WO1998038849A1

WO1998038849A1 - Integrated system for energy supply and energy use in greenhouse horticulture

Info

Publication number: WO1998038849A1
Application number: PCT/NL1998/000130
Authority: WO
Inventors: Dirk Hendrik Recter; Marco Robin Hessem; Anthony Van Waveren; Antonius Johannes Maria Van Wingerden
Original assignee: Nv Tot Gasvoorziening Gelders Rivierengebied Mij
Current assignee: Nv Tot Gasvoorziening Gelders Rivierengebied Mij
Priority date: 1997-03-06
Filing date: 1998-03-05
Publication date: 1998-09-11
Anticipated expiration: 1999-09-06
Also published as: EP0969713A1; NL1005459C2

Abstract

The invention relates to an integrated system for energy supply and energy storage in glasshouse horticulture, at least including a greenhouse, equipped with a heating system, a gas-fired heat pump and a system for heat storage in the soil, whereby during a period of heat excess, heat can be stored in the soil, while in a period of heat demand, using the heat pump, heat is extracted at least partly from the soil.

Description

Title: Integrated system for energy supply and energy use in greenhouse horticulture.

This invention relates to an integrated system for energy supply and energy use in greenhouse horticulture, whereby through a combination of a number of systems for energy supply and energy use, in combination with C0₂-fertilization of the greenhouse horticulture area, an optimum use of energy sources, more particularly natural gas, is obtained. It is well known that in glasshouse horticulture (greenhouses) , a variable energy demand exists, depending on the outside temperature. At low outside temperatures, there is a necessity for heating, while at high outside temperatures, an excess of heat may exist. Further, for a number of crops, it is desired to accelerate the growth thereof during dark periods (night time; winter) using assimilation illumination. In the current situation, mostly -use is made of natural gas-fired boilers, in combination with a heat buffer for a short period, for instance one day.

Another aspect of glasshouse horticulture that is related to energy use is the C0₂-fertilization of the greenhouse. It is conducive to the growth of the crop that sufficient C0₂ is supplied to the greenhouse. For that purpose, a number of systems are known, including the supply of pure C0₂ (from an external storage) . It is also known to generate C0₂ by combustion of natural gas or by employing gas engine off- gases for this purpose, provided they are sufficiently pure or sufficiently purified.

The need for C0₂ in the greenhouse is normally in opposite phase to the heat demand in the greenhouse, both viewed over a period of a day and viewed over the seasons . In general, it can be stated that the following energy-related products are relevant to a greenhouse:

1) heat for the low-temperature heat network

2) heat for the high-temperature heat network

3) power for drying out 4) cold for cooling (cultivation, cold store)

5) electricity (illumination, cooling)

6) C0₂ for fertilization

7) steam (for sterilization) It is a first object of the invention to provide an energy management system for a greenhouse, wherein through a balanced combination of measures these products can be furnished with minimal energy consumption.

It is a second object of the invention to provide a system for integrated energy supply and energy storage for greenhouse horticulture, wherein through an optimum combination of measures a clear energy saving can be realized.

It is a third object of the invention to provide an integrated system of the above-described nature, in which the investment costs are minimal .

Further objects of the invention .will become apparent from the following description of the invention and the figures. The invention is based on the surprising insight that the use of a gas-fired heat pump, in combination with an optimized heat storage in the soil, yields a surprisingly large saving of energy over the conventional situation.

The invention accordingly comprises an integrated system for energy supply and energy storage in greenhouse horticulture, at least comprising a greenhouse comprising a heating system, a gas-fired heat pump and a system for heat storage in the soil, wherein during a period of heat excess, heat can be stored in the soil, while in a period of heat demand, using the heat pump, heat is at least partly extracted from the soil.

According to the invention, a gas- fired heat pump is used. The task of this heat pump in the system of the invention is twofold. Firstly, the task of the heat pump is to supply sufficient heat to the greenhouse in periods of heat deficiency, at a sufficient temperature level. To that end, heat of a low temperature is extracted by the heat pump from the heat storage and upgraded to a higher temperature level, for instance for heating.

Secondly, the task of the heat pump is to make a contribution to the storage of heat in the soil, for instance heat coming from solar collectors, ambient heat or heat coming from the C0₂ production.

Surprisingly, it has been found that by the use of a gas- fired heat pump combined with long-term heat storage, a synergistic effect can be achieved, specifically in the production of C0₂ by burning gas and long-term heat storage in one season and the possibility of utilizing this stored heat again in another season. The use of the heat pump guarantees a very high energy efficiency of this process. In a preferred embodiment, the gas-fired C0₂ production and heat storage and the energy consumption occur spread over several seasons without significant energy loss.

Suitable types of heat pumps for use with the invention utilize, for instance, the absorbent lithium bromide. Such heat pumps are commercially available. The energy for driving the heat pump can be supplied by a built-in (natural gas) boiler, but it is also possible to derive this energy, for instance in the form of hot water or steam, in whole or in part, from elsewhere in the system. Obviously, any residual heat and/or C0₂ coming from the drive of the heat pump can be usefully employed again elsewhere in the system.

The long-term storage in the soil preferably occurs thermoneutrally, which means that in a year on average as much heat is stored in the soil as is extracted from it . The temperature level at which the heat is stored is of importance, to the extent that it is preferred to store the heat at a comparatively high level, since this has a highly positive effect on the efficiency of the heat pump.

More specifically, it is preferred that the temperature of the soil is, on average, at least 5°C above the normal soil temperature. The advantage of higher supply temperatures for the heat pump is that it can work with a higher efficiency. At higher average soil temperatures, it is possible to save more energy. In general, it is preferred that at the end of winter, that is, when substantially all of the previously stored heat has been extracted from the soil, the average soil temperature is back to the normal level again. During heat storage in the summer, the average temperature of the soil then rises again, to reach a maximum value when the maximum amount of energy is stored. In the technology concept presented, this long-term heat storage plays an essential role. The fact is that using the heat storage in combination with the gas-fired heat pump and optionally a heat collector, a large amount of energy can be saved. Practically speaking, there exist two forms of heat storage in the soil, viz. soil heat exchangers and aquifer.

A soil heat exchanger consists of,, a closed system of pipes through which water is cooled or heated by exchange with the soil . Limiting factor in this system is the heat exchanging surface. If, owing to a too small heat exchanging surface of the soil heat exchanger, a steep temperature gradient develops around the soil heat exchanger, the system works suboptimally. This holds both for supply and extraction of heat. When too much power is being demanded, freezing phenomena can arise in the soil.

An aquifer is a porous soil stratum (for instance sand) , confined between two non-porous soil strata (for instance clay or rock) . At different points of the aquifer, one or more hot and one or more cold wells are dug. When hot water is supplied to a hot well, at the same time cold water is extracted from a cold well vice versa . An advantage of the aquifer over a soil heat exchanger is that fewer pipes are necessary and the aquifer construction can be made better accessible for maintenance. An aquifer can function at a variety of temperatures. These temperatures can vary from approximately 0°C to about 120°C.

Employing soil heat exchangers on a MW scale sometimes requires disproportionally large investments. It is therefore preferred to make use of an aquifer construction. The eventual form of the long-term heat storage, however, is highly dependent on the local soil conditions. In designing a plant according to the invention, therefore, an inventory of the soil conditions and optionally a soil analysis should be made. If no natural aquifer is present in the soil, the artificial construction of an aquifer is not directly preferred in view of the large amounts of costly earth-moving work. With this energy management system according to the invention, a surprising saving can be obtained by cleverly using heat which is released as a by-j?roduct of electricity and/or C0₂ production; heat from climatic conditions, such as solar heat and ambient heat; heat from secondary sources, such as surface water. Also eligible for utilization in the system is heat coming from other sources, for instance generated by cooling units and the like.

The growth of many crops in greenhouses can be positively influenced by the supply of additional C0₂. Specifically in periods with much sunlight, a plant has much need for C0₂. By additional C0₂ supply, the production per hectare can be considerably increased.

Additional C0₂ fertilization up to a level of 750 ppm or more yields considerable improvements in the production of the greenhouse, as appears from the following table. The gain is expressed relative to non-additional dosage.

Production in kg/πf and production gain at different minimal C0₂ concentrations .

Savings can be made on the additional burning for C0₂ production by making use of a heat buffer. By a heat buffer, a hot water storage is meant. These buffers are suitable for heat storage for a few days. They are not suited, however, for storage over a longer period. In fact, by making use of such a heat buffer, firing can be better adjusted to follow CO₂, so that less additional gas is necessary. From the literature it is known that for cucumbers and tomatoes a heat buffer of 80 to 100 m³/ha yields significant savings. Larger buffers yield little additional gain.

In a system in which a C0₂ concentration of at least

750 ppm is utilized and a (short-term) heat buffer is used, about 9 m³ of additional gas per square meter per year is burned for the C0₂ fertilization. For a business of, for instance, 10 hectares, this means 900,000 m³ a year.

A high efficiency boiler extracts 90%, at higher value, of heat from gas. 9 m³/m²/year therefore means an amount of heat of 305 MJ/m²/year. This heat becomes available specifically at times when there is much sunlight, which implies that at that time energy can also be generated by solar collectors, if any are present, while, furthermore, there will be an outside temperature that is suitable for recovering ambient heat . It is precisely when a heat pump for storing heat in the soil is used, that this concurrence of conditions provides advantages, since the heat pump then has the most effect . By storing additional heat in the soil, greater savings can be achieved. Using the heat pump, this additional energy can be recovered. Through the heat storage in the soil, the efficiency of the gas-fired heat pump will increase further. When the supply temperature remains greater than 10 °C, an efficiency (c.o.p.) of about 1.8 can be achieved with a lithium bromide gas- fired heat pump.

When the procedure to keep the soil temperature above 10 °C is thermoneutral , a c.o.p. of about 1.8 will not simply suffice to meet the entire heat demand. If a heat equivalent of 9 m³/m² is stored, no heat equivalent of, for instance, 15 m³/m² can be extracted from the soil without the soil temperature falling below the normal value of 10 °C. When the supply temperature for the heat pump falls below 10°C, however, the c.o.p. of about 1.8 is not achieved anymore.

A solution is that in summer additional heat is stored. With the heat pump, heat can be derived from cold stores and cultivation cooling. Heat can also be derived from solar collectors, the greenhouse air or the outside air. The solar collectors or outside-air heat exchangers do not need to be advanced constructions. At summer temperatures, for instance, freezing phenomena do no exist .

The c.o.p. of about 1.8 requires further comments. A 7 MW lithium bromide heat pump achieves a c.o.p. of about 2.1 when the supply temperature is 11°C on the cold side (soil) and the delivery temperature is 35 °C on the warm side. The value of about 1.8 utilized herein holds for supply and delivery temperatures of 15°C and 44°C, respectively. For heating the greenhouse, water temperatures of about 45°C are desirable. It is noted in this connection that the given values relate to an example of a current gas-fired heat pump in a specific exemplary situation. It is, of course, always necessary to determine the correct values for the configuration to be eventually chosen. Starting from a certain amount of heat which is to be stored, it can be determined, by way of example, what volume of soil must be heated to store this amount of heat. For that purpose, a number of premises are to be made. It is assumed that an aquifer consists of water-saturated sand. It is further assumed that the maximum temperature increase that is achieved is 20°C. Given a 20°C temperature jump, the heat absorption of the soil is 61MJ/m³. Accordingly, with an amount of energy of 30.5TJ, 500,000 m³ of soil are heated. This is equivalent to a cube with sides of 80 meters.

Assuming heat migration over a distance of 10 meters in all directions, the cube will be 100 meters in all directions after six months. By then, the heat has spread over 1,000,000 m³ of soil. What is then left of the jump in temperature is 10 °C. This energy can then be absorbed again in winter, whereby the temperature of the soil falls to the original value again.

The system according to the invention, as already indicated above, further comprises a cheating system and optionally a heating boiler. This heating system serves to introduce heat into the greenhouse, for instance by means of a pipe system for hot water or a hot air supply. The heating boiler is preferably present in the case where high powers and/or high temperatures are desired. The off-gas of the boiler can also play a role in C0₂ fertilization.

The technology concept according to the invention is innovative for the glasshouse horticulture, compared with current applications, by the use of the gas-fired heat pump in combination with heat storage in the soil and optionally storage of additional outside air heat and/or solar heat. In the context of this invention, it is noted that the parts in the plant are always indicated in singular. This should be understood to mean, however, that, unless otherwise indicated, at least one such component should be present, it being very well possible for more than one component, for instance a heating boiler and the like, to be present. Hereinbelow follows a brief discussion of the various components of the system according to the invention, insofar as this has not already been done above.

Regarding the heating boiler, research has revealed that the desired thermal peak load power can best be supplied with (high-efficiency) natural gas-fired boilers. As has already been noted above, the option exists of designing the system without heating boiler. In that case, in the simplest system, the firing system of the heat pump is used as main source of heat. It is preferred, however, to make use of a separate gas boiler and/or H/P plant.

In the context of the invention, it may be advantageous to employ a so-called total energy plant, also referred to as combined heat and power plant (H/P plant) . The choice of whether or not to use such a plant depends to a large extent on local conditions, such as electricity rates, electricity demand (illumination) and the like. In the case where such an H/P plant is used, it can be advantageous to use the CO₂ coming from the combustion of natural gas for driving the plant, in the CO₂ fertilization.

What electric power is to be placed, typically depends to a large extent on the arrangements made with the electricity supplier about terms of return delivery and investment costs.

A plant of 400 KW can operate cold store, cultivation cooling and operating load in the case of a 10-ha business. Because this plant then runs practically at full continuity, it is certainly profitable.

Cultivation cooling and the cold store can perhaps be cooled with the heat pump. If not, about 0.4 MW of cooling units will be needed for a business of the above-indicated size .

In the above, it has been indicated that it may provide advantages to use a short-term heat buffer. In this case, this involves storage of heat in a basin with water. Studies show that a buffer of 100 m³/ha properly meets the requirements . Collectors for outside air heat and solar heat can be used in the invention with advantage.

When with the 8 MW heat pump outside air heat or solar heat is to be stored, a collector with a power of 3.6 MW is needed. Here, use can be made of simple, black heat exchangers .

The use of the system according to the invention occurs in the various seasons as follows, referring to the appended figure . In the figure, the situation in winter is outlined. Using the gas-fired heat pump, heat is passed from the soil storage to the low-temperature network. If desired, the heat of the boiler and the H/P plant can be passed directly into a high- temperature network. Optionally, the heat of the H/P plant can also supply the gas-fired heat pump. The electricity of the H/P is supplemented with purchased electricity for the peak loads .

Cooling (cold store or cultivation) can be generated both with the heat pump and with electricity. In principle, CO₂ is derived from the boiler. Optionally, it is also possible to use C0₂ from the H/P. The heat collector will mainly work in summer, but with the heat pump, also at low temperatures additional heat can be absorbed from outside.

In summer, by CO₂ fertilization, a heat surplus may form. The function of the heat pump can then be inverted. Heat is pumped into the soil. Source for the heat pump can then also be the solar heat or outside air heat collector.

In the figure, the inlet of the heat buffer is applied to the high temperature. The heat from the heat buffer can be supplied to the high- as well as the low- temperature network, depending on the temperature in the heat buffer.

Claims

1. An integrated system for energy supply and energy storage in glasshouse horticulture, at least comprising a greenhouse including a heating system, a gas-fired heat pump and a system for heat storage in the soil, whereby during a period of heat excess, heat can be stored in the soil, while in a period of heat demand, using the heat pump, heat is at least partly extracted from the soil .

2. A system according to claim 1, wherein the system for heat storage in the soil is based on an aquifer.

3. A system according to claim 1 or 2 , wherein further a gas boiler and/or a combined heat and power plant is used.

4. A system according to claims 1-3, wherein use is made of C0₂ fertilization of the greenhouse.

5. A system according to claims l-4-_> wherein the C0 fertilization takes place to a minimum level of at least 650 ppm.

6. A system according to claims 1-5, wherein a short-term heat buffer, more particularly a water storage, is used.

7. A system according to claims 1-6, wherein a heat pump based on lithium bromide is used.

8. A system according to claims 1-7, wherein further facilities are present for recovering heat from sunlight, outside air, surface water, electricity production and/or CO₂ production.

9. A system according to claim 8, wherein solar collectors are used.

10. A system according to claim 8 or 9, wherein the thus recovered heat is stored in the soil, utilizing a heat pump.

11. A system according to claims 1-10, wherein the heat storage and heat recovery in the soil occur substantially thermoneutrally.

12. A system according to claims 1-11, wherein an average soil temperature of minimally 10°C is employed.

13. A system according to claim 12, wherein the average soil temperature is at least 2.5, preferably at least 4.5°C above the normal soil temperature.