Loading...
Please wait, while we are loading the content...
Similar Documents
Overall Energy Analysis of ( Semi ) Closed Greenhouses
| Content Provider | Semantic Scholar |
|---|---|
| Author | Zwart, Harald |
| Copyright Year | 2008 |
| Abstract | Natural ventilation to discharge excess heat and vapour from the greenhouse environment has serious drawbacks. Pests and diseases find their way through the openings; carbon dioxide fertilisation becomes inefficient and the inescapable coupling of heat and vapour release results often in sub-optimal conditions for either temperature or humidity. The present trend, therefore, is to reduce ventilation as much as possible, also in Mediterranean conditions. This relies obviously on improved means for diminishing the heat load and proper use of cooling equipment. Especially the latter can combine the benefits of cooling the greenhouse air with serious energy conservation. However, opposite to the clear benefits there are also serious investments associated with active cooling of greenhouse. Therefore, there is a growing demand for some computational tool that enables quantitive comparisons between the vast number of alternatives with respect to the different components of (semi) closed greenhouse systems. The benefits in terms of improved production (quality, ornamental value and quantity) are quite difficult to quantify, due to the complexity of the biological processes involved. On the energy side of the balance, however, since the physics of greenhouses, climate controllers and horticultural hardware can be described very well, it is quite possible to develop such a tool for predicting the energy consumption of a (semi) closed greenhouse for a wide range of horticultural and outside climate conditions. This paper gives an outline of such a tool and discusses some results. Just as an illustration, a number of quantitative effects are shown of changing the fraction of closed greenhouse surface in a 1 hectare enterprise that consists of closed and non-closed compartments. This analysis is made for both a Dutch climate situation and a Mediterranean weather data set. INTRODUCTION Dutch horticulture aims to decrease the fossil energy input to horticulture. One of the promising means to do so is to transform the greenhouse in a so called “Energy producing greenhouse”. There are some innovative ideas of achieving this goal by adding PhotoVoltaic elements in the construction (Sonneveld, 2006), but most of the concepts that contribute to this goal are based on extracting heat from the greenhouse air or the cover (Opdam et al., 2004; Bot et al., 2004; Campen et al., 2001). The amount of surplus heat ready to harvest is large. Even in the northern latitudes, in non shadow screened greenhouses, at least 1500 MJ/(m2 yr) of surplus heat is carried off by opened windows during sunny days. Additionally, around 500 MJ/(m2 yr) is ventilated away on dull days, when greenhouse air is exchanged with outside air for dehumidification purposes. Due to the botanic ‘comfort zones’, the heat extraction, particularly when serving a dehumidification demand, takes place at a low air temperature level. In addition, because the costs associated with realizing low cooling water temperatures are high, the typical temperature differences between air and the cold heat extracting surface are limited to some 5 to 15°C. This means that, when aiming at serious heat extraction capacities of around 300 W/m2, the heat exchanging surface must be large and/or provided by a high heat exchange coefficient. It is obvious that the heat exchanging 811 Proc. IS on Greensys2007 Eds.:S. De Pascale et al. Acta Hort. 801, ISHS 2008 surface and the heat exchange coefficient are more or less interchangeable. Therefore, each design will need a survey to determine the optimal configuration. This optimum will not only be dependent on static parameters, determining the fixed costs of the one or the other heat exchanging device, but will also depend on energy prices since forced convection by ventilators is a powerful means of enhancing the heat exchange coefficient. Another article presented on GREENSYS 2007 (de Zwart, 2007) focuses in detail on all aspects having to do with characterizing heat exchangers. However, even after a selection procedure has found the heat exchanger that promises to provide the cheapest way of gathering heat out from surpluses of thermal heat (a greenhouse being too hot) or from surpluses of latent heat (too high a humidity), there is another, much more complicated optimization to perform. This second optimization has to balance all costs of cooling the greenhouse with the benefits. The costs come from the capital costs associated with the investments and from electricity for making cold water and driving pumps and ventilators. From these components, the determination of the costs of making cold water is the most complicated because, when heating a greenhouse with a heat pump in winter, a certain amount of cold water is produced as ‘waste product’. This means that, providing that the driving power for the heat pump is addressed as heating costs, at least part of the cooling water can be get for free. With respect to the benefits of cooling, the present work only rates the advantage of an elevated CO2-concentration on photosynthesis. This paper presents a brief outline of the optimisation instrument called the Synergy-Compass and the simulation tool that it uses. Then, as an illustration, this tool is used to show the effect of closing a certain fraction of a greenhouse area on energy consumption, canopy production and variable costs and benefits in Dutch and in Mediterranean weather conditions. BRIEF DESCRIPTION OF THE SYNERGY-COMPASS In Figure 2, the main input sheet of the Synergy-Compass is presented. On this sheet, amongst some administrative settings, three prominent sets of input data can be found, namely the greenhouse climate requirements, the building characteristics and the description of the devices that serve the climate control. These data comprise the main input for a simple but quite complete simulation model describing the greenhouse climate and heating and cooling demand on an hourly base. Besides, the simulation model describes the main energy conversion processes in the boiler house in order to translate a heating and cooling demand to primary energy sources like natural gas and electricity. The base of the simulation model is to compute an hourly stationary energy balance that satisfies the user defined temperature set point as a function of outside weather conditions and the actual heat loss coefficient (the u-value). This u-value depends on user defined thermal screen characteristics and control (see Fig. 2), but also on ventilation. Ventilation is partly uncontrollable (leakage), and partly controlled in order to carry off moisture from the greenhouse, having a user defined maximal humidity. On sunny days, however, the ventilation is predominantly a controlled air exchange rate in order to prevent too high a greenhouse temperature. Solar radiation absorbed by the canopy is split into a sensible heat flux and an evaporation rate (representing a latent heat flux). It is assumed that 40% of the absorbed radiation is turned into vapour but there is always a minimum evaporation rate of 5 Watt times the LAI. The remainder of the absorbed radiation is transformed to sensible latent heat, which means that during the night the canopy absorbs some 15 W sensible heat per m2 greenhouse area. By defining temperature excess driven ventilation as a last resort action, the insertion of a certain active cooling capacity will show the effects of greenhouse cooling on a diminished ventilation rate and the related increment of CO2-concentration (if CO2dosing is provided). The benefits of an increased CO2-concentration are computed in terms of an increased carbon fixation, computed by a standard photosynthesis algorithm (Gijzen, 1992). |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://library.wur.nl/WebQuery/wurpubs/fulltext/52992 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
