heat and power installations and heat buffers in horticultural greenhousesCalculationElectron Agriculture(2019)
- E. Cuce et al.Renewable and sustainable energy saving strategies for greenhouse systems: A comprehensive reviewRenew SustainableEnergy Rev (2016)
- Greenhouse heating using heat pumps with high coefficient of performance (COP)BiosystEng (2010)
- L. Aye et al.Evaluation of heat pump system for greenhouse heatingIntJ Therm Sci(2010)
