Is it possible to use a heatsink without a peltier plate?
so i assume that the rest of 26W is transformed into heat


This is troubling.  When you said 24W of heat I was assuming a 100W street lamp.

The best LED for this project, in my opinion, would be used would be the Cree XP-G3. 70% of the electricity would be converted to light and 30% heat. 

XP-G3 2.82V @ 350mA produces 730 mW of light when powered with 987 mW (2.62V x 350 mA) of electricity.



Note: Royal Blue is used in all white (phosphor pushed) XP-G3 LEDs. 

The Samsung LH351B is a viable alternative.  Half the price at an efficacy  cost of 6 lumens per watt.

Since it is solar/ battery powered project a peltier cooler is out of the question.  Your only option is a passive heatsink.  You could add a fan, when needed, to improve efficiency. That would only be on hot days. 

Denmark is a good place for LED street lights with a high temperature of 32°C. 

The only place to use solar power is where no other source is available.  The cost of deep discharge batteries makes solar a poor choice over most other sources of electricity.  


  but in datasheet is stated that it should use 14W,


Do not buy products from vendors that lie. Cree or Samsung are the only vendors for this project.  A battery powered LED must be efficient.  Cree currently makes the the most efficient high power LED.  Samsung makes the most efficient white mid-power LED (LM301B 228 lm/W) but would require too much real estate for a street light. 

A 25W street light could get you 9,000 lumens using Cree XP-G3 LEDs.  

Street lights have been 100-400W HPS.  

Below is a table from a 2011 US Dept. of Energy street light study. Lumen output ranges from 6,000 to 32,000 lumens. 9,000 lumens is not that bright. 



In December Denmark has 12.25 hours of night plus 2 hours for Astronomical Dusk and Dawn.  That means a minimum of 14 hours battery charge.  At 25W you will need 2A from a 12V lead-acid battery which equates to 30 Ah AGM battery.
You will need at least a 100W solar panel. You will need more capacity because it takes a long time (14-16 hours) to charge lead acid. and on a sunny day you will have less than 8 hours of sunlight sufficient for charging.  For cloudy days you should have batteries with about 3 days worth of charge. 

Your biggest issue is keeping the batteries warm in winter and cool in summer.  The capacity of batteries are calculated at 25°C. Heat is very bad for batteries.  Keep in mind in summer the capacity of a lead acid battery drops 6.25% for every degree over 25°C.  A battery damaged by heat cannot be repaired.

Sounds to me you guys are in over your head.