Use of process integration techhology in greenhouse energy systems

摘要

This paper introduces the concept of process integration techniques, as developed by others, for application to commercial greenhouse systems. Process integration was originally developed by Linnhoff et al (1984) within the chemical process industryand subsequently successfully applied in many other sectors. The technique involves comparing energy streams in a qualitative and quantitative manner to produce a theoretical minimum energy input value, with options for assessing various changes to plant functionality for maximum energy savings. This has been extended over the years to permit economic factors to be built in to the whole. The system developed is designed to be operated as either a stand alone item for an audit purpose, or in the futureto be linked to the environmental computer such that an on-going analysis can be maintained showing actual performance against the theoretical minimum. The software described has been developed as a holistic one, considering all system inputs and outputs. The former inputs are primarily raw energy (gas, electricity, etc), but include secondary items such as fertiliser, water and packaging. System outputs are mainly in the form of product, packaging and waste materials. Energy streams are considered to be as one of three types -conventional Temperature Enthalpy (T-H) streams (eg, heating), quantitative input streams (eg, water), and energy consumption (process) -enthalpy - product quantity streams (eg, volumetric output). In addition to these complications, there are several external restraints, for example limitations on recycling of water due to pest and disease issues. Some elements, such as thermal buffer tanks, effectively take multiple roles depending on state. The software has been developed using Object Orientated Programming (OOP) techniques in C++ using Microsoft Foundation Class (MFC) for the Windows front end and ChartFX for graphing. Data exchange to environmental computers is at present via Excel sheets, but it is proposed that XML be adopted for this purpose. The programme outputs are in the form of a theoretical minimum energy input, the current energy input and the practically achievable one. Suggestions for improvements to energy efficiency are listed and ranked in order of energy saving potential, but with no linking to economic considerations as yet. These are via the output screen, with options for graphical representations in terms of data summaries and reports. Data from environmental computer systems in commercial blocks forboth edibles and ornamentals have been used to undertake initial evaluation of this approach, and results with these test data sets are given. The need for benchmarking and for objective assessment of energy efficiency has recently been highlighted within the UK by the need for the industry to demonstrate efficiency improvements in order to maintain the Climate Change Levy (CCL) reduction. This currently presents obvious difficulties in comparing year on year data without taking into account meteorological and cropping variations. The approach outlined permits such an objective assessment although is to a certain extent dependent on more subjective data in terms of yield response to different cropping regimes.