Gas turbine exhaust gases as a source of heat and carbon dioxide for the production of greenhouse tomatoes

Abstract

The long held belief that hydrocarbon energy sources are limitless is rapidly being disproven. While non-renewable resources such as gas, oil, and coal become limiting, the fear of an energy shortage is becoming very real. This energy scare is resulting in campaigns to make the public more concerned with how remaining sources of energy are used.

An examination of the major industrial users of hydrocarbon fuels reveals that tremendous amounts of energy in the form of heat are released into the environment every day. This heat is released in the gaseous state or as a heated effluent. In many cases, the high quality of this heat would allow it to be used for beneficial purposes.

The concept of using waste heat is not new. Many attempts have been made in the past or are presently being undertaken to put waste energy to work. In many cases, these attempts have not been successful due to technical, economic, or political problems. Dwindling energy supplies and a conservation-minded public mean that greater efforts must be made to solve these problems.

There are two basic reasons for undertaking projects which make use of waste heat sources. The first is economic and the second is ecological. The economic reasons become obvious when the rapidly rising cost of conventional energy sources is considered. The ecological reasons are based on concern for the environment when large quantities of thermal waste are released into it. Besides these basic reasons, many would argue there is a social or moral obligation to make more efficient use of natural resources than is presently the case.

Often steam-electrical generating stations or other industrial plants release large quantities of heated water into lakes or rivers. This can have disastrous effects on ecological relationships. Thus, projects may have as their main concern the dissipation of heat in a way that is not harmful to the environment. Usually the most economic way of dissipating this heat in an ecologically sound manner"is to find a practical use for it. Systems must be designed to utilize the waste heat to economically improve the conversion efficiency of the fuels burned and at the same time reduce environmental risks.

The major problem in utilizing waste heat from the cooling water of steam-electrical generators is to develop efficient transfer systems that remove the heat. Often the heat is of low quality so the design of the transfer system is critical. Some designs use closed-system heat exchangers and others make use of contact-type heat exchangers.

Although the heat released as part of gaseous exhausts is one-third of that released as heated water, it still has great potential as an alternative energy source. Exhaust gases also present an ecological concern because many of the by-products of hydrocarbon fuel combustion contain pollutants that are harmful to the atmosphere. Systems that utilize the heat of the exhaust, as well as removing harmful components, would serve a dual purpose.

When the hot exhaust gases of hydrocarbon combustion are considered, the temperature of the gases released is often much higher than is the case with heated water, but the noxious components of the exhaust present problems. Heat exchangers that are part of a closed system and prevent the entry of noxious materials must be used or else the noxious component must be removed from the exhaust to allow its direct use. If the latter method is possible, a benefit other than heating could be realized. The exhaust from hydrocarbon fuels contains large quantities of carbon dioxide which could be used to enhance the growth of crops located in protected environments.

In order to test this concept, experiments were conducted from August 1974 to January 1976 by the Departments of Horticulture Science and Mechanical Engineering to study the feasibility of using the exhaust from a natural gas-fired turbine for greenhouse heating and carbon dioxide enrichment. The merits of the system were evaluated using the tomato as a test plant.