Geothermal energy for greenhouse applications gains a lot of interest in the Netherlands. This new concept which utilizes the heating source much better by means of heat pumps can increase the efficiency of this capital intensive energy source considerably.
In order to be able to pump warm water from earth layers and pump it back again, millions of euros are invested for the process, which demands much electricity for the drive of the pumps. So it is essential that the pumped cubic meters are used as efficient as possible. Further cooling down of the spring water can increase the utilization degree of a geo-source heavily.
Current greenhouse temperatures and existing greenhouse heating installations restrict the measure of cooling down. In general it appears that spring water (of for example 85 degrees on a depth of 2800m) cannot be cooled down any further than approx. 35 degrees Celsius.
Indirect pump system
VB Group member VB Projects and Carrier have been cooperating and have designed a concept with which spring water can be cooled down further (from 35 to 10 degrees). They use an indirect system with heat pumps. With the withdrawn energy, return water from the greenhouse can be heatened from 35 degrees to a higher useful temperature of for example 70 degrees. By doing this in four small steps following each other, the system with the existing heat pumps can be justified. With the extra cooling down a geothermal source can deliver 50 percent extra heat already.
VB Projects and Carrier have calculated that the investments in heat pumps in order to increase the value of the geothermal source can be earned back in a short period of time. Estimated to be a period of 1,5 years.
On application combined with geothermal sources, Carrier uses a system with 4 standard heating pumps type 30 XWH with venerable screw compressors, which are placed behind each other. This design functions efficient and because of the use of equivalent engines it is flexible and robust. When more capacity is needed, a redoubling to eight heating pumps for example is possible. The heating pump power stations are available from three to twelve Megawatt thermal.
A coordinating management system drives the heating pumps mutually and takes care for a perfect tuning and flexible functioning. By this, one can work on partial load between 100 and 15 percent. Also it can happen with a lower desired heating temperature that only 3 of the 4 heating pumps will be switched on. The management system calculates the optimum use of the engines himself, based on the adjusted heating desires.
It is true that for the heat pump system extra electrical energy is needed, but through a smart combination with a Cogen-installation (CHP) a maximum energy achievement can be delivered. The heat of 70 degrees produced by the heat pump can be increased with the thermal power of the Cogen to 80 or 85 degrees. With this temperature the (existing) heat buffer systems can be easily used without disturbance.